Strategic Priority: Exploring atmospheric, Earth system, and solar processes, variability, and change

Paleoclimate

Figure. This figure shows annual air temperature simulated by the NCAR Community Climate System Model (CCSM3) for four different past time periods: a warm period approximately 250 Mya – the Permian-Triassic, a period of abrupt warming approximately 55 Mya – the Paleocene-Eocene Thermal Maximum, a glacial period approximately 21 kya - the Last Glacial Maximum, and a cold period approximately 500 years ago - the Little Ice Age. This image illustrates the large range of climates under natural forcings. A comparison of the CCSM3 simulations with geologic data confirms that this model captures the magnitudes of change about right justifying its use of future climate projections. High resolution figure

Paleoclimates offer a unique perspective to understand both the Earth's climate sensitivity and stability. NCAR climate models have been used to study past natural variability of the Earth system since the 1970's with the pioneering work of Jill Williams, Eric Barron, John Kutzbach, and Warren Washington. The development of a coupled climate model, the Climate System Model (CSM), in the 1990's included a lower resolution (but otherwise equal) version of the model, PaleoCSM, which was particularly useful for the long simulations required to study past climates. PaleoCSM was successfully used to study mechanisms responsible for changes in the coupled climate system and to determine associated magnitudes of changes for various climatic variables. Simulations covered a large range of applications, including the last millennium, Holocene ENSO, the Last Glacial Maximum, Eocene, and Cretaceous. These simulations highlighted the importance of considering feedbacks among the atmosphere, ocean, land surface, and sea ice in establishing the magnitudes of past climate change to changes in past forcings. These simulations not only acted as a benchmark for CSM but allowed testing of various hypotheses of mechanisms to explain proxy records of past climate change.

A strong test of the Community Climate System Model (CCSM) is to simulate past climate against records from ice cores, tree rings, and other proxy data. Magnitudes and rates of past change also provide an important context for future climate changes. Within ESSL, we are exploring past changes over many different time periods: from the distant geologic past, with radically different continental configurations, when the Earth's surface temperature and latitudinal gradients were significantly different from present and levels of atmospheric carbon dioxide, methane, and other greenhouse gases reached levels up to ten or more times present levels; the last million years, when the Earth experienced a waxing and waning of ice ages and levels of atmospheric carbon dioxide, methane, and other greenhouse gases during the ice ages were reduced by half or more from present levels; and the last few millennia with colder periods extensively documented in the proxy record associated with solar fluctuations and volcanic eruptions. Each of these geologic periods gives us an improved understanding of the natural variability of the Earth system and our ability to model feedbacks in the climate system. By comparing climate simulations of Earth's past to the data from geological and geochemical archives, we can evaluate the accuracy of climate models such as CCSM that are used to look at Earth's future. At the same time, geologists have started to use CCSM to understand how their specific data can be understood in a more large scale, dynamical context. CCSM has become a valuable partner to field-based geological research.

CCSM has been applied to all these different time periods. The Permian-Triassic (PT) boundary, approximately 250 Ma, marks the largest extinction recorded in Earth's history, where across this boundary marine and terrestrial species were lost. Simulations of the atmospheric chemical state of the Permian were carried out with the Middle Atmosphere Community Climate Model (MACCM). These studies indicate that a large release of hydrogen sulfide from the anoxic oceans would lead to a large decrease in OH, the hydroxyl radical. This reduction in OH, in turn, leads to a large increase in methane lifetime (~250 years). The means methane can build up in the Permian atmosphere causing a collapse of atmospheric ozone, and a seven-fold increase in surface UV-B radiation. This massive increase in UV-B would lead to a die off of terrestrial life at that time. Proxy records for the deglaciation that started 21 thousand years ago indicate events with large freshwater inputs to the Atlantic Ocean basin as iceberg discharges into the high-latitude North Atlantic, Laurentide meltwater input to the Gulf of Mexico, or meltwater diversion to the North Atlantic via the St. Lawrence River and other eastern outlets. The climate responded, in the North Atlantic region and globally, to these freshwater events, but the responses varied among the events and are not completely understood. The sensitivity of the climate system to the magnitude and location of freshwater input into the North Atlantic has been studied using the fully coupled version of CCSM3 for glacial conditions. The results suggest that the response of the North Atlantic meridional overturning circulation is proportional, though not linearly, to the size of the freshwater added. On the other hand, the southward migration of the ITCZ over the tropical Atlantic displays a threshold response to the amount of freshwater forcing and hysteresis in the response versus recovery from the event. This has implications for detecting freshwater events using the Cariaco Basin records.

2008 and Beyond

Future plans a CCSM3 simulation of the climate of the Late Ordovician, a time of great cold and one of the earliest mass extinctions of life. Future plans also include deep time simulations of the Cretaceous time period prior to the massive asteroid impact that led to the demise of the dinosaurs. There will also be a focus on simulating the magnitudes and rates of past climate change on many time scales using the planned NCAR Earth System Model, which will allow us to explore more completely feedbacks with vegetation and ice sheets, atmospheric chemical changes, and the carbon and nitrogen cycles.

return to top

HIRDLS recovery and application

Figure. HIRDLS/Aura-EOS observations of temperature (top), ozone (middle) and nitric acid (bottom) during January 23, 2006 along the single satellite orbit (141° - 143° W). On the color plates the white contours represent contours of PV (potential vorticity, 1.25 and 1.75 PV-unit) indicating the position of the dynamical tropopause (1.75 PV-unit), and active areas of the air-mass exchanges as seen in the O3 and HNO3 spatial distributions. Several HIRDLS temperature vertical profiles with features of the double tropopause phenomenon are observed along this and adjacent HIRDLS orbits.

High resolution figure

The High Resolution Dynamics Limb Sounder (HIRDLS) is a 21 channel infrared limb scanning radiometer, jointly developed by ACD, the University of Colorado, and the Physics Department of Oxford University. It is designed to make observations of temperature, ozone, water vapor, and 8 other trace species, as well as PSC's, aerosols and cirrus clouds, from the upper troposphere to the mesosphere, with higher vertical resolution than has previously been available from space observations. HIRDLS was launched on the Aura spacecraft in July 2004. Despite an obstruction that limited the view to the atmosphere to a small fraction of the width of the optical beam, HIRDLS scientists demonstrated that there is recoverable atmospheric information in the signals. Initial results for temperature, ozone, PSC's and cirrus were very encouraging.

For example, episodes of stratospheric air intrusion in the troposphere seen by HIRDLS and MLS instruments have been analyzed by HIRDLS/ACD group. The Figure illustrates a clear episode of intrusions of air masses in the upper troposphere/lower stratosphere (UT/LS) using the high vertical probing of the atmosphere by the HIRDLS instrument. Indications of UT air intrusions from low latitudes into mid-latitude LS and indications of mid-latitude LS ozone and nitric acid transport into the tropical UT are remarkably seen in the Figure along the selected HIRDLS orbit (141° - 143° W) on Jan 23 2006. The effect of a sudden strong polar warming on the distribution of LS chemical species in mid- and high latitudes amplifies the intrusion of HNO3- and O3-rich air in the subtropics and mid-latitudes. In the NH hemisphere sub-tropics and mid-latitudes the remarkable double tropopause structures have been observed by HIRDLS, SABER and MLS instruments and reproduced by the meteorological analyses in the UT/LS. The HIRDLS science team is now providing analysis of dynamics and chemistry during this and other episodes highlighting importance of the high vertical resolution of satellite data in order to predict and resolve such phenomena in the UT/LS. Other observed episodes of stratospheric air mass intrusions into the UT are localized in their latitudinal and longitudinal spreads. However, the unique spatial resolution of HIRDLS temperature, ozone and HNO3 retrievals (~ 1 km in vertical and ~1 degree along the orbit) helps to study these events with consistent vertical and horizontal sampling along the orbits. It is expected that the assimilation of HIRDLS temperature and constituents retrievals into the global models will bring data analysis to a new level in UT/LS transport and chemistry studies.

In the next year the algorithms to correct the radiances will be improved to allow additional species to be recovered. In addition, emphasis will be placed on the use of the released data for scientific studies, especially of gravity waves, and UT/LS processes, notably strat-trop exchange. In particular, we expect to be heavily involved in the START08 experiment.

return to top

Solar Dynamo Modeling And Solar Cycle Prediction

Torsional oscillations (color levels) at 0.985 R and butterfly diagram (contour levels) for different levels of random forcing (ratio of 1 sigma fluctuation to mean) as indicated in the lower left corner of each panel. Torsional oscillations with clear association to the magnetic cycle are visible for fluctuations not exceeding more than 50% of the mean. The coherence of the magnetic field is lost for fluctuations exceeding 200% of the mean.

High resolution figure

 

Comparison of simulated polar flux (blue curves), cross-equatorial flux (red curves), and spot-producing toroidal flux integral (yellow shading) for surface poloidal flux input data averaged over (a) 27 rotations, and (b) 1 rotation. Meridional circulation used varies according to observations since 1995 (steady flow case is shown in dashed curves). Note particularly how much short term variability remains in the 1 rotation average case in the cross-equatorial and polar fluxes, compared to the spot producing toroidal flux integral. Also note slow-down in flow during 1995-2005 causes increase in cross-equatorial flux (solid red curve has a higher peak than dashed red curve), but the same slow-down in that flow causes decrease in polar flux (solid blue curve has a smaller peak than a dashed blue curve).

High resolution figure

The magnetic fields that are ultimately the source of the activity that takes place in the solar atmosphere have their origin inside the Sun, where convective, rotational, and other flows of highly electrically conducting plasma contribute to the operation of the dynamo. Work by HAO scientists has shown that the meridional circulation, a large-scale flow directed from the equator to the N and S poles at the solar surface and completing the circuit from the poles to the equator near the bottom of the convection zone, plays a fundamental role in in the dynamo process. It continuously transports poloidal magnetic fields from the surface to the tachocline at the base of the convective envelope where they are differentially stretched by the rotational shear therein to produce new toroidal magnetic fields. Buoyant magnetic flux tubes formed from these fields become the source for new poloidal fields at the surface, thus completing the dynamo cycle. HAO researchers have pioneered the development of the so-called flux-transport dynamo model that is based on this physical picture, and have had remarkable success in applying it to the problem of simulating and predicting solar cycle amplitudes.

During the last year, notable progress was made in continuing efforts to further develop and apply the flux-transport dynamo model to the interpretation and simulation of the Sun's cyclic magnetic activity. M. Dikpati and P. Gilman described the procedure that needs to be followed in order that dynamo model runs attain a calibrated equilibrium which can serve as the initial condition for meaningful simulations of the solar cycle. Their results show that runs covering nearly 60,000 years of model time are required to reach such a state. Following up on plans to test and refine the predictive capabilties of their model, Dikpati and Gilman conducted runs for the purpose of separately simulating and predicting the peak cycle amplitudes in the northern and southern solar hemispheres. They found that the model calculations could successfully reproduce the differences between the peak amplitudes in the two hemispheres, provided those differences exceeded several percent. The accuracy of these simulations was only slightly lower than that obtained for runs in which the hemispheric surface magnetic field data input to the model was combined rather than treated separately. Dikpati, Gilman, and G. deToma also carried out extensive tests of the relative skill of three different predictors of the solar cycle. In the tests, the behavior of the integrated tachocline toroidal flux derived from calibrated flux-transport dynamo model runs was compared with that of the cross-equatorial and polar magnetic fluxes obtained from the same simulations. Their results indicate that the toroidal flux has the highest predictive skill, with the correlations between the computed peaks of the three predictors and the observed solar cycle peaks being 0.96, 0.76, and 0.48, respectively, for the toroidal, cross-equatorial, and polar fluxes. Also, in 2006-2007, M. Rempel pursued plans to further study the behavior of his non-kinematic flux-transport dynamo model, with particular attention paid to the solar torsional oscillations. This model combines mean-field descriptions of turbulent transport and dynamo processes, and includes treatment of the Lorentz force feedback on the differential rotation and meridional circulation in the convection zone. Rempel investigated the effect of a highly variable meridional flow on model by adding random fluctuations with a given correlation time and length scale to the model's parameterized description of the turbulent angular momentum flux. For a correlation time of about 10 days, he found that the dynamo maintained coherence for fluctuations as large as 200% of the mean amplitude. However, a tighter constraint on the permitted level of variability is provided by the torsional oscillations; Rempel's results show that these variations in rotation lose coherence for fluctuations exceeding 50% of the mean amplitude. Rempel also conducted a general study of the possible origins of the torsional oscillations, finding that the poleward propagating branch at high latitudes could be the result of either mechanical or thermal forcing, but that the equatorward propagating branch at low latitudes likely has a thermal origin.

During the next year, Dikpati, Gilman and collaborators will continue work on the development of data assimilation techniques for use in simulating and predicting the solar cycle. Particular attention will be paid to sequential data assimilation which will enable simultaneous prediction of the amplitude, timing, and shape of the solar cycle. They will also investigate the sensitivity of cycle simulations to different assumed meridional circulation profiles. Rempel will continue to study the solar torsional oscillations, focusing on whether the thermal forcing of low-latitude oscillations is the result of quenching of the internal convective flux or enhanced radiative cooling at the solar surface. In collaboration with Yuhong Fan, he will use the fields derived from non-kinematic flux-transport dynamo simulations to investigate the formation and rise of flux tubes in the solar convection zone.

return to top

Upper Troposphere and Lower Stratosphere (UTLS) Initiative

The primary science objective of the UTLS program is to investigate transport processes that impact the chemical-microphysical distribution of the Ex-UTLS. The Ex-UTLS region has multi-scale dynamics (i.e., stratospheric circulation and tropospheric weather systems ) coupled with chemistry. It controls and redistributes a suite of radiatively significant species (ozone, water vapor, particles...).

High resolution figure

The Upper Troposphere and Lower Stratosphere (UTLS) initiative is focused on understanding and simulating UTLS processes that affect chemistry and climate. It brings together process based observing studies from a range of Earth Observation platforms (aircraft, satellites) to help improve critical process representations in models from the microscale to clouds scale to global scales. During FY07, the UTLS Initiative team made significant progress in field campaign planning, understanding chemical and microphysical measurements in the UTLS region and the representation of UTLS process in state-of-art chemistry-climate models.

FY07 is a year marked by first publications of science discoveries from two early field campaigns.  Among the three papers presenting results of data analyses using the START05 campaign during the HIAPER Progressive Science Missions, chemical mixing behavior of the tropopause region is characterized in detail for the first time with a contrast of difference meteorological conditions [Pan et al., 2007].  New particle formation was also found in the region of frontal zone [Young et al., 2007]. Also, the concept of unstable manifold in theoretical fluid dynamics was investigated using HIAPER chemical observations [Bowman et al., 2007]. These results brought new understanding to the dynamical, chemical, and microphysical behavior of the UTLS region. 

UTLS initiative team has made progress on the planning of two large field campaigns, START08 and DC3. During FY07, the START08 completed its proposal and funding approval process. The field campaign is gathering momentum to occur in Spring 2008.  More detail is given in a separate section of this report (link to the subtitle START08 field campaign). Deep Convective Clouds and Chemistry (DC3), planned for the summer of 2010 is an unprecedented effort cross cut between the chemistry and meteorology community.  At the end of the second year of planning, the scientific program overview document (SPO, led by Mary Barth) and the experiment design overview document (EDO, led by Chris Cantrell) are ready for submission.

Among the new progress in UTLS measurements, Andy Heymsfield is leading the progress of understanding the ice and humidity measurements from aircraft. The central finding from this research is the sensitivity of the ensemble-mean fallspeeds to the properties of the ice crystals, most importantly the ice crystal masses and indirectly the ice crystal shape(s). Bill Randel, Andrew Gettelman, and Laura Pan are leading the effort of integrating satellite data into planned UTLS studies using HIAPER. Two papers are published this year for UTLS ozone measurements from AIRS on Aqua satellite. 

Integration of observations and models is another on going activity supported by the UTLS initiative. Andrew Gettelman contributed to the 2006 WMO Ozone assessment, as a co-author of the modeling chapter, a co-author on two papers published regarding simulations of ozone in the future, as well as co-coordinator of the SPARC Chemistry Climate Model Validation Project (CCMVal). Laura Pan also contributed to the CCMVal project for model diagnostics in the UTLS region, including a paper that proposed a method of a standard diagnotic. As part of the CCMVal activity, ASP postdoc Simone Tilmes has made progress of evaluating NCAR community model WACCM using aircraft data. Her work is also a significant part of the START08 pre-campaign activity. Using the NCAR CTM MOZART-3, tracer relationships for major transport pathways are examined. The results as part of model prediction for UTLS dynamics and chemistry coupling, will be tested during the field campaign.  Additional modeling activities include the installation of the full stratosphere and troposphere version of the CLaMS model [Konopka et al., 2007] at NCAR. Bill Hall, in collaboration with Rolf Mueller and Paul Konopka made initial successful runs using this model to simulate the mixing observations from the START05 flights.

START08 field campaign will take the center stage of FY08 UTLS activities. Field campaign will also motivate involvement of ESSL staff and their collaborators from university and other national laboratories to participate in a focused investigation on climate related processes near the extratropical tropopause, including chemical transport and microphysics. 

return to top

Simulations And Observations Of Magnetic Flux Emergence And CME Initiation

Figure 1 : From Fan and Gibson (2007, ApJ v.668, p.1232). The top two rows of images show two MHD simulations of the eruption of a twisted flux rope in the coronal triggered by the onset of the torus instability where the erupting flux rope mainly shows an outward expansion (1st row of images), and the onset of the kink instability where the flux rope shows significant rotation as it erupts (2nd row of images). The bottom left panel shows an image observed by SoHO EIT in He II emission at 304 A of two large solar prominences erupting nearly simultaneously at the two limbs, one showing mainly outward expansion and the other showing significant rotation, similar to the two types of eruptions modeled by the simulations. The three remaining panels in the bottom row show snapshots of a CME that began at 01:30 UT on Feb 18 2003 observed by HAO's MLSO MK4 in white light, where the core prominence of the CME shows significant kinking motion.

High resolution figure

Figure 2 : Gibson & Fan (2006) model of a flux rope eruption that results in the rope breaking in two. The left image shows sample field lines for the pre-eruption flux rope. Red and black field lines are dipped field lines intersecting the central, vertical axis, pink-lavender are dipped field lines grazing the photosphere, and dark green and blue are additional rope field lines. Note that all of these field lines are rooted in the magnetic boundary of the ``rope bipole'', (red and blue circular poles, seen most clearly in ``Surviving rope" image). Not shown in this image are the arcade field lines, which extend over the rope and which are rooted in an extended linear bipolar arcade boundary (the blue, negative pole of which can be seen as the extended linear structure to the front of the image ). The right two images show the bifurcated rope later in its eruption. The top right image shows sample field lines of the escaping rope, which are rooted in the arcade boundary, and which demonstrate that the escaping rope's axis has rotated at least 90 degrees. The bottom right image shows sample field lines of the surviving rope, color-coded in the manner of the original rope. The surviving rope is rooted in the rope bipole, but some adjacent field lines (e.g., the orange ones in this image) have one footpoint in the rope-bipole boundary, and one in the arcade boundary.

High resolution figure

Solar-driven space weather can have significantly adverse consequences for the Earth and near-Earth environment. Coronal mass ejections (CMEs) are the principal solar drivers of space weather. It is therefore important to understand the origin and dynamic evolution of CMEs and what makes a CME geoeffective.

Using 3D magnetohydrodynamic simulations of the coronal magnetic field driven by the emergence of a twisted flux rope, HAO scientists have made significant advances in understanding the precursor structures and initiation mechanisms for coronal mass ejections (Fan and Gibson, ApJ, 668, 1232). Depending on how rapidly the ambient coronal magnetic field declines with height, the emerging flux rope may lose confinement and erupt through either the onset of the torus instability or the kink instability (see figure 1). In the former case the erupting flux rope primarily shows an outward expansion while in the latter case the flux rope develops significant writhing or rotation. Both types of eruptive behavior have been observed. In both cases a current sheet of sigmoid morphology is found to form prior to the onset of the eruption. The current sheet intensifies during the eruption and reconnections in the current sheet produce post-reconnection loops with cusp-shaped tops. Some sigmoid shaped dipped fields are found to remain beneath the post-reconnection loops during the early phase of the eruption in both cases. These results explain the observed presence of X-ray sigmoids in CME source regions, and the transition from X-ray sigmoid brightening to cusp-shaped post-flare X-ray loops , which in some cases are seen to straddle a remaining under-lying X-ray sigmoid, during an eruption.

Eruptions triggered by the torus instability or the kink instability can result in magnetic clouds which differ in their geoeffectiveness due to the different amount of rotation of the escaping flux rope that ultimately affects the orientation of the magnetic field impacting the Earth's magnetosphere. In the course of both types of eruptions, the coronal flux rope reconnects internally as well as with the surrounding fields in a manner that alters its magnetic connectivity, helicity and, arguably, even its topology. Moreover, in the case of the kinked eruption, the rope breaks in two during eruption, with a significantly twisted, surviving portion of the rope remaining behind which is a candidate for future eruptions (see attached figure 2 and Gibson and Fan 2006, JGR, 111, A12103). These changes would impact observations of CMEs embedded in the solar wind, in particular by complicating connections between such interplanetary CMEs and ``sources'' back at the Sun, and by leading to multiple eruptions from a single solar source.

With the successful launch of the Hinode satellite, which are now for the first time providing high quality data of photosphere vector magnetic field evolution in emerging active regions, simulations of the coronal magnetic field driven by the observed flux emergence and photospheric shearing/twisting motions will be carried out. These observationally driven event studies will allow more direct and quantitative comparison of model results with coronal multi-wavelength observations of CME events. These studies are the first step towards building a predictive model for the onset and the eruptive properties of coronal mass ejections.

return to top

Globalization of air quality and intercontinental transport

Figure 1. Decrease in springtime (March-April-May) mean surface ozone (ppbv) in the HTAP receptor regions from 20% emission reductions of ozone precursors NOx, VOC and CO individually within the source regions, and applying all perturbations together (ALL). Each group of five bars includes the 4 perturbations experiments; the 5th bar is the sum of the 3 "foreign" impacts. The bats denote the multi--model mean response (colored by source region) and whiskers span the full range of the individual model responses.

High resolution figure

Figure 2. Dependence of fire related TOA shortwave radiative forcing (color coded) and AOD (defined by symbol size) on the atmospheric burden of carbon aerosols and the emissions ratio (ER) of CB to OC. Isolines derived from linear regression analysis for TOA (solid lines) and AOD (dotted lines) are indicated.

High resolution figure

Figure 3. The November 2006 global average distribution of atmospheric carbon monoxide at an altitude of about 3 km as measured by the MOPITT instrument aboard NASA's Terra satellite. High levels of CO pollution are shown in red. The large plume of pollution from Indonesian fires is clearly visible. Some carbon monoxide from fires in northwestern Australia and end-of-season fires in the South American Amazon and southern Africa is also apparent, as are persistent plumes of industrial pollution leaving China and out over the Pacific Ocean toward North America. MOPITT data courtesy of the NCAR and University of Toronto MOPITT teams.

High resolution figure

The Task Force on Hemispheric Transport of Air Pollution (TF HTAP) was set up in 2004 by the United Nations Economy Commission for Europe (UNECE) Convention on Long-range Transboundary Air Pollution (LRTAP Convention). The TF HTAP was charged to obtain a better understanding of intercontinental transport of air pollution and to provide estimates of source-receptor (S/R) relationships for intercontinental air pollution. The first set of coordinated experiments under the HTAP model intercomparison examined the global impacts of 20% emission reductions of relevant anthropogenic pollutants in four regions of the world: North America, Europe, South Asia, and East Asia for various pollutants. This effort was international in scale and included 9-14 models. ACD scientists led the NCAR effort using CAM (Community Atmosphere Model) with chemistry (CAM-chem). CAM-chem is the next generation chemistry climate model within ACD for examining tropospheric chemistry and its interaction with climate. The development of CAM-chem has represented a major effort within the tropospheric global modeling group in ACD, and allows the incorporation of chemistry into CAM in both online and offline modes. The HTAP experiments allowed CAM-chem to be evaluated against other chemistry transport models and to contribute to an international assessment of the importance of intercontinental transport to air quality. Figure 1, from Chapter 5 of 2007 HTAP interim report summarizes this impact. Model intercomparisons are extremely important and the HTAP work will continue in FY2008 and will lead to a final report in 2009. The work is funded by NSF/NCAR.

In another study of chemistry climate interactions, Pfister et al. (Impact of the Summer 2004 Alaska Fires on TOA Clear-Sky Radiation Fluxes, J. Geophys. Res., accepted) performed a study where they estimated the radiative impact of wildfires in Alaska during the record wildfire season of 2004 by integrating model simulations with CAM-Chem and satellite observations of the top of the atmosphere (TOA) radiative fluxes from CERES and aerosol optical depth (AOD) from the MODIS and MISR instruments. Results for the summer of 2004 were compared to results for the summer of 2000 when fire activity in the boreal zone was low. Both observations and model show a decrease in TOA clear-sky fluxes over the Alaska fire region during summer 2004 of -7 +/- 6 W m-2 and -10 +/- 4 W m-2, respectively. About 2/3 of the change occurs in the longwave and 1/3 in the shortwave spectral range. Based on detailed model analysis it was estimated that changes in the longwave flux are predominantly due to a higher surface temperature in summer 2004 compared to 2000. The change in the shortwave flux is largely caused by scattering of solar radiation on organic carbon (OC) aerosols emitted from the 2004 fires. This cooling is somewhat mitigated by the warming effect due to absorbing black carbon (BC) aerosols emitted from the fires and to a lesser extent by ozone and other greenhouse gases produced and released from the fires. Sensitivity studies with varying aerosol emission scenarios indicate that the ratio of black to organic carbon aerosol emissions of the boreal fires used in this study needs to be increased considerably to match both observations of AOD and TOA radiation fluxes or the biomass burning aerosols must be considerably more absorbing than parameterized in the model. This dependence of fire related TOA shortwave radiative forcing (color coded) and AOD (defined by symbol size) on the atmospheric burden of carbon aerosols and the emissions ratio (ER) of CB to OC is shown in Figure 2. Isolines derived from linear regression analysis for TOA (solid lines) and AOD (dotted lines) are indicated. While this study cannot resolve the cause of this discrepancy it presents a powerful methodology to constrain aerosol emissions. This methodology will benefit from future improvements in measurements and modeling techniques. Work in FY2008 will focus on a similar analysis of Idaho fires in 2007. This work is funded by NASA and NSF/NCAR.

In support of the analysis of the MIRAGE campaign, ACD scientists have performed with CAM-chem a set of simulations to isolate the radiative forcing of the emissions (of ozone precursors and aerosols) from Mexico City. This preliminary analysis indicates that, based on the emissions of this single city, the shortwave radiative forcing is, locally, on the order 0f 0.5-1 mW/m2. While these values are quite small, it is important to recognize that many cities or regions around the world contribute emissions similar or larger to Mexico. In particular, we are in the process of evaluating how different these results are in the case of New York City emissions. We expect these analyses to be continued and finalized in FY2008. This work is funded by NASA, DOE, and NSF/NCAR.

The last few months of 2006 saw very poor air quality over Indonesia due to intense smoke from numerous fires on the islands of Sumatra and Borneo. The smoke consists of particulate pollutants (aerosols) such as soot in addition to other trace gases such as carbon monoxide (CO). Carbon monoxide is a very good pollution indicator since it is produced during combustion processes such as the burning of fossil fuels in urban and industrial areas as well as by wildfires and agricultural burning. Carbon monoxide and other fire pollutants also increase the concentration of ground-level ozone.

Satellite remote sensing provides a useful way to investigate the impact of these intense fires on the regional and global air quality. Figure 3 is a false-color image showing the global view of CO pollution levels at an altitude of roughly 3 km (700 millibars) as seen from space, with data collected during the month of November 2006 by the joint Canadian/U.S. Measurements Of Pollution In The Troposphere (MOPITT) mission aboard NASA's Terra satellite. A large plume of CO is seen flowing from the Indonesian Islands and spreading out into the Indian Ocean. This results in the most intense pollution feature in the world at this time.

Some burning takes place in Indonesia every year, although the number and intensity of the fires varies significantly, and consequently the amount of pollution that is generated changes year-to-year. MOPITT CO measurements over the Indonesian region showed distinct spikes from September to November at the end of 2002, 2004, and 2006. The high CO pollution emissions from the large number of Indonesian fires in these years often traveled very great distances and determined the inter-annual variability of pollution levels throughout the Southern Hemisphere. Drought periods over Indonesia are often brought on by the shift in the atmospheric circulation over the topical Pacific associated with El Nino conditions. Significant El Nino conditions closely match the occurrence of high CO pollutant loadings in 2002, 2004, and 2006. Even though the 2006 El Nino is considered mild, it led to significant pollution over Indonesia and neighboring countries. FY08 work will continue the use of satellite data, including MOPITT data, to examine factors affecting regional and global air quality. This work is funded by NASA and NSF/NCAR.

return to top

Model for Ozone And Related chemical Tracers (MOZART): Global chemistry-transport modeling

Figure 1. MOZART-4 has been used to calculate the impact of emissions from a specific region on the regional O3 concentrations. Top panel: Mexico City emissions were tracked for the period of the MIRAGE campaign (March 2006) and the resulting ozone mixing ratios have been averaged over the surface to 400 hPa altitude. Bottom panel: Ozone column amounts produced by Asian emissions in simulations for the INTEX-B campaign (April-May 2006).

High resolution figure

Figure 2. Average model contribution (fraction) of the individual source types to the tropospheric ozone column. Results are shown for each IONS site, averaged over all sites, and averaged over the entire contiguous US. Only days with sonde launches are included, except for the results for the contiguous US where the average is taken over 1 July - 15 August, 2004.

High resolution figure

The Model for Ozone and Related chemical Tracers (MOZART-4 [link to sp4.htm#14d]) is a three dimensional global chemical transport model designed to study processes that determine the chemical composition of the troposphere. ACD scientists use MOZART-4 to assist in the analysis of satellite observations, aircraft experiments and ozonesondes. The model is generally driven by NCEP meteorology, allowing simulation of specific time periods. The chemical mechanism can be modified to track the contributions of different types of sources to tropospheric composition.

MOZART-4 simulations have been run at various resolutions to assist in the analysis of the MIRAGE field experiment. The typical MOZART horizontal resolution of 2.8 degrees is too coarse to capture much of the variability observed by the aircraft around Mexico City, so simulations at 1.4 and 0.7 degrees have been performed, resulting in better agreement with the observations. To understand the relative importance of the various types of emissions in Mexico, the different sources (auto emissions, biomass burning) of CO have been "tagged" in the model. A mechanism has also been developed for MOZART that keeps track of the ozone production from a specified source of NO. This mechanism has been used to quantify the contribution of ozone produced by Mexico City emissions to the regional composition, as shown in Figure 1.

In collaboration with Anne Thompson (Penn State University), MOZART is being used in the analysis of ozonesonde measurements. During the ICARTT/INTEX-A (International Consortium on Atmospheric Research on Transport and Transformation/ Intercontinental Transport Experiment) campaign nearly 300 ozone and radiosonde profiles were collected over North America from 1 July-15 August 2004 as part of IONS (INTEX Ozonesonde Network Study, 2004). This data is being used for evaluating MOZART and for examining the ozone budget over the US. For this purpose, ACD scientists are tagging the ozone production from various NOx sources in the model (stratosphere, lightning, Alaskan/Canadian wildfires, anthropogenic and biomass burning sources in the contiguous US, and Asian anthropogenic and biomass burning sources). Figure 2 shows average model contributions of the individual source types to the tropospheric ozone column for the IONS sites as well as for the entire contiguous US. The average budget over the IONS sites reflects the general features of the budget for the entire contiguous US demonstrating that the sample of stations and launch days is a good representation of the large-scale picture. On average, anthropogenic and biomass sources in the US contribute about 28% to the tropospheric O3 column over the contiguous US, stratospheric ozone 20%, lightning 11%, fires in Alaska and Canada 3% and Asian sources 8%. An O3 laminae analysis has been applied to the soundings to infer gravity and Rossby wave influences on soundings (Thompson et al., 2007). The four budget terms derived from this analysis include a stratospheric ozone term, a boundary layer ozone term, an ozone term due to interactions of regional pollution with convection plus photochemical reactions from lightning generated NO and an advection term (recent transport and aged ozone). The results from this analysis have been compared with the information from the MOZART model. At most sites model and sondes agree well and the model represents the spatial variability in the IONS locations fairly well.

ACD scientists have studied the sensitivity of isoprene emission calculations in the MOZART model to input land cover characteristics and analyzed the impacts of changes in isoprene on the tropospheric budgets of atmospheric key species. MOZART has been updated with the online calculation of isoprene emissions based on the Model of Emissions of Gases and Aerosols from Nature (MEGAN) and the emissions model has been driven by three different land parameter inputs. They also included a tagging scheme in MOZART, which keeps track of the production of carbon containing species from isoprene oxidation. They found that the amount of tropospheric carbon monoxide (CO), formaldehyde (HCHO) and peroxyacetylnitrate (PAN) explained by isoprene oxidation is 9-16%, 15-27%, and 22-32%. Dependent on the isoprene emissions scenario, the changes in the global tropospheric burden were found to be up to 10% for CO, 15% for HCHO, and 20% for PAN. Changes for ozone are small on a global scale, but regionally differences are up to 3DU in the tropospheric column and up to 5 ppb in surface concentrations. Their results demonstrate that a careful integration of isoprene emissions and chemistry in global models can be of high importance for simulating the budgets of a number of atmospheric trace gases. They further demonstrated that the model tagging scheme has the capability of improving conventional methods of constraining isoprene emissions from space-borne HCHO column observations, especially in regions where a considerable part of the variability in the HCHO column is not related to isoprene (Pfister et al., 2007).

During FY08 MOZART will continue to be used to evaluate trace gas budgets and for analysis of field campaign data. This work is funded by NSF/NCAR and NASA.

return to top

Figure. This figure shows time series of the inferred changes over time in various quantities associated with tropical cyclones around the world based upon simulations with a high resolution WRF model of hurricane Katrina using values within 400 km of the eye of the storms as it evolved over time, and how the various quantities varied as the intensity of the storm varied. The quantities tracked here are the storm precipitation (green, right scale), and the surface heat exchanges in the form of sensible heat (SH, cyan) and latent heat (LH, dark blue) or equivalently evaporation, and the total of these (black) in 1021 Joules per year. The best track data for the tropical cyclones observed each year classified by maximum wind speed was used to relate to the corresponding model values. The dotted lines are linear trends. What this figure shows is an overall upward trend in all of these quantities, signaling an increase in tropical storm activity. However, the changes also reveal interannual variability and the two most active years are 1997 and 1992 which were El Niño years when the dominant activity was in the Pacific. It suggests that climate change is indeed playing a major role and that heavy rainfalls in hurricanes will be a major risk for flooding. From Trenberth et al (2006c). High resolution figure

Following the very active 2004 hurricane season in the North Atlantic, there has been much discussion about the likely role of global warming in contributing to increases in intensity of tropical cyclones. This was in part in response to suggestions by some in the community that it was all due to natural variability. Publications of findings of increases in intensity and duration of tropical storms, and in particular increases in category 4 and 5 storms since 1970, further inflamed the debate. Nature also played a major role by bringing the record breaking 2005 season, with its four category 5 storms, including the devastating Katrina. In the past year, this has been followed up by considerable work within ESSL devoted to understanding the changes in the environment and their effects on tropical cyclones. Several investigators working under different funding (mainly from NSF, but also DOE and NOAA) have been involved.

Causes of changes in tropical cyclones have been explored in several studies. The origins of the record breaking 2005 Atlantic hurricane season have been analyzed. Sea surface temperatures (SSTs) in the tropical (10° to 20°N) North Atlantic region critical for hurricanes were at record high levels in spite of all the hurricane activity. However, about half of the SST anomaly was related to global SST changes, and thus global warming. ENSO accounted for another important portion, but North Atlantic SST variations, as given through the Atlantic Multi-decadal Oscillation, contributed in only a minor way. The latter was important for the lull in activity from 1970 to about 1990, however. Climate models have been used to study the possible causes of SST changes in Atlantic and Pacific tropical storm cyclogenesis regions. The observed SST increases in these regions range from 0.32°C to 0.67°C over the 20th Century. The climate models examined suggest that century-timescale SST changes of this magnitude cannot be explained solely by unforced variability of the climate system. For the period 1906-2005, there is an 84% chance that external forcing explains at least 67% of observed SST increases in the two tropical cyclogenesis regions. Model "20th Century" simulations, with external forcing by combined anthropogenic and natural factors, are generally capable of replicating observed SST increases. In experiments in which forcing factors are varied individually rather than jointly, human-caused changes in greenhouse gases are the main driver of the 20th Century SST increases in both tropical cyclogenesis regions. The observed best track hurricane record has been analyzed for time varying sampling biases that might account for trends in intensity, but the signature of increased duration of weaker storms that might arise from less frequent sampling in the past is not present in the data. In another published study this past year, ESSL scientists outlined issues related to hurricane changes with climate change. This article provides some much needed balance to counter some quite misleading articles and claims that natural variability alone is responsible for the observed changes.

Another major topic has been the energy and water cycles of hurricanes and their role in the climate system. A further study has computed how much moisture that ends up as rain in hurricanes comes from local evaporation in the storm versus large-scale convergence. This has been analyzed in a model framework using WRF at high resolution for simulations run for observed storms, in particular Ivan in 2004 and Katrina in 2005 that are realistic. Model sensitivity runs have also been made with SSTs increased and decreased by 1°C. Results demonstrate the overwhelming dominance of moisture convergence into the storms, in spite of the critical role of the surface evaporative source, and have implications for the changing environment on hurricanes as climate changes. In another study, these model results have been related empirically to the maximum sustained wind in the model and the results used with the "best track" global observed data on tropical cyclones to deduce how surface fluxes and precipitation in hurricanes have changed since 1970. Hurricanes appear to play a key role in climate and that role is increasing over time as SSTs rise. These sorts of studies are expected to continue especially as models improve and can be run at higher resolution and coupled with ocean models.

return to top

Climate Variability and Predictability World Climate Research Program (CLIVAR)

CLIVAR: Climate Process Teams (CPTs)

Figure 2. Upper-ocean vertical profiles of zonally-integrated, time-mean total advective (Eulerian mean + eddy-induced) heat transport at 49.4°S from integrations with (NSEF) and without (CONTROL) the new near-surface eddy flux scheme and from an eddy resolving model (ER). The latter is used to represent "truth." Arguably, the most dramatic effects of the elimination of the strong near-surface eddy-induced circulations with the new scheme are evident in this figure. Although both CONTROL and NSEF have similar vertically integrated total advective transports, their vertical structures differ substantially. In particular, the NSEF profile is in remarkably good agreement with the profile from ER, both in magnitude and shape. In contrast, the CONTROL profile has alternating northward and southward transports in the upper 200 m, reflecting the dominance of the spurious near-surface eddy-induced circulation. Supported with the ER data, we believe that the NSEF profile is more realistic and will improve the upper-ocean structures of all tracers.

Figure 1. Time-mean salinity distributions (in psu) at a depth of 1100 m in the North Atlantic. The observational climatological distribution is given in panel a, clearly showing the spreading of the Mediterranean outflow water into the interior Atlantic Ocean to form the Mediterranean Salt Tongue (MST). Without the parameterized Mediterranean overflow, the MST is completely absent (panel b). With the new parameterization, the MST is recovered as the dominant pattern in the North Atlantic in both uncoupled (panel c) and coupled (panel d) ocean simulations. High resolution figure

ESSL scientists remain involved in leadership of the Climate Variability and Predictability (CLIVAR) initiative of the World Climate Research Programme (WCRP) through membership on various national and international CLIVAR panels, as well as through contributions to CLIVAR goals and objectives as research scientists. The purpose of CLIVAR is to investigate climate variability and predictability on time-scales from months to decades and the response of the climate system to anthropogenic forcing. CLIVAR, as one of the major components of the WCRP, started in 1998 and has a lifetime of 15 years. It focuses on the role of the coupled ocean and atmosphere within the overall climate system, with emphasis on variability, especially within the oceans, on seasonal to centennial time scales. CLIVAR intends to explore predictability and how to improve predictions of climate variability and climate change using existing, reanalyzed, and new global observations, enhanced coupled ocean-atmosphere-land-ice models, and paleoclimate records.

A major effort of the U.S. CLIVAR program has been the introduction and fostering of Climate Process Teams (CPTs). A CPT is a team of theoreticians, observationalists, process modelers, and coupled climate modelers formed around specific issues or key uncertainties. They aim to link process-oriented research to modeling for the purpose of addressing key uncertainties in coupled climate models. Within ESSL, major ocean model developments are proceeding under the auspices of the CPTs on both gravity current entrainment and eddy mixed layer interaction in collaboration with the external university and laboratory community.

The CPT on gravity current entrainment has resulted in a new parameterized Mediterranean overflow scheme based on the marginal sea boundary condition of Price and Yang (1998, in Ocean Modeling and Parameterization, Kluwer Academic, 155-170.) This new parameterization has been implemented in the ocean component of the Community Climate System Model version 3 (CCSM3) to represent exchanges through the Strait of Gibraltar, associated entrainment and intrusion of overflow product water into the Atlantic. Previously, in coarse resolution model versions with a closed Strait, this physics has been either missing in uncoupled configurations or both only partially and unphysically treated as a surface salt exchange when fully coupled. The two major criteria satisfied by the implementation in a fully coupled climate model and a global ocean model are stable solutions and projection of the overflow signal across the Atlantic basin at about 1100 m depth. In both configurations, the transports of inflow, source and entrainment water are all within the range of observed estimates. The properties of the product water differ little from observed estimates and both the uncoupled and coupled models develop a Mediterranean salt tongue that spreads west and south from the Strait with a signature reminiscent of the observed hydrography (see Figure 1). In the coupled solution, the impact of the improved overflow physics on the global climate is minimal, with North Atlantic sea surface temperatures and heat fluxes changing generally by less than 1°C and 15 W m^-2, respectively.

The CPT on eddy mixed layer interaction has resulted in the implementation of a new near-surface eddy flux parameterization and a new prescription for the surface intensification and abyssal reduction of the tracer diffusivities in the CCSM3 ocean component. The former involves the near-boundary eddy flux parameterization of Ferrari and McWilliams (2008, J. Climate, in press). This scheme includes the effects of diabatic mesoscale fluxes within the surface layer. The experiments with the new parameterization show significant improvements compared to a control integration that tapers the effects of the eddies as the surface is approached. Such surface tapering is typical of present implementations of eddy transport in some current ocean models. The comparison is also promising versus available observations and results from an eddy-resolving model. These improvements include the elimination of strong, near-surface, eddy-induced circulations and a better heat transport profile in the upper-ocean (see Figure 2). The experiments with the new scheme also show reduced abyssal cooling and diminished trends in the potential temperature drifts. Furthermore the need for any ad-hoc, near-surface taper functions is eliminated. The impact of the new parameterization is mostly associated with the modified eddy-induced velocity treatment near the surface. The mixed layer, i.e. the regions of weak stratification at the ocean surface, is found to be a good proxy for the sum of the boundary layer depth and transition layer thickness.

The surface intensification and abyssal reduction of the tracer diffusivities are simply achieved using a stratification dependent vertical profile. Experiments with vertical variations of diffusivities as large as 4000 m² s^-1 within the surface diabatic layer, diminishing to 400 m² s^-1 or so by a depth of 2 km yield solutions that compare more favorably with the available observations. These include an improved representation of the vertical structure and transport of the eddy-induced velocity in the upper-ocean North Pacific, a reduced warm bias in the upper ocean, including the equatorial Pacific, and improved southward heat transport in the low- to mid-latitude Southern Hemisphere. There is also a modest enhancement of abyssal stratification in the Southern Ocean.

The ocean CPTs have been documented in three published / in press articles: Wu, Danabasoglu, and Large (2007, Ocean Modelling, v19, 31-52) for the Mediterranean overflow parameterization; Danabasoglu, Ferrari, and McWilliams (2008, J. Climate, in press) for the near-surface eddy flux parameterization; and Danabasoglu and Marshall (2007, Ocean Modelling, v18, 122-141) for the vertical variations of the diffusivities.

For the CPT on gravity current entrainment, the current work involves the adaptation of the Mediterranean overflow scheme to the Denmark Strait and Faroe Bank Channel Overflows, proper representations of which are believed to have significant climate impacts. For the CPT on the eddy mixed layer interaction, implementation of a new submesoscale parameterization is underway. The climate impacts of these new parameterizations, a primary purpose of these CPTs, will be investigated next year (the final year of these projects).

return to top

Convection, Flux Tubes, And Waves In The Solar Interior

Convective patterns in a simulation of solar convection. Shown are (a) the radial velocity (b) the radial vorticity, (c) the horizontal divergence and (d) the temperature perturbation near the outer surface.

High resolution figure

The energy liberated through the nuclear burning of hydrogen in the core of the Sun is transported outward by radiative diffusion for radii less than about 0.7 R_sun, and by convection within that portion of the interior located between 0.7 R_sun and the photosphere. The structure and dynamics of this outer convective envelope, the nature of the interface between it and the underlying stably stratified radiative interior, and the hydrodynamical and magnetohydrodynamical (MHD) processes that take place within these layers are critical to understanding the operation of the solar dynamo, the transport and emergence of dynamo-generated magnetic fields, and the properties of the Sun's differential rotation and meridional circulation. During 2007, HAO researchers made substantial progress on plans to improve numerical models of turbulent solar convection, simulate the buoyant rise of magnetic flux ropes, and investigate the properties of waves and instabilities that can exist in the region below the convection zone.

M. Miesch and collaborators A. S. Brun (CEA/Saclay), M. K. Browning (UC, Berkeley), J. Toomre and B. Brown (both JILA/CU) have continued to use high-resolution 3D simulations on scalably parallel supercomputers to study the properties of solar convection. Recent results have provided information about the structure, lifetime, and propagation of convective patterns in the upper part of the convection zone, and about how convective flows can turbulently pump magnetic flux into the tachocline (the rotational shear layer at the bottom of the convection zone) where it is organized into toroidal bands. Miesch and co-workers have also used the computational tools they developed for simulating solar convection to investigate the structure of convection in rapidly rotating solar-type stars. Y. Fan and M. Rempel have studied the formation and rise of buoyant magnetic flux tubes, using the toroidal and poloidal magnetic field distributions derived from a flux-transport dynamo model as the initial state for their simulations. For conditions corresponding to solar cycle maximum, the tubes that reach the top of the convection zone have properties that agree well with observations, and exhibit a field line twist whose sense is influenced by the interaction between the rising tube and the mean poloidal field present in the envelope. These calculations were carried out as part of a project supported by the NCAR Director's Opportunity Fund.

M. Dikpati, P. Gilman and M. Miesch have examined the linear and nonlinear evolution of global instabilities of toroidal magnetic fields and differential rotation in the solar tachocline. Their 3D analyses extend previous 2D results, and delineate the circumstances under which the so-called clam-shell and tipping instabilities grow and saturate. T. Rogers and K. MacGregor have conducted 2D numerical simulations of gravity wave-driven flows in the tachocline, solving the complete hydrodynamic equations rather than relying on a quasilinear description of the interaction between WKB waves and the mean flow, as has generally been done heretofore. They find that gravity wave forcing can, under idealized cicumstances, lead to the development of oscillatory shear flows containing critical layers, but that flows of this type might be more difficult to produce under conditions more typical of the solar interior.

In the coming year, convection simulations will be performed to quantify the efficiency of magnetic pumping and the generation of toroidal fields by rotational shear. Stochastic forcing of the upper boundary will be introduced to explore the influence of supergranulation of deep convection. Flux tube studies will proceed with simulations of the formation and rise of tubes at different solar cycle phases to examine the cycle variation of the twist of emerging tubes. Studies of global MHD instabilities of the tachocline will continue in order to further investigate their nonlinear evolution in 3D and to examine their relation to active longitudes on the solar surface. Studies of gravity wave-driven flows inside the Sun will focus on determining what role internal waves may have played in establishing the uniform rotation of the solar radiative interior.

return to top

Spectro-polarimetric studies of magnetic fields in the lower solar atmosphere

High resolution figure

The Lower Solar Atmosphere (LSA) section studies the evolution of the solar magnetic field and its interactions with the plasma from its emergence through the photosphere up to the chromosphere. The magnetic flux rises from the interior due to buoyancy and becomes directly measurable at the photospheric level by means of spectro-polarimetric observations. In this layer the dynamics is dominated by convective plasma motion and the magnetic field is forced to follow these flows. As it moves upwards into the upper photosphere and lower chromosphere, a transition occurs to a completely different physical regime in which magnetic forces take over and dominate the dynamics. Understanding the implications of this transition is the main challenge of the LSA section, which is heavily driven by observations of the polarimetric signatures imprinted by magnetic fields on photospheric and chromospheric spectral lines.

The most significant event for the LSA during the past year has been the launch and operation of the Hinode spacecraft. HAO has contributed in a very significant manner to the development of Hinode, mainly through the design, construction and calibration of the Spectro-polarimetric (SP) instrument whose goal is to provide the best existing datasets of the photospheric magnetic field. During these early operations, LSA has worked on the development of community data analysis and inversion algorithms, primarily through (but not limited to) the CSAC strategic initiative. LSA staff have been heavily involved in the first scientific results of this important mission, leading the pioneering work that is being done with the SP instrument. Highlights of such groundbreaking results are the detection of an ubiquitous horizontal component of the magnetic field (nearly as strong as the vertical component), the discovery of scattering polarization signals in the photospheric 630.2 nm lines and the detailed observation (resolved in space and time) of small-scale flux emergence through the photosphere. Besides Hinode, LSA continues supporting work for other instruments. For example, SPINOR is nearly finalized and is already supplying data of great interest. In particular, a couple of novel observations from SPINOR during this year may have provided the final clue to settle the controversy surrounding the so-called "solar oxygen crisis" in favor of the reduced chemical abundances. Other instrumental projects include the balloon experiment Sunrise, which recently completed a test flight with success, the Prominence Magnetometer (ProMag) and the Advanced Technology Solar Telescope (ATST).

For next year we expect that there will be studies using the Hinode SP in combination with a chromospheric imager and/or the soft X-ray telescope. This would allow us to investigate the coupling between photosphere, chromosphere and corona with an unprecedented spatial resolution. LSA has started a collaboration with the Solar Interior and Variability section to compare state of the art numerical simulations of magnetoconvection with Hinode data. Some preliminary results of this effort may be available during that period. Finally, with the completion of SPINOR, instrumental efforts will shift to ProMag and Sunrise.

return to top

MHD Physics Of The Solar Corona And Wind

The solar corona kept by heating mechanisms to million-degree temperatures is essentially fully ionized. Embedded with a magnetic field of about 10 Gauss at the coronal base, this atmosphere is an excellent conductor of both heat and electric currents. Outward thermal conduction of heat, aided by MHD and plasma waves, enables the outer corona to expand continuously into the solar wind filling interplanetary space. High electrical conductivity enables the magnetic fields to store significant amount of energy that is released through thin sheets of electric currents that must dissipate resistively despite the high conductivity. It is widely believed that this MHD process heats the corona ubiquitously and produces the occasional impulsive flares. The corona is not static, of course. The coronal magnetic field changes in time, reversing its global polarity once every 11 years by the dynamo actions in the solar interior. Thus, the dynamo rejuvenates the corona in each cycle, producing flares and sending daily coronal mass ejections into the more slowly varying solar wind. This part of the solar atmosphere controls the near-Earth solar-wind conditions as well as the rate of mass ejections that impact the Earth's magnetosphere violently.

BC Low and his collaborators investigate the basic MHD processes of the corona that have remained physically not well understood, one of the obstacles to advancement in space-weather prediction. The theory of coronal heating by current sheets, due to E. N. Parker, University of Chicago, poses nonlinear mathematical problems involving variations in all three spatial dimensions. A sophisticated numerical approach is premature because the physical questions relating to magnetic topology need to be put in quantitative forms. Low (2006a) has discovered a general mathematical description of twisted, three-dimensional magnetic fields in terms of the topology of their flux surfaces. This description generalizes the conventional concepts of magnetic helicity as a measure of magnetic twist (Woltjer 1958, Berger & Field 1984). Among the first applications of this development is the precise definition of a topologically untwisted field whose MHD properties render the Parker problem less intractable than the more general ones posed by the twisted fields. It became possible for Low (2006b, 2007) to demonstrate explicitly how current sheets are produced spontaneously as result of the tendency to trap magnetic flux within each parcel of plasma. This development has also revised our physical understanding of nonlinear models for extrapolating coronal magnetic fields from their observed values at the coronal base, models in great demand in the university community. With Natasha Flyer (IMaGe), Low investigated a long-standing but poorly understood issue with these models. They showed that a proper mathematical formulation of the model requires an explicit treatment of the physics of spontaneous current sheets (Low & Flyer 2007). With Piotr Smolarkievicz (MMM), Low investigates the formation of current sheet formation in a three-dimensional, time-dependent MHD numerical model, with a novel use of the results in Low (2006a) to describe magnetic fields in terms of their flux surfaces.

In a separate project with Aase Marit Janse, graduate student, Oslo University, Low investigates the implosion of coronal magnetic structures following a significant magnetic-energy release (Janse & Low 2007). This interaction between a magnetic flux with its surrounding is a general feature of magnetic-field evolution in the corona, generally neglected in the current research thinking, but will be investigated further by a broadly defined group of collaborators that include Janse and Ramit Battacharyya, both ASP Post-doctoral Fellows; Flyer; Smolarkievicz; Bengt Fornberg, Colorado University; Kenneth Miller, Wichita State University; Mei Zhang, Beijing Astronomical Observatory and NCAR Affiliate Scientist. We define basic physical questions suggested by observation, build quantitative mathematical models to address them, and pursue broad areas of concerns systematically as described in two recent reviews on the solar interior-atmospheric system (Zhang & Low 2005, Athay, Low, & White 2007). The above report reflects our present emphasis on current sheet formation.

Resources:

• R. G. Athay, B. C. Low, O. R. White 2007, "The Solar Interior-Atmospheric System", in Proceedings of the National Solar Observatory Workshop on Surface and Atmospheric Influences on Solar Activity, ed. R. Howe and R. Komm, 2007.
• A. M. Janse & B. C. Low 2007, Astron. & Astrophys. 472, 957
• M. Berger & G. Field 1984, J. Fluid Mech. 147, 133
• B. C. Low 2006a, ApJ 646, 1288.
• B. C. Low 2006b, ApJ 649, 1064
• B. C. Low 2007, J Phys. Plasmas, submitted.
• B. C. Low & N. Flyer 2007, ApJ 688, 557
• L. Woltjer 1958, Proc. Natl. Acad. Sci. 44, 489.
• M. Zhang & B. C. Low 2005, Ann. Rev. Astron. Astrophys. 43, 103

return to top

Chemistry and dynamics of the middle atmosphere

Amplitude of the diurnal tide at the equator (top) showing that enhanced maxima occur during the vernal equinox in years where the QBO wind in the lower stratosphere (bottom) is in the westerly phase.

High resolution figure

This plot shows the total column (bottom panel) and vertical profile from mid-troposphere to mid-stratosphere (upper panel) of hydrogen fluoride (HF). Each point is a daily observation from the Thule site. The high springtime column values are due to cooling and subsidence of the Arctic atmosphere during polar night. The ascent of the airmass to the nominal summertime state is shown by the profile sticks where lower HF mixing ratio values move upwards into the lower stratosphere. HF is the predominant reservoir for fluorine, which largely enters the atmosphere as CFCs. We may expect that due to the Montreal Protocol limits on CFC production, fluorine will stabilize which may be the case shown by the quadratic fit to summertime values (red curve). The linear fit (blue) though continues to show a modest increase in fluorine.

High resolution figure

A significant fraction of the solar energy deposited in the upper part of the middle atmosphere is realized as heat as a consequence of exothermic chemical reactions, including reaction between ozone and atomic hydrogen. The heating varies with time in response to variations in composition due to photochemistry and to vertical advection by tides and other processes. ACD scientists are involved in a number of studies to examine the nature of variability in temperature, ozone, and tides.

ACD scientists, in collaboration with J. Xu (Chinese Academy of Science), H.-L. Liu and Q. Wu (HAO), M. Mlynczak (NASA) and J. Russell (Hampton U.), investigated the variations in middle atmosphere temperature using observations from the SABER instrument on the TIMED satellite. The results characterize the semi-annual, annual, quasi-biennial and longer term changes over a layer extending from the lower stratosphere to the mesopause. In low latitudes, the semi-annual and quasi-biennial variations are dominant while in middle and high latitudes, the annual cycle is dominant. The longer term changes include a solar cycle but may also include other trends or changes. In another study, the same group of collaborators looked at the variations of the diurnal tide. The diurnal tide is very large in the upper mesosphere and reaches amplitudes of more than 20K and tens of m/s in horizontal wind speed. The tide was found to have a significant interannual modulation associated with the quasi-biennial oscillation in the lower tropical stratosphere. This is an example of the dynamical coupling between the stratosphere and mesosphere. Analysis of SABER temperature is ongoing.

ACD scientists, in collaboration with D. Pancheva and N. Mitchell (Univ. Bath, UK), M. Mlynczak (NASA) and J. Russell (Hampton U.), also investigated variability of the semidiurnal (12-hour) tide and its dependence on such factors as planetary waves, background winds, and tide-tide interactions. This study found a persistent correlation between the semidiurnal tide measured by radar in high northern latitudes and the planetary wave variability in the stratosphere of the southern hemisphere. The planetary waves were observed by the SABER instrument on TIMED. The correlation implies that there is a global scale interaction and response between planetary wave and the tide.

In collaboration with ACD visitor K. Matthes (also at Free University of Berlin) ACD scientists have used the SOCRATES model to investigate the response of stratospheric ozone and temperature to the solar flux variations in the presence of a quasi-biennial oscillation (QBO) in tropical winds. Model integrations demonstrate that the QBO contamination of the solar response cannot be completely removed even with analysis periods of more than 40 years.

An ongoing investigation by ACD scientists is focused on understanding large differences in observed ozone in the upper mesosphere and that in numerical models (WACCM and ROSE models, as well as by other non-NCAR models). In particular, the observations indicate that the ozone concentration is substantially higher than appears in the models. This is a complex problem that involves dynamical transport of atomic oxygen and water, molecular diffusion for interchange of air between the thermosphere and middle atmosphere, and sensitivity to photolysis and chemical rate coefficients. The investigation is using data from a number of satellites to constrain the dynamics, diffusivity and photochemistry of the models.

ACD scientists are also involved in longer-term remote sensing measurements of atmospheric constituents. As part of the international Network for the Detection of Atmospheric Composition Change (NDACC), ACD scientists operate an infrared Fourier transform spectrometer (FTS) at the Thule, Greenland (76.53N). The NDACC is a network of high quality ground based observing stations each with a suite of instruments for early measurement of changes in the composition and state of the stratosphere and troposphere and determination of their causes. Operation of the spectrometer at Thule is autonomous, but can be controlled from Boulder via internet link. Solar observations are typically made on 40% of the possible 225 sunlit days at the high Arctic site. Spectra are analyzed for column amounts of both stratospheric gases important in ozone chemistry and tropospheric gases related to climate change. These data products are then archived for community use. The broad-band, high resolution spectra contain signatures of upwards of 25 trace species which can provide course vertical profiles of some gases. The spectra are a long term record of the evolution of the Arctic atmosphere. During the 2007 IPY a 15 day observational intensive was undertaken in which 38h of data were recorded. Such a baseline data set will contribute to our knowledge of the detectability and upper limits of many trace species. In conjunction with other observations from the network, composed mostly of research teams from nations other than the US, the ACD Thule measurements are being used in the validation activities of recently launched satellite-borne instruments. Collaborations are ongoing with instruments aboard the ENVISAT (EU) platform, the NASA EOS-Terra satellite (US, Canada, Japan), the NASA EOS-Aura satellite (US, UK, Netherlands, Finland) and the Atmospheric Chemistry Experiment (ACE) instrument aboard the Canadian SCISAT-1 satellite.

return to top

UTLS dynamics, trends, and composition

Figure 1. Trends in tropopause height derived from quality-controlled radiosonde observations over 1980-2005. Each point represents a particular radiosonde station, and error bars denote the two-sigma statistical uncertainty in the trends. From Seidel and Randel, JGR, 2006.

High resolution figure

Figure 2. Near-global observations of carbon monoxide (CO) at 100 hPa obtained from Microwave Limb Sounder (MLS) observations during July-August 2005. The strong maximum over Asia is associated with the monsoon anticyclone in the upper troposphere, as identified by the wind vectors (from NCEP reanalysis data). From Park et al, 2007.

High resolution figure

Figure 3. Frequency of occurrence of clouds near the tropical tropopause derived from satellite measurements. HIRDLS data are from 2005 – 2007 while the HALOE data are from 1998 – 2005. These clouds are involved in processes that dehydrate the upper troposphere and lower stratosphere.

High resolution figure

Figure 4. Correlation diagram of ozone and water vapor measured December 1, 2005, onboard HIAPER during the START experiment (top), used to identify air masses with stratospheric (red), tropospheric (green) and mixed characteristics (blue). These identifications are mapped back to the flight track (bottom), together with the background meteorological field (the grey shaded region denotes location of the jet core). This analyses shows that the mixed air mass is minimal on the anticyclonic (equatorward) side of the jet, but extensive on the cyclonic (poleward) side of the jet. From Pan et al, 2007.

High resolution figure

ACD scientists have focused on understanding the chemical and dynamical behavior of the tropopause region, and its long-term variability, to help quantify processes which contribute to coupling in the upper troposphere - lower stratosphere (UTLS).

Climate models predict a rise in global tropopause height in response to anthropogenic climate change, but observations of tropopause changes have been limited by uncertainties in the data record. ACD scientists, in collaboration with Dian Seidel of the NOAA Air Resources Laboratory, performed an analysis of global tropopause trends based on a carefully selected set of historical radiosonde data spanning ~1960-2005. Results showed a systematic increase in height of the tropopause by ~100 m/decade, spanning most of the globe (Fig. 1). Further analyses showed that the tropopause height trends were significantly correlated with cooling of lower stratospheric temperatures, but not with changes in tropospheric temperatures; the implication is that the tropopause may not be an unambiguous indicator of global warming, as proposed by other researchers. ACD scientists also published new observational analyses of the so-called extratropical tropopause inversion layer (TIL), using high vertical resolution GPS radio occultation temperature data. This work was in collaboration with Piers Forster of the University of Leeds. The TIL is characterized by a systematic temperature increase of 3-5 K above the tropopause, first identified in high resolution radiosonde measurements. The GPS data show that the TIL is a global phenomenon evident in both hemispheres and all seasons. This work furthermore proposes a novel radiative forcing mechanism for the TIL, based on the detailed vertical structure of ozone and water vapor near the tropopause.

ACD scientists used Microwave Limb Sounder (MLS) satellite observations to study constituent variability in the Asian summer monsoon anticyclone in the UTLS region. The monsoon anticyclone is a hemispheric-scale circulation associated with persistent deep convection over India and Southeast Asia during Northern Hemisphere summer (June-August). The strong circulation acts to confine and isolate air within the anticyclone in the UTLS region (~12-18 km). Carbon monoxide (CO) measured by MLS shows a persistent maximum within the anticyclone throughout summer (Fig. 2), which results from the vertical transport of near-surface air into the upper troposphere by deep convection. This association is supported by strong temporal correlation between CO and the underlying deep convection. MLS measurements of ozone and water vapor confirm this overall paradigm of upward transport and horizontal confinement. Ongoing work involves quantifying pathways for transport into and out of the anticyclone, plus studying the influence of the anticyclone on stratosphere-troposphere coupling.

The High Resolution Dynamics Limb Sounder (HIRDLS) satellite experiment has 21 spectral channels, several of which are sensitive to the observation of clouds in the stratosphere and troposphere. ACD scientists and HIRDLS team members used several channels to characterize cloud behavior near the tropopause using HIRDLS data, and made comparisons to previous observations. The statistical behavior of clouds observed by HIRDLS near the tropopause (at 82 hPa) is in good agreement with previous observations by the Halogen Occultation (HALOE) experiment (Fig. 3), with most frequent occurrence over the Maritime Continent, South America, and Africa. HIRDLS is providing daily global observations that will allow detailed analyses of these clouds and their links to processes that dehydrate the upper troposphere and lower stratosphere.

ACD scientists also investigated chemical transport and mixing near the extratropical tropopause, using aircraft measurements made on NSF/NCAR G-V during the Stratosphere Troposphere Analyses of Regional Transport (START) experiment, conducted during December 2005. Ozone and water vapor measurements from HIAPER are used to identify air masses with stratospheric and tropospheric characteristics, together with air with mixed behavior, based on a tracer correlation technique (Fig. 4). This analysis allows isolation of the spatial structure of the mixing region, revealing a depth of mixed air (~5 km in vertical distribution) on the cyclonic (poleward) side of the subtropical jet, where the thermal gradient is relatively weak. Away from the jet or on the anticyclonic side of the jet, where the stability gradient was strong, the chemical transition across the tropopause was more abrupt with minimal mixing. The results of this analysis suggest that, if the extratropical tropopause is treated as a transition layer, the thickness of the layer appears to have strong spatial variation. The depth of the transition layer will be a subject of further investigation in upcoming experiment START08 (planned for spring 2008).

ACD scientists and colleagues from the Institute for Atmospheric Physics (IAP) in Beijing, China, performed analyses of ozonesonde measurements from Beijing, China, covering a 3 year time period (September 2002 to July 2005). These Beijing measurements are based on a new Global Positioning System (GPS) ozone measuring system. This work included detailed comparisons with satellite ozone retrievals from the Atmospheric Infrared Sounder (AIRS) and the Microwave Limb Sounder (MLS). The results show relatively small mean biases between the ozonesonde and satellite data sets, and furthermore demonstrate that both satellite data sets can reproduce the vertical gradients and variability of ozone in the UTLS region (over 400 -70 hPa).

FY08 work will continue analysis of satellite data to characterize the chemistry and dynamics of the UTLS region. This work is funded by NSF/NCAR and NASA.

return to top

Gravity Waves

Figure 1. Two-dimensional (ω, k) power spectra of vertical velocity in dB near the model top for the Rayleigh damping (RD), diffusive damping (DD), vertical velocity damping (WD), and no damping (ND) simulations.

High resolution figure

The Weather Research and Forecast Model (WRF) is a state of the art model that can be used for studies of convection and mesoscale systems. WRF would be an excellent model for studies of gravity waves generated by convection and mesoscales systems as it can be run over large areas and with realistic background conditions. However, the current top boundary condition in WRF causes a lot of wave reflection back into the model domain, making WRF unsuitable for gravity wave studies.

In collaboration with J. Klemp (MMM), J. Dudhia (MMM), J. Richter (CGD), and H.-L. Liu (HAO), TIIMES visitor, Alex Hassiotis (graduate student, Pennsylvania State University) explored various upper boundary conditions in WRF that would expand WRF’s gravity waves damping capabilities. Hassiotis performed two-dimensional idealized squall line simulations with a) sponge layer using horizontal diffusion, b) Rayleigh damping, and c) vertical-velocity damping. He found, that when a broad spectrum of gravity waves is present, as is the case with convective sources, the Rayleigh and diffusive damping schemes in WRF remove only a portion of the wave spectrum. In contrast, vertical-velocity damping with a sufficiently high damping coefficient proved highly effective at dissipating almost the complete spectrum of waves (Figure 1).

Vertical velocity damping looks promising for permanent implementation in WRF for gravity wave damping, however tests with real-data cases must be still performed to confirm this finding.

Figure 2. Brunt-Vaisala frequency (s-¹) and maximum wind shear allowed by dynamic stability (in parentheses, unit: ms-¹ km-¹) from TIME-GCM simulations under September equinox (a) and December solstice (b) conditions at 0° longitude. Contour interval: 0.0025 s-¹> (5 ms-¹ km-¹). Shaded region: N > 0.03-¹ (S^max > 60 ms-¹ km-¹). (c) Profiles of maximum wind shear allowed at 18 longitudes (20° apart) and 2.5°N for day 355. The profiles are in good agreement with the measured maximum wind shears.

High resolution figure

Unexpectedly strong winds/shears and fast meridional transport above the mesopause have been revealed from previous rocket and satellite observations. These strong winds/shears and fast meridional transport are not accounted for in current global models. In this study we examine the possible causes using analytical and diagnostic studies.

Using Richardson number criteria for dynamic stability, H.-L. Liu (HAO) estimated the maximum wind shears allowable by the background static stability, which peaks above the mesopause. These maximum shears are in general agreement with the large wind shears inferred from the rocket measurements at low and mid-latitudes, indicating the close relationship between the latter and the the stability constraint set by the background atmosphere (Figure 2). The large shear may come from gravity waves, whose vertical wavelength decreases in the region of maximum static stability. Diagnostic calculations also indicate that the meridional transport in this region may not be well understood solely by examining the mean meridional circulation, and large amplitude tides/planetary waves can play an important role in the bulk transport of tracers. Strong stochastic winds, presumably due to gravity waves, do not seem to significantly change the large scale pattern of the transport but may extend the range of the tracer movement.

H.-L. Liu will work with university collaborators to examine lidar/radar observations of winds, temperature, and gravity wave activities to test the theoretical understanding.

Figure 3. Streamlines and turbulence dissipation rate, Ε^1/3 (m^2/3 s-¹), over Owens Valley in the lee of the Sierra Nevada derived form in situ data collected by the University of Wyoming King Air during the first T-REX IOP 3 research flight on 9 March 2006. Horizontal lines (black dashed, blue and red solid) mark the aircraft flight tracks, and times indicated the start and end times of each of the flight segments included in the analysis. Large values of turbulence, marking the rotor, are found behind the leading updraft of the lee wave over Owens Valley.

High resolution figure

Vanda Grubišić has been an ASP Faculty Fellow visitor in TIIMES since January 2007 working on gravity wave (GW) research as part of her post-field-campaign T-REX data analysis work. She has been involved in the formulation of the proposal for the GW strategic initiative headed by Han-Li Liu (HAO), and have participated in the proposal for CONCORDIASI led by Dave Parsons.

The core of this research has been devoted to the analysis of T-REX observational data including in situ aircraft, ground-based remote sensing, and in situ surface sensors from the T-REX wave and rotor events. The analysis has been focused on the lower tropospheric wave response and, of course, rotors. The group has also pursued an idealized numerical study of lee wave generation over double bell-shaped orography. These results point to the existence of a special non-linear resonant lee wave response over double bell-shaped orography for atmospheric profiles of wind and stability and mountain profiles resembling those found in the T-REX experimental activity area (Figure 3).

return to top

Weather Research and Forecast model coupled with Chemistry (WRF-Chem)

Nitric oxide (NO) mixing ratios (pptv) across the observed 10 July 1996 STERAO storm anvil at t = 2316 to t = 0036 UTC and simulated nitrogen oxide (NO + NO2) mixing ratios at t = 6000 s from eight different models. The location of the cross-section is 50 km downwind of the convective storm core. The solid black line is cloud particle concentration equal to 0.1 per liter. Objective analysis of the aircraft measurements (upper left panel) are from Skamarock et al., (2003). This intercomparison of models emphasizes the importance of including production of NO from lightning. Other models do not include this important process and substantially underestimate NOx mixing ratios. The comparison brought out that the key processes: lightning flash rate which depends on the storm kinematic and microphysical characteristics, lightning type (cloud-to-ground or intracloud), amount of the NO source per flash, and the location of the NO source within the storm, are crucial to include in lightning-NOx parameterizations. Uncertainties in these same processes contribute significantly to the variability seen in NOx mixing ratios for other models. The role of convection as a source of nitrogen oxides is important for understanding the sources and sinks of O3 in the upper troposphere where O3 affects the radiation balance and oxidizing power of the atmosphere.


High resolution figure

The Weather Research and Forecasting (WRF) model coupled with Chemistry (WRF-Chem) is being developed by NOAA scientists, in collaboration with the WRF community including NCAR/ESSL scientists. The model is used for investigation of regional-scale air quality, field program analysis, and cloud-scale interactions between clouds and chemistry. ESSL scientists and staff provide support by integrating and maintaining the chemistry components in the evolving WRF modeling system, as well as contributing new code in the development of WRF-Chem. Models such as WRF-Chem can be used to further the understanding of precipitation and chemical processes, including multiscale atmospheric chemical constituent transport, dispersion and transformations.

Atmospheric chemical and aerosol transport, dispersion and transformation depend on accurate specification of the dynamics and physics across a wide range of scales, from the microscale to the mesoscale. WRF-Chem is being utilized to develop a deeper understanding of the dynamics, physics and chemistry affecting these constituents. Because WRF-Chem is able to simulate the coupling between dynamics, radiation, chemistry and aerosols, science issues that depend on these interactions are being pursued. These applications include transport of tracers from urban regions, processing of chemical constituents by deep convection, analysis of field measurements with WRF-Chem configured for the regional scale, and studies that examine the interactions between aerosols and clouds and their impacts on precipitation, climate, and chemistry.

To analyze field program observations and understand the processes that contribute to observed concentrations, WRF-Chem has been improved to include boundary and initial conditions that are produced by the global chemistry transport model MOZART. A simple dust module has been implemented and is currently being evaluated. Special analysis tools are being incorporated that will elucidate the important processes controlling the concentrations of key chemical species. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) has been put into WRF-Chem allowing scientists to study interactions between the biosphere and atmosphere with impacts on air quality and climate. Existing and new parameterizations describing the production of nitrogen oxides (NOx) from lightning are being implemented in WRF-Chem for studies on the impact of convection on the upper troposphere composition and chemistry. An intercomparison of convective-scale cloud chemistry models with lightning-NOx production schemes highlights the uncertainties in currently-used schemes and emphasizes the need for additional measurements that should be taken in upcoming field campaigns, such as the Deep Convective Clouds and Chemistry (DC3) experiment that is being planned. WRF-Chem will continue to be used for field campaign analysis of MIRAGE and INTEX-B and for preparing for DC3. Other applications include investigations of aerosol-cloud interactions for exploring the role of dust in tropical cyclogenesis and in orographically-produced wave clouds, for studying the interactions between emissions, thunderstorms, and precipitation, for examining the impact of urban areas on fog, and for studying the importance of aqueous-phase organic chemistry on the production of organic aerosols. This work is supported by the NSF.

return to top

Stratospheric ozone recovery [Whole-Atmosphere Community Climate Model (WACCM)] group

Figure. Calculated ozone trend (% per decade) over the period 1980-2050.

High resolution figure

The Whole Atmosphere Community Climate Model (WACCM) was used to investigate the recovery of global stratospheric ozone in the 21st century due to the Montreal Protocol and later agreements on curbing the release of chlorofluorocarbons, halons, carbon tetrachloride, methyl chloroform and other halogens.

The recovery of ozone in the 21st century is found to depend, not only on changes in the concentration of chlorine and bromine compounds, but also on changes in the stratospheric (Brewer-Dobson) circulation brought about by increases in greenhouse gases (GHG).

As GHG increase so does Eliassen-Palm (EP) flux divergence of waves propagating from the troposphere. Maxima in the trend of EP flux divergence are found in the subtropical lower stratosphere. These trends drive an acceleration of the Brewer-Dobson circulation, which in turn leads to a decrease in ozone in the lower stratosphere, as ozone-poor air is transported upward from lower altitudes.

The figure shows the calculated ozone trend (% per decade) over the period 1980-2050. While ozone increases throughout most of the stratosphere, as expected from the decreasing load of halogen compounds, it actually decreases in the lower stratosphere, between about +-45° of latitude as a result of the acceleration in the Brewer-Dobson circulation. The overall ozone column trends are still positive, but the decrease in the lower stratosphere reduces the rate of recovery of the ozone column, especially in the tropics and subtropics.

FY08 work will continue the use of WACCM to evaluate the factors contributing to stratospheric ozone recovery. This work is funded by NSF/NCAR.

return to top

WACCM

Solar cycle variability in the latitudinal distribution of PMC ice-mass (t/5 deg. latitude). PMC ice-mass is higher during solar minimum due to cooler temperatures, and a reduction in water vapor loss via photolysis.

High resolution figure

While cold mesospheric temperatures dictate that polar mesospheric clouds (PMC) will usually form during the summer months, substantial variation in their global frequency and distribution has been observed by both ground-based and satellite instruments. Likely sources of PMC variability include global-scale dynamics, solar irradiance changes, and long-term trends in atmospheric constituents and temperatures. To explore this variability, a parameterization of PMC microphysics has been incorporated into NCAR's Whole Atmosphere Community Climate Model (WACCM), a chemistry climate model the covers the height range from the surface to ~140 km. The new model is capable of reproducing much of the observed PMC variability, and initial comparisons with data from the recently launched Aeronomy of Ice in the Mesosphere satellite ( http://aim.hamptonu.edu/ ) show good agreement.

Quicktime movie of PMC albedo over the Northern Hemisphere, showing variability due to planetary-scale waves near the mesopause.

FY08 work will continue evaluation of sources of PMC variability This work is funded by NASA and NSF/NCAR.

return to top

Intense photochemistry in the Antarctic troposphere

Figure 1. Plot of H2SO4 from ANTCI 2005. This flight passed through the plume of Mt. Erebus. The spikes in H2SO4 at high altitudes are the plume encounters. The fact that the values are lower than those seen at lower altitudes is due to the fact that in the volcano plume the SO2 hasn't had time to convert to H2SO4 - fresh emission. Down low the large H2SO4 is due to station emissions of SO2 which have had a long time to convert to H2SO4.

High resolution figure

The third field study of The Antarctic Tropospheric Chemistry Investigation (ANTCI) program was conducted in Antarctica during November-December 2005. The primary goal of the ANTCI program is to enhance understanding of the processes that control tropospheric levels of HOx , NOx , sulfur, and other trace species over the Antarctic continent. The project included ground based measurements at the South Pole and an airborne component on the NSF Twin Otter based out of McMurdo Station with a short deployment out of the South Pole. ACD scientists were responsible for the design and layout of the experimental package used on the twin otter aircraft, as well as for measurements of OH, H2SO4 , and MSA, along with aircraft state parameters.

Highlights of the study include measurements made on the Antarctic plateau, in glacial valleys, and along the coast.. Based upon observations of elevated concentrations of NO and OH from previous studies at the South Pole, it has been hypothesized that on the polar plateau, NO is produced from NO3 - photolysis in the surface layer of snow. As air follows the katabatic flow towards the coast, this NO is converted back into HNO3 and redeposited back to the snow surface where NO3 - photolysis can occur again. However, preliminary results suggest that while larger concentrations of NO (and OH) were observed on the plateau and at the upper entrances to glacial valleys, these concentrations were not making it down the valleys to the exit at the coast. Further analysis will focus on these results as well as the details of photochemistry and nitrogen cycling in the various regions of Antarctica.

The study also included the first airborne measurements of the plume emitting from Mt. Erebus, a volcano located near McMurdo Station. Measurements of H2SO4 and SO2 inside the plume revealed extremely elevated concentrations compared to upwind of the plume. Following the plume downwind of the volcano, these concentrations fell as expected, giving both chemical and dynamic (dilution) information about the plume. The Figure shows H2SO4 from the plume. . The spikes in H2SO4 at high altitudes are the plume encounters. The fact the values are lower than those seen at lower altitudes is due to the fact that in the volcano plume the SO2 hasn't had time to convert to H2SO4, which is an indication of fresh emissions. At lower altitudes, the large H2SO4 is due to station emissions of SO2, which have had sufficient time to convert to H2SO4.

FY 2087 work will focus on continued data analysis and modeling and will include collaborative efforts with participating university colleagues. This work is funded by NSF/NCAR and NSF Polar Programs.

return to top

Severe atmosphere convection

Figure 1. Results from a theoretical and numerical study of gravity currents in a deep atmosphere. The abscissa denotes the normalized depth of the atmosphere, with 0 indicating the shallowest possible depth, and 1 indicating the largest possible depth. (Previous studies assumed a value of 0, whereas the Earth's atmosphere corresponds to 0.5). The ordinate denotes a normalized propagation speed. Solid lines show results from theory, with the black line indicating the maximum possible speed and the red line indicating the speed for inviscid flow. Blue dots are results from a series of numerical simulations, which confirm the theoretical results. Overall, this study found that cold pools in the Earth's atmosphere are shallower and slower (by about 30%) than previously thought.

High resolution figure

Severe convective weather, such as tornadoes and severe wind gusts, impact life and property throughout the world. In the United States, severe convective weather is frequent, and can result in hundreds of deaths every year. ESSL scientists study the processes by which thunderstorms produce severe weather with the goal of understanding and predicting their occurrence.

Using data collected during the Bow Echo and MCV Experiment (BAMEX), previous research identified two key processes concerning severe-wind-producing convective systems: 1) surface-based cold pools in these systems often extended to 4-5 km above the surface, which is surprisingly deep; and 2) bow-shaped convective systems were common at night. Plans for FY07 were to study these two findings from a theoretical and modeling perspective to understand their implications for prediction of severe weather, and to evaluate previously derived theories for severe weather production. In addition, ongoing development of cloud-scale nonhydrostatic modeling systems was planned to be a primary focus, with emphasis on both real-time forecasting applications and idealized research studies. Support of these modeling systems for the broader scientific community was planned to be a key part of this development.

Based on the findings from BAMEX, the dynamics of cold pools needed to be re-examined to account for their unexpected depth. To this end, a theory was developed for steady cold pools (also known as gravity currents) in a deep atmosphere. Results were shown to depart significantly from the classic theory, which was based on the incompressible equations (a simplified set of equations valid for shallow flows). For conditions typical of severe storm environments, maximum possible propa