Regional Carbon Cycle

TIIMES measurement and instrument development projects have advanced our understanding of regional carbon fluxes, particularly in the mountain west, and provided key NCAR contributions to the multi-agency North American Carbon Program (NACP). These projects include:

Regional Atmospheric Continuous CO₂; Network in the Rocky Mountains (Rocky RACCOON)

In order to improve our understanding of regional carbon fluxes in the Rocky Mountain West, TIIMES scientists have developed and deployed autonomous, inexpensive, and robust CO₂; analyzers (AIRCOA) at five sites throughout Colorado and Utah, and plan additional deployments on the Navajo Reservation, Arizona in September 2007 and atop Mount Kenya, Africa in November 2007 (http://raccoon.ucar.edu) CO₂; differences between the sites reflect the influence of regional carbon exchange, primarily by mountain forests (Figure 1). A one-dimensional CO₂; budget equation, following Bakwin et al. (2004), to estimate regional monthly-mean fluxes from the continuous CO₂; concentrations. These comparisons between the measurements and estimates of free-tropospheric background concentrations reveal regional-scale CO₂; flux signals that are generally consistent with one another across the Rocky RACCOON sites (Figure 2). The timing and magnitude of these estimates combined with information on atmospheric transport provide insights into regional variations in CO₂; fluxes.

 

These measurements will be included in future NOAA CarbonTracker assimilation runs and other planned model-data fusion efforts. However, questions still exist concerning the ability of these models to accurately represent the various influences on CO₂; concentrations in continental boundary layers, and at mountaintop sites in particular. Analysis of the diurnal cycles in CO₂; concentration and CO₂; variability at these sites provide insight as to when and under what conditions mountaintop CO₂; signals are regionally representative, as well as first-order constraints on boundary-layer heights and flux rates for use in evaluating model fidelity. Because of coarse representation of topography and boundary-layer mixing biases, forward model CO₂; diurnal cycles can be 180 degrees out of phase with respect to assimilated mountaintop CO₂; observations if care is not taken in the choice of model level used.

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Airborne Carbon in the Mountains Experiment (ACME-07)

ACME-07 built on results from the ACME-04 campaign, which made the first attempt to measure carbon exchange in mountainous terrain using airborne techniques and explored a wide range of approaches. The ACME-07 campaign was designed to test a series of specific hypotheses about the scaling of carbon fluxes as a function of time of year, and evaluate hypotheses developed from long-term ground based data using airborne measurements over the seasonal cycle. Over 60 hours of flights were conducted on the Wyoming King-Air between April and August of 2007 (Figure 3). Semi-Lagrangian budgeting techniques were used to constrain regional fluxes over the Colorado Rocky Mountains and aircraft profiles in the early morning in large intermountain valleys explored the large "pools" of CO₂ that accumulate from nocturnal respiration and drainage flows. 

The ACME-07 payload included the NCAR Community Airborne Oxygen Instrument (AO2) which was completed this year by TIIMES scientists. This instrument is based on a vacuum-ultraviolet absorption technique and has a precision equivalent to detecting the removal of one O₂ molecule from 1 million molecules of air in 5 seconds.  Because of the unique relationships between industrial, terrestrial, and oceanic exchange of carbon and oxygen, this instrument promises valuable insights into these processes.

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Future plans to address regional carbon fluxes include the participation of TIIMES scientists in the Brazilian Amazon Regional Carbon Airborne study (BARCA) scheduled for September 2008, as well as in depth analysis of ACME-07 and Rocky RACCOON data and synthesis with modeling efforts. TIIMES scientists will deploy a sixth AIRCOA unit as part of RACCOON at Roof Butte on the Navajo Reservation in Northeastern Arizona and a seventh unit at Mt. Kenya in Africa in fall 2007.

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Landfall impacts of hurricanes

The 2005 Atlantic hurricane season is a vivid reminder of the economic and societal consequences of landfalling tropical cyclones. Improved forecasts of hurricane intensity change, and time-extensions of skillful track prediction are vital for evacuation strategies. Furthermore, accurate assessment of the uncertainty in hurricane forecasts is critical in a variety of economic sectors. Progress requires solving difficult problems such as the inner-core hurricane dynamics and how it affects intensity, quantifying the net enthalpy flux from the ocean in high-wind-speed conditions, and incorporation of a variety of remotely sensed data into model initial conditions. The purpose of ESSL/MMM research in hurricane simulations is to create the next generation hurricane-prediction system, and a community hurricane-prediction model that can be used for process and predictability studies.

During the past year, analysis of WRF simulations of Atlantic hurricanes from the 2005 and 2006 seasons was conducted. Improvements in the representation of surface fluxes were tested and implemented, as well as new methods of initialization including the ensemble Kalman filter and 3DVAR methods. These are currently being tested during the 2007 hurricane season and have shown considerable skill in predicting the intensification of Dean and Felix, both Category 5 storms, while correctly not allowing appreciable intensification of Ingrid which never attained hurricane strength.

Detailed analysis of the inner-core structure of hurricane Katrina (2005) revealed simulated mesovortices in the eye wall that did not occur in the observations. The existence of these structures depends strongly on physical parameterizations in the model. Additional analysis of simulated tropical cyclones moving into higher latitudes and environments with more vertical wind shear was performed. This revealed the remarkable ability of WRF to simulate emerging frontal structures and asymmetries in rainfall patterns. These patterns were related to the precession of upper and lower portions of the hurricane vortex about each other in an attempt to offset the deleterious effects of vertical wind shear that acted to tilt the hurricane.

In the coming year work will continue to improve data assimilation for improved initialization of hurricanes, as well as advancements in the treatment of convection and the air-sea interface. Some new applications of hurricane work will begin, including the use of high-resolution wind fields in insurance industry loss models. Seasonal and interannual simulations of hurricane activity will be examined through the Nested Regional Climate Model, with an emphasis on hurricane formation mechanisms. Finally, MMM scientists will participate in the T-PARC field campaign in the western Pacific Ocean beginning in August 2008. The emphasis is both on high-resolution radar observations of tropical depression formation and the transition of structure that accompanies storms moving into higher latitudes (extratropical transition). The latter component will be tied to verifying structures predicted by the WRF model.

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Global and Regional Water Cycle

The Water Cycle Program has conducted research related to the Regional and Global water cycle since 2001.  Using the diurnal cycle of precipitation as a focus, research has shown that current climate models do not accurately simulate the frequency, intensity,  and timing of summer time convection over continental regions.  Much of the reason for this error is in the poor simulation of propagating systems of convection in the lee of major mountain ranges by current convective parameterization schemes.  This error is reflected in the high degree of uncertainty of current climate model runs in these regions (see white circled regions in Figure 1 which shows regions of > 66% model disagreement on the sign of precipitation change with climate change).   

The goals of the Water Cycle program are: 1) to reduce this uncertainty through focused research on the mechanisms leading to propagating research,  2) testing and improving new parameterizations of convection that attempt to simulate this phenomenon, 3) improve our understanding of the coupling between land surface, boundary layer and convective parameterizations in climate models. The latter goal aims at examining whether the often tight coupling between precipitation and soil moisture in climate models is realistic, and its role in modulating and initiating propagating convection.

Current research has focused on the following areas supporting the goals mentioned above:

1. Investigating initiation modes for propagating convection due to various factors including the role of short waves.

2. Determining the percentage of precipitation over the continental United States due to propagating systems.

3. Improving convective parameterizations in climate models to be able to handle propagating convection.

4. Conducting high resolution model simulations using boundary conditions with short waves removed.  Investigate the role of the mountain-plains circulation in maintaining and initiating the propating convection as well as other mechanisms.

5. Percent of precipitation over the United States attributable to propagating convection.  Twelve year radar dataset does not reveal a correlation of the intensity or phase of the propagating convection to ENSO.

6. Diagnosing the water cycle in climate models and observations and in retrospective datasets.

7. Simulation of the low-level jet in climate models.

8. Improving the hydrological cycle in CLM3.

9. Examining the role of the land surface in modulating propagating convection.

10. Planning for a new project in the Water Cycle called Colorado Headwaters to examine the Upper Colorado Basin snowpack under climate change using high resolution models.

Results to date show that short waves are not the dominant factor in initiating convection.  Research is now focused on other mechanisms.  Examination of the amount of precipitation attributed to propagating precipitation over the continental U.S. suggests that up to 70% of central U.S. precipitation is due to propagating systems.  Research on the role of land surface impacts suggest that the land surface plays a secondary role in the formation and propagating of long-lived convective systems. Research related to the low-level jet over the central U.S. shows a strong coupling of the corridors of propagating convection to the low level jet.  In support of improved simulation of these propagating systems in climate models, a new cross-NCAR convective parameterization scientific interest group has been formed.  The group has met three times during FY07, and plans to continue to meet into FY08.   Scientific efforts in this area have focused on Jaga Richter and Changhai evaluating the new Moncrieff and Liu convective scheme as to it ability to properly simulate propagating convection.

The Water Cycle Program will focus on the following areas during the next few years:

1. Continued emphasis on improving convective parameterizations in climate models, including the testing of various candidate schemes (including the new Moncreiff and Liu scheme developed under Water Cycle sponsorship) and continuation of the scientific interest group on convective parameterization.

2. Continue to investigate mechanisms that lead to the initiation of propagating systems.

3. Continue with Regional and Global Diagnostic studies of the water cycle.

4. Evaluate how to improve the coupling of land surface, boundary layer, and convective schemes in climate models.

5. Initiate the Colorado Headwaters snowpack study.

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Polar Climate

Overview, historical context, and significance

Arctic sea ice has undergone rapid retreat in recent decades and climate models project a continued decline into the foreseeable future, with the possibility of summer ice-free conditions being reached later this century. Considerable effort is underway to examine these observed and projected changes in the sea ice system and the consequences of a seasonally ice-free Arctic ocean for the climate system. Ongoing research indicates that the observed winter ice cover anomalies show less impact from the North Atlantic Oscillation in recent years and exhibit a more spatially uniform decrease since 2000. An analysis of projected changes in the future ice cover suggests that gradual, linear changes are unlikely. Instead, Community Climate System Model (CCSM) integrations exhibit abrupt reductions in the future summer sea ice cover, with the most extreme event going from 80% September ice coverage to 20% coverage in approximately 10 years (Figure 1). The mechanisms responsible for these transitions include: 1) an increased efficiency of open water production as the Arctic ice thins, 2) rapid increases in ocean heat transport that trigger the events, and 3) the surface albedo feedback, which accelerates the ice retreat. The role of natural versus external forcing in driving these transitions and the potential predictability of these events is currently being assessed using CCSM3 experiments. An analysis of additional models participating in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4) indicates that about 50% of them exhibit similar abrupt reductions in future Arctic summer ice cover for some future forcing scenarios, although the length and magnitude of the events vary. Reasons for the inter-model variations in future Arctic ice loss are being assessed through an analysis of the sea ice mass budget changes in the IPCC-AR4 models, their relationship to changing surface heat budgets, and their impact on the timing and transition to seasonally ice-free conditions. The presence of seasonally ice-free conditions has potential impacts on the Arctic and global systems and CCSM experiments are being used to assess the impacts on global atmospheric circulation, the hydrological cycle and ocean conditions. The implications for future polar bear habitat loss are also being assessed. This collaborative work involves numerous scientists within ESSL, at a number of different universities and at various government laboratories.

Associated with reductions in sea ice and Arctic warming are indications that the permafrost is warming and thawing. Large-scale thawing of permafrost is likely to induce a number of feedbacks to the hydrologic and carbon cycles of the Arctic system. Of particular concern, especially from a global perspective, is how permafrost thaw will affect the carbon balance in the Arctic. A high-latitude terrestrial climate change feedback project has been initiated to investigate this issue. This interdisciplinary project aims to improve our ability to simulate, understand, and predict high-latitude terrestrial climate feedbacks in CCSM with a particular goal to develop a version of CCSM that can address the critical carbon issues in the Arctic tundra. These issues include, but are not limited to, the accumulation and loss of carbon in organic or peatland type soil profiles, the partitioning of carbon emission between methane and carbon dioxide, hydrologic cycle change related to permafrost degradation, and the interaction between temperature, nitrogen cycling and the transition between herbaceous tundra and woody arctic shrubland.

Efforts over the past year, conducted in collaboration with university partner permafrost process specialists, have focused on improvements in the simulation of the thermal and hydrologic state of permafrost in the Community Land Model (CLM), namely by explicitly representing the (geographically non-uniform) thermal and hydraulic properties of organic or peatland soil and significantly deepening the soil column from 3.5 to 50 m. The revised model shows a marked improvement in soil temperature dynamics (Figure 2). The sensitivity of model projections of permafrost degradation (which in CCSM3 were severe) to these changes have also been evaluated. Although permafrost degradation is somewhat slower due to the insulating effect of the organic soil and the dampening provided by the cold deep soil layers, century-timescale projections of near-surface permafrost degradation remain severe (Figure 3). This research is sponsored by NSF and DOE.

Work is underway to incorporate a dynamic wetland model that is capable of simulating the anticipated changes in wetland distribution associated with permafrost thaw induced soil subsidence. Additional efforts will focus on an integration of the revised model with the prognostic soil carbon calculated in the CLM carbon cycle model (CLM-CN) and incorporation of a boreal shrub plant functional type into the CLM Dynamic Global Vegetation Model (CLM-DGVM-CN) so that the anticipated transition from tundra to woody arctic shrubland can be simulated and its impact assessed.

Changing sea ice and permafrost conditions have important implications for the Arctic hydrological system change. Because of the proximity to deep water formation regions within the northern North Atlantic, this in turn can modify the global thermohaline circulation. Future projections of the Arctic freshwater budgets and their influence on deep water formation regions have been assessed in IPCC-AR4 simulations. Additional experiments are underway to further examine the role of changing ice-ocean freshwater exchange on ocean circulation changes in future climate conditions. Additionally, the influence of the Bering Strait through-flow on the thermohaline circulation sensitivity to freshwater flux perturbations has been examined and found to play an important stabilizing role. Additional experiments to further examine and quantify these effects are underway.

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Megacities Impacts on regional And global Environments/Megacity Initiative: Local and Global Research Observatories (MIRAGE/MILAGRO)

The MIRAGE (Megacities Impacts on Regional and Global Environments) initiative aims to understand the fate of urban emissions in the downwind atmosphere. Mexico City was selected as a case-study, with intensive observations made during March 2006. NCAR led the measurements of atmospheric gases and aerosols, radiation, and meteorology, with collaborators from universities, Mexican and U.S. government agencies, and other institutions. The preliminary results listed below are providing a unique look into the interactions between atmospheric gases and aerosols. Such multi-phase environments may be the norm, rather than the exception, for megacities in rapidly growing economies, and may require different approaches than U.S. or European cities.

Important new results include:

• Measurements from the C-130 indicate that the air downwind from Mexico City continues to be chemically active, with production of ozone (O3) and organic particles continuing for several days.
• Organic reactivity is dominated by hydrocarbons near the surface in Mexico City, but by oxygenated organics (particularly aldehydes) in the outflow.
• Reactive nitrogen appears to be lost more rapidly than expected, and a significant fraction is unidentified.
• Biomass burning may add significantly to the regional burden of aerosols, and to a lesser extent of some gases. Models (MOZART, WRF-Chem) have examined the relative importance of biomass burning and urban emissions.
• New particle formation is seen commonly near the city. Downwind, growing aerosol particles perturb the radiation field, with increasing absorption at shorter wavelengths.

In general a picture is emerging of Mexico City as a strong source of both volatile organic carbon and nitrogen oxides, the latter being in excess locally and exported regionally as organic or inorganic nitrogen. Production of ozone and secondary aerosols continues in the plume well downwind of the urban area. Aircraft-based measurements of many species, including hydrocarbons and their oxidation intermediates, are being compared with emission inventories and model predictions. A wealth of data on secondary organic aerosol is being used to examine why current models under-predict them. Biomass burning also influences regional air quality, particularly for aerosols. Radiative impacts of aerosols and other pollutants are being evaluated.

The MIRAGE data analysis and interpretation is still in its earliest stages, and is expected to continue over the next several years. This phase is expected to involve model simulation and evaluation (both process-level and 3d chemistry-transport models), and to be increasingly collaborative in using the very rich data set. A conference special session (AGU, Fall 07) and a journal special issue (ACP) are planned. It is clear that there exists great interest in carrying out similar intensive observational missions for other megacities, and discussions are underway for future collaborative work with researchers from South America, Asia, and Europe.

This work is funded by NSF/NCAR.

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Biosphere-Atmosphere Exchange of Aerosols with Cloud, Carbon, and Hydrologic cycles including Organics and Nitrogen (BEACHON) program objectives and plans

The Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H₂O, Organics & Nitrogen (BEACHON) project is a broadly collaborative and interdisciplinary research effort being developed by researchers at NCAR and the university community. BEACHON will improve predictability of earth system behavior over the time scale of months to a decade based on a better understanding of the coupling between water, energy and biogeochemical cycles in a multi-scale modeling framework.  This will be accomplished through coordinated modeling, observations and process studies that explicitly address the coupled water, energy and biogeochemical cycles at multiple temporal and spatial scales.  The main goal of the BEACHON project is to provide a detailed and quantitative characterization of biosphere-hydrosphere-atmosphere interactions and to use that characterization to improve regional and global models of the earth system.  A major focus will be measurement and interpretation of surface fluxes of energy, aerosols, CO₂, water, and organic and nitrogen compounds.  Investigations will also address other fundamental processes including atmospheric aerosol production and growth processes, oxidant and cloud processes; and the response of ecological, hydrological, and physiological processes to land-use change and ecological disturbances. BEACHON will focus on water-limited ecosystems but will include studies in tropical rainforests and other ecosystems to compare and contrast over a large range of water availability.

An initial BEACHON science plan was drafted in FY07 and distributed to the university community. A workshop was organized for the purpose of developing a detailed implementation plan. FY07 field measurements focused on potential field sites and approaches for integrating interdisciplinary measurement activities. A scoping study at Manitou Experimental Forest demonstrated that this location is a suitable site for long-term measurements of biosphere-atmosphere trace gas and aerosol exchange.  EOL and TIIMES scientists collaborated with Roni Avissar (Duke University), Brian Lamb (Washington State University) and Brad Baker (Sacramento State University) to extend the CHATS study to include canopy LIDAR measurements, helicopter flux measurements, tracer dispersion measurements, and ozone, nitrogen and VOC flux and concentration measurements. The resulting dataset is an initial step towards the suite of observations required to address BEACHON science questions.

BEACHON FY08 objectives include model (1D, LES and regional) development and application, analysis of the CHATS and Manitou Experimental Forest field study data, initial field measurements at BEACHON sites, and laboratory process studies. Model development will include significant advances in land surface model parameterizations and implementation of improved biogenic emission, aerosol formation and growth, and cloud microphysics in the WRF regional model. Field observations will be used to characterize the processes controlling biosphere-atmosphere exchange and to evaluate model simulations of these processes.  Laboratory studies will emphasize observations that will improve quantitative descriptions of atmospheric and ecological drivers (e.g., drought stress, insect infestation, climate driven ecosystem changes, solar radiation, and temperature) of biogeochemical cycling and their impact on atmospheric distributions of trace gases and aerosols.

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Global Biogeochemical Cycles

As is clearly articulated in the Fourth Assessment report of the Intergovernmental Panel on Climate Change (IPCC, 2007), there is increasingly strong motivation to examine terrestrial and oceanic carbon fluxes on regional to continental scales, to understand the coupling of the carbon cycle to the climate system and to other biogeochemical cycles, to understand the processes responsible for present uptake of anthropogenic carbon, to predict future trends in these fluxes under various climate change scenarios, and to assess potential strategies for increasing carbon uptake and storage into the future.  The challenges of scaling up from local measurements and scaling down from global constraints are being addressed in TIIMES through the development and application of advanced observational and modeling tools.

Example 1:

An analysis of airborne atmospheric carbon dioxide measurements has revealed that forests in northern mid- to upper-latitudes are removing much less carbon dioxide from the atmosphere than previously thought, and that intact tropical forests are removing much more (Figure 1).  These results, reported in the 22 June 2007 issue of Science, are a major step towards resolving the missing carbon sink and will help others improve climate change projections and assess potential mitigation strategies.

Scientists know that over the decade of the 1990’s 40% of the 8 billion tons of carbon we emitted each year stayed in the atmosphere and that roughly 30% was taken up by the worlds oceans and 30% by land plants (IPCC AR4 WG1, 2007), but don’t understand where and why this uptake on land is occurring well enough to predict its behavior in the future.  For the past 15 years, computer models of the atmosphere using information on winds and measurements of carbon dioxide taken near the ground have indicated that most of these 2.6 billion tons of carbon are being taken up in the Northern Hemisphere, but researchers have been able to account for only about half of this uptake on the ground.  The remainder is often referred to as the "missing carbon sink."  We also know that the tropics are a large source of carbon to the atmosphere as a result of deforestation, but it's been unclear how much carbon the intact forests are taking up.  Local-scale studies suggesting strong uptake in intact tropical forests have been intensely scrutinized, partly because they were in disagreement with the atmospheric model predictions.

TIIMES scientists analyzed measurements of carbon dioxide concentration made on flasks of air collected by profiling aircraft approximately biweekly at 12 sampling locations around the world.  Several of these records extended over 20 years, but until recently there were not enough sites and the data had not been synthesized to provide a clear picture of carbon fluxes on global scales.  The researchers compared these measurements to the predictions of atmospheric carbon models participating in the recent TransCom 3 intercomparison project and found that in general, the models underestimated carbon dioxide concentrations at altitude over the Northern Hemisphere.  It appears that the atmospheric models have a tendency to mix too much carbon dioxide down in summer when photosynthesis is removing it at the ground and in some cases not enough up in winter when respiration and fossil-fuel burning are releasing it, which was only apparent by comparing to the aircraft measurements.  The few models that most accurately reproduced the observed annual-mean vertical CO₂ gradients predict that around 1 billion tons less carbon per year is going into northern forests than previously thought and that in the tropics the balance between emission from deforestation and uptake in intact forests is emitting almost 2 billion tons of carbon less per year than previously thought.

Stephens et al., Weak northern and strong tropical land carbon uptake from vertical profiles of atmospheric CO₂, Science, 316, 22 June 2007.

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Example 2:

Nutrient cycling affects carbon uptake by the terrestrial biosphere and imposes controls on carbon cycle response to variation in temperature and precipitation, but nutrient cycling is ignored in most global coupled models of the carbon cycle and climate system.  TIIMES scientists demonstrated that the inclusion of nutrient cycle dynamics, specifically the close coupling between carbon and nitrogen cycles, in a terrestrial biogeochemistry component of a global coupled climate system model leads to fundamentally altered behavior for several of the most critical feedback mechanisms operating between the land biosphere and the global climate system.  Carbon-nitrogen cycle coupling reduces the simulated global terrestrial carbon uptake response to increasing atmospheric CO₂ concentration by 74%, relative to a carbon-only counterpart model (Figure 2).  Global integrated responses of net land carbon exchange to variation in temperature and precipitation are significantly damped by carbon-nitrogen cycle coupling.  The carbon cycle responses to temperature and precipitation variation are reduced in magnitude as atmospheric CO₂ concentration rises for the coupled carbon-nitrogen model, but increase in magnitude for the carbon-only counterpart.  These results suggest that previous carbon-only treatments of climate-carbon cycle coupling likely over-estimate the terrestrial biosphere’s capacity to ameliorate atmospheric CO₂ increases through direct fertilization.  The next generation of coupled climate-biogeochemistry model projections for future atmospheric CO₂ concentration and climate change should include explicit, prognostic treatment of terrestrial carbon-nitrogen cycle coupling.

Thornton, P.E., J.-F. Lamarque, N.A. Rosenbloom, N. Mahowald, in press. Inclusion of carbon-nitrogen feedback fundamentally changes response of land carbon model to CO₂ fertilization and climate variability. Global Biogeochemical Cycles.

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Future plans to address the global carbon cycle will include the involvement of TIIMES scientists in the HIAPER Pole-to-Pole Observations of Carbon Cycle and Greenhouse Gases Study (HIPPO). In collaboration with Harvard, Scripps, and NOAA, HIPPO will measure cross sections of atmospheric concentrations approximately pole-to-pole, from the surface to the tropopause, 4-6 times during different seasons over a 2.5 to 3-year period. A comprehensive suite of tracers of the carbon cycle and related species will be measured: CO₂, O₂:N₂ ratio, CH₄, CO, N₂, ¹³CO₂:¹²CO₂, H₂, SF₆, COS, CFCs, HFCs, HCFCs, and selected hydrocarbons. HIPPO will transect the mid-Pacific ocean and return either over the Eastern Pacific, or over the Western Atlantic. The program will provide the first comprehensive, global survey of atmospheric trace gases, covering the full troposphere in all seasons and multiple years. HIPPO will quantify the sources of major carbon cycle and greenhouse gases by region at the global scale. Hypotheses to be tested include, as examples:

a) Northern mid-latitude terrestrial ecosystems are a major sink for CO₂

b) The Southern Ocean is a major sink for CO₂ and the driver for global seasonality of the O₂:N₂ratio

c) Amazonia is a major source region for CH₄, CO, and N₂O

d) Upper tropospheric data can be used to falsify models used to derive inverse analysis of the global carbon cycle

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Bioemissions and photochemical processing

Chemicals produced by the biosphere include volatile compounds that are emitted into the air where they can have a substantial impact on the chemistry of the atmosphere. These biogenic gases are dominated by volatile organic compounds (VOCs) both in total mass and number of compounds. The important role of biogenic VOCs in controlling global oxidant (e.g., OH and ozone) distributions has been demonstrated using global models while regional air quality models have shown that it is necessary to included biogenic VOC emissions in modeling efforts to develop regional ozone pollution control strategies. ACD and TIIMES scientists are investigating the processes controlling biogenic emissions and their role in tropospheric photochemistry and are developing numerical schemes for including this information in regional and global earth system models.

The response of biogenic VOC emissions to climate change is being studied by ACD/TIIMES scientists and colleagues. This work is conducted in greenhouse and growth chamber facilities in the NCAR Foothills Atmospheric Chemistry Laboratory (FL0). These laboratory experiments have focused on several categories of biogenic VOC including isoprene, sesquiterpenes, and oxygenated VOC emissions. The ACD/TIIMES researchers are working with Paul Palmer (U. Edinburgh) and Jed Sparks (Cornell University) to characterize the response of isoprene emission to elevated ozone. The response of oxygenated VOC (e.g., methanol, acetone, and acetaldehyde) to climate change is being investigated in a plant growth chamber with precise environmental (e.g., light and temperature) controls in collaboration with David Hanson and Nou Chang (Augsburg College) and John Mak and Kolby Jardine (SUNY Stonybrook). ACD/TIIMES scientists have also teamed with Detlev Helmig (U. Colorado) to characterize the potential impacts of landuse change on sesquiterpene emissions by quantifying variations among different plant species.

In FY07, ACD/TIIMES scientists conducted canopy scale field studies of biosphere-atmosphere chemical exchange in Australia and California. Canopy scale emissions, losses, and transformations were quantified and are being used to investigate the processes controlling surface-atmosphere exchange of trace gases and aerosols. NO/NOY, SO2, ozone and biogenic VOC concentrations and fluxes within and above a eucalyptus forest canopy were measured by ACD/TIIMES scientists during the Eucalyptus Forest Aerosols and Precursors (EUCAP) study near Tumbarumba Australia. In addition, ACD/TIIMES scientists measured particles, NO/NOY, SO2, ozone and biogenic VOC concentrations and fluxes within and above a walnut plantation canopy in California during the Canopy Horizontal Array Turbulence study (CHATS). Leaf and branch level VOC emissions were also quantified at both sites. The observations are being analyzed and interpreted using canopy scale chemistry and transport models.

Observations of OH and other trace gases at rural and remote sites suggest that OH losses are considerably higher than what can be accounted for by the measured OH sinks. ACD scientists collaborated with Prof. Yoshizumi Kajii's research group in Tokyo Metropolitan University to investigate OH lifetime and sinks in August 2006 at the Tomakomai Experimental Forest in Hokkaido, Japan and in August 2007 at an urban site in Tokyo Japan. Emissions and ambient concentrations of a wide range of compounds were measured to quantify their contributions to OH loss. Previous studies relied on model estimates of some compounds that were directly measured during these campaigns. The initial results suggest that additional measurements can account for some, but not all, of the missing OH sinks.

There are thousands of organic compounds that have been identified within biological organisms but only a small fraction of these have been identified as important emissions into the atmosphere. Salates are organic compounds found in the waxes covering the leaves of some plants. They are thought to be a natural sunscreen protecting the leaves from ultraviolet rays. ACD/TIIMES scientists developed and applied sampling and analytical techniques that provide the first evidence that emissions of salates could be important biogenic VOC emissions into the atmosphere. Field measurements with Mark Potosnak and Maria Papiez (U. Nevada Desert Research Institute) showed that these compounds are emitted at substantial rates from plants growing in the Mojave Desert (see Figure). Additional measurements show that these compounds are emitted at much lower rates from plants growing in Colorado, Australia and Japan. These semi-volatile compounds may be a significant source of organic aerosol.

Tropical landscapes are thought to be responsible for about 80% of global biogenic VOC emissions and yet are among the least understood. Global chemistry and transport models often perform poorly when using the biogenic VOC emission rates recommended by current emission models. This could be due to uncertainties in emissions but could also be a result of inaccurate characterization of boundary layer meteorology and/or chemistry. Isoprene emissions, boundary layer heights and trace gas distributions were measured by ACD/TIIMES scientists during an aircraft study in central Amazonia. These observations were used to demonstrate that photochemical models can substantially underpredict atmospheric OH concentrations. The regional scale aircraft measurements also showed that isoprene emissions for this region are not overestimated and may even be underestimated.

The oceans also act as a source of biogenic organic species to the atmosphere. Among the important oceanic emissions are species that contain iodine (e.g., methyl iodide, CH3I, and larger compounds). Because these species tend to be very short-lived and because iodine can act as a catalyst for ozone destruction, these species can be important contributors to the oxidative capacity of the marine boundary layer. ACD scientists, in collaboration with scientists at the Georgia Institute of Technology, Auburn University and the Ford Motor Company, have been studying the atmospheric oxidation pathways of these compounds using a variety of experimental and theoretical techniques. Earlier work has shown that alkenes are significant products of the atmospheric oxidation of these iodinated species. The current focus is on an as-yet unexplained large yield of organic peroxyacids from the oxidation of these species, and on the mechanism of iodinated alkyl radicals with O2.

Both hydroperoxy radicals and acetylperoxy radicals play important roles in the tropospheric oxidation mechanisms of biologically emitted compounds. HO2 radicals are central to the production of ozone, and the generation of hydroxyl radicals, through their interaction with NO. Acetylperoxy radicals, formed in the oxidation of acetaldehyde, are required to form peroxyacetyl nitrate (PAN). Both these reactions require the presence of nitrogen oxides (NOx). However, at low NOx, acetylperoxy radicals and hydroperoxy radicals react together. Until a few years ago, this reaction was thought to result solely in the formation of acetic and peracetic acids. In 2003, ACD scientists, in collaboration with Alam Hasson of Fresno State University, showed that the reaction has another major pathway, leading to hydroxyl and methyl radicals, with a consequent reduction in the formation of acids. In 2007 the collaboration was continued, in order to investigate the reaction system at other temperatures (250-345 K). Acetic acid and peracetic acid were measured by infrared spectroscopy, while peracetic acid and methyl hydroperoxide were measured by liquid chromatography. No major change in mechanism was found over thee extended temperature range. The experiments at low temperature also allowed the products of the addition of HO2 to acetaldehyde to be studied. This reaction may be important at the colder temperatures of the upper troposphere, and could lead to the formation of formic and acetic acids, the levels of which are larger than can currently be explained.

The most definitive measurements of reaction rate constants are usually made through time-resolved methods. However, this requires the application of a sensitive, specific detection technique for the free radicals. ACD scientists have developed a flash photolysis system with a solid-state diode laser operating at 1.4 micrometer to detect HO2 radicals. The experiment uses partially silver-coated mirrors to couple both a UV photolysis beam and the IR detection beam into the reaction cell. In order to quantify the concentration of HO2 radicals, it was first necessary to derive IR absorption cross sections. This was accomplished using acetylene as a photochemical actinometer. Having derived the absorption cross sections, it was possible to measure the rate of the HO2 self reaction, which is in good agreement with literature measurements. Experiments are now in progress to measure the rate of addition of HO2 to acetaldehyde and to acetone, two atmospherically important carbonyl compounds. The results of these time-resolved experiments will be coupled with the chamber measurements described above, to fully understand the oxidation mechanisms of these compounds.

FY2008 work will continue laboratory and field investigations of factors affecting biogenic emissions. Ongoing laboratory studies of kinetics will also continue. This work was funded by NSF/NCAR and EPA.

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Ecosystem - biogeochemistry - climate interactions

Overview, historical context, and significance

Increased understanding and appreciation of interactions between physical and biological aspects of the Earth system has opened new frontiers for observation, experimentation, and modeling. The goal of much recent effort in the fields of ecology, biogeochemistry, and climate science has been to diagnose the past and present behavior of ecosystem - climate interactions, and to build, test, and apply models capable of predicting likely future trajectories for these interactions.

A major research focus for terrestrial science is natural and human-mediated changes in land cover and ecosystem functions and their effects on climate, water resources, and biogeochemistry. The industrial age and growing human population has produced large changes in land surface characteristics, particularly deforestation, cultivation of cropland, and urbanization. Change in land cover from human uses of land is increasingly being recognized as an important forcing of climate. The influence of historical land cover change on climate needs to be considered as a climate forcing in addition to traditional forcings such as greenhouse gases, aerosols, solar variability, and ozone. Future projected land cover changes due to human land uses are also likely to alter climate, especially in the tropics, subtropics, and semiarid regions.

Change in climate, land use and disturbance regimes are driving increasingly evident changes to ecosystems. The impacts of these three related forces are complex and non-linear and current state of the art coupled carbon-climate models show divergent results. Reducing this uncertainty requires improved parameterizations of key ecosystem processes and better estimates of pertinent rate constants. Developing and parameterizing models to capture these interactions and forecasting them accurately has forced the development of a new family of modeling techniques, known in the literature as "data assimilation" and "model data fusion", drawing on techniques pioneered in control theory, statistics and weather forecasting. While the underlying process-level science and numerical techniques associated with climate and ecosystem simulations are relatively mature, our capacity to efficiently apply these data assimilation and error analysis techniques to complex models and to applications where significant non-linear behaviors occur is currently not well developed.

Several current ESSL research efforts are directly addressing these questions and problems. Within the Community Climate System Model (CCSM), several working groups are actively pursuing research topics related to ecosystem - biogeochemistry - climate interactions, resulting in new and more sophisticated model components, and new analyses of coupled system behavior. An intensive airborne measurement campaign is underway to assess these interactions at the landscape scale, focusing on the complex terrain of the Rocky Mountain Front Range. A number of exploratory projects are also underway that focus on particular mechanisms of ecosystem - climate coupling, for example the interactions between land-use and land-cover change and climate and carbon cycle, the interactions of climate and fire dynamics, and the long-term influence of climate and biogeochemical cycling on global-scale biogeography.

The CCSM Biogeochemistry and Land Working Groups have placed a strong emphasis on the development and testing of new CCSM component models that improve the representation of known interactions between ecosystems, biogeochemistry, and climate in our fully-coupled model. Accomplishments over the past year include the adoption of a new land model component (CLM-CN) for the interim model version CCSM 3.5. CLM-CN includes a fully prognostic treatment of both carbon and nitrogen cycles within terrestrial ecosystems, and retains all of the mechanistic detail from CLM, developed and tested over many years, related to physical and biophysical mechanisms of land surface - climate interaction. ESSL scientists and their collaborators are actively documenting the improved biogeochemical mechanisms in CLM-CN (Dickinson et al., 2006, Thornton and Zimmermann, 2007, Lawrence et al., 2007, and Thornton et al., in press).

In collaboration with a broad group of university investigators, ESSL scientists have recently completed a series of fully-coupled simulations linking new land (Thornton et al., in press) and ocean (Moore et al., 2004) ecosystem components to the physical components of CCSM. This system has been used to investigate the influence of climate-biogeochemistry coupling on climate-carbon cycle feedback finding very significant impacts due to the new coupling.

The coupled modeling activity includes a simple prognostic treatment of fire occurrence and fire effects on the physical and biogeochemical aspects of land ecosystems. In collaboration with PI Jim Randerson, University of California, Irvine, ESSL scientists are participating in a project to evaluate the predictions of the current fire model against remote sensing and in situ observations, and to improve the fire model for future coupled climate-biogeochemistry experiments.

An important aspect of the focus on evaluation and improvement of ecosystem - climate interactions in the CCSM framework is an on-going project to compare multiple models of climate-carbon cycle dynamics against a broad range of remotely sensed and in situ observations. This project (Carbon Land Model Intercomparison Project, or C-LAMP) has been underway for about two years with the goal of documenting a standard intercomparison protocol and how simulations from these models compare with each other and observational benchmarks. ESSL scientists are in discussion with the broader Coupled Climate Carbon Cycle Model Intercomparison Project (C4MIP) community to have the C-LAMP protocol adopted as the new standard for ecosystem-climate model intercomparisons.

ESSL Scientists in the CGD Terrestrial Sciences Section have initiated studies of the climate forcing associated with land use. This work has the goal of documenting (a) how changes in land use and land cover have altered present-day climate and are likely to alter future climate and (b) the importance of the land use and land cover change forcing relative to other IPCC SRES forcings. This work involves developing parameterizations of urban land cover, agroecosystems, and soil degradation for use with the CLM. It also involves development of historical and future datasets of land cover change. Accomplishments to date include: the implementation and testing of an urban land cover parameterization for CLM; development of historical land cover change datasets; and participation in the LUCID (Land-Use and Climate, IDentification of robust impacts) project under the auspices of IGBP-iLEAPS and GEWEX-GLASS.

Plans for FY08

Over the coming year ESSL will continue to document the behavior of the fully-coupled model, with several additional manuscripts in preparation or in planning stages at present. In addition to human-mediated land use changes, large-scale changes in the geographic distribution of vegetation as a result of past and future climate changes feed back to alter climate. Scientists in TSS have initiated a project to merge the carbon and nitrogen cycling capability of the CLM-CN model with a previously developed global dynamic vegetation model (CLM-DGVM). ESSL scientists will also be exploring the introduction of coupling between the carbon, nitrogen, and phosphorus cycles. Staff recently participated in a workshop aimed at coordinating the development of new scenarios within the integrated assessment community and the needs and capabilities of the new generation of coupled climate-biogeochemistry models. There are also efforts aimed at exploring the interactions of carbon and nitrogen cycling with land-use and land-cover change, where some preliminary experiments have already been conducted in the CCSM framework. As with most activities in ESSL, our work is highly collaborative and ESSL staff continue to explore opportunities for collaboration, such as with another NSF-sponsored project on hydrologic synthesis, led from the University of Illinois, Champaign-Urbana.

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Numerical simulation of turbulence

Large-Eddy Simulation (LES) has been widely used to examine turbulent processes in the PBL, however most LES applications have been limited to PBLs over horizontally uniform surfaces or idealized strip-like heterogeneous surfaces, which are uncommon in nature. The most logical way to expand our LES study for more complex and realistic PBLs is by nesting an LES code inside a regional mesoscale model. Nesting a turbulence-resolving model (like LES) inside a turbulence-parameterizing model (like mesoscale or climate models) is difficult because at the nest boundaries the simulated flow fields change abruptly from non-turbulent to turbulent flows, or vice versa. Last year we began tackling this two-way nesting problem by examining the feasibility of nesting a finer-grid LES inside a coarser-grid LES using the WRF’s dynamic core. We showed a promising result that both LESs (the outer and the nest domains) generate similar turbulent motions and transport if they are driven by the same forcing and environment. Our strategy is to systematically introduce complexity into the problem by next nesting an LES inside an idealized mesoscale flow (such as a land-sea breeze circulation or a flow over an idealized hill) before trying it out with the real-world setting.

Another more complex and important PBL regime is the one under cumulonimbus). We know precipitation can generate cold pools in the PBL due to evaporation, but how these cold pools affect turbulent transport in the PBL and how the change in the PBL affects the life cycle of deep convection remain questionable. As part of the research funded by the new NSF Science and Technology Center for Multiscale Modeling for Atmospheric Processes (CMMAP), which is based at Colorado State University, we began exploring the interaction between deep and shallow convection, and the interaction between deep convection and the PBL. A benchmark will be generated for this study by performing a large-domain (about 500 km x 500 km x 20 km), high-resolution (a grid mesh about 100 m x 100 m x 20 m) LES of a tropical deep convection system in which deep and shallow convection, as well as large turbulent eddies (inside the PBL and inside the convection) are all resolved. We plan to use a new Vector Vorticity Model developed by Jung and Arakawa (CMMAP scientists) to perform such an LES. This will be the first attempt to simulate tropical deep convection with its associated shallow convection and turbulence motions explicitly calculated as well.

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Exploring the Role of Aerosols

The effects of aerosols on climate represent the single largest source of uncertainty in our understanding of global warming. In order to reduce the uncertainty of the role of aerosols in climate and weather, ESSL scientists are conducting both experimental and modeling studies of aerosol formation, composition, and direct as well as indirect radiative forcing.

ACD researchers are working with the research group of P. McMurry (U. Minnesota) in order to understand how particles form in the atmosphere and how they grow to become important participants in atmospheric chemistry and climate. During 2007 this team analyzed data from the 2006 MILAGRO campaign in Mexico City, where they studied of the chemical composition of the particles in the peak of the growth mode in the size distribution following new particle formation, as shown in Figure 1. These measurements were performed using the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) [refer to 17b]. On this day, nascent particles contained far more nitrates and organics than sulfates: molar ratios of nitrate, organics and sulfur species were respectively ~50%, ~45% and ~5%, indicating that sulfur species were a relatively small fraction of the total. The TDCIMS data show that the particulate organic species included methyl and dimethyl amine, organic nitrates, and organic acids. Independent calculations show that sulfuric acid condensation could have accounted for only 5-10% of the growth that was observed on this day, which is consistent with the TDCIMS measurements of composition. It follows that nitrogen compounds and organic acids must certainly contribute to the high growth rates that were observed. This is the first direct measurement of size-resolved aerosol composition following new particle formation, and will used in developing state-of-the art GCM aerosol-cloud interactions modules in order to assess the aerosol indirect effect.

ACD scientists are also working with Tanarit Sakulyanontvittaya, Jana Milford and Detlev Helmig (U. Colorado) to investigate the contribution of biogenic VOC emissions to secondary organic aerosols (SOA) in the U.S. An improved model for simulating sesquiterpenes, monoterpenes, and isoprene emissions was developed and used to estimate U.S. monthly emission distributions. Initial estimates of sesquiterpene contributions to regional SOA distributions were generated by developing and implementing a module to simulate sesquiterpene oxidation and SOA yield. Results from the emission and SOA model indicate that sesquiterpenes, monoterpenes and isoprene can all make significant contributions to U.S. organic aerosol.

Plans for 2008 include completing the analysis of the MILAGRO data set and publishing results, and conducting further studies of the chemical composition of particles formed from nucleation in support of model development for predicting particle growth.

This work is funded by NSF, DOE, NOAA and USEPA.

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Convective Organization: Observational analysis and resolved simulations

The propensity for deep, moist convection to organize and project onto larger spatial and temporal scales requires numerical simulations spanning convection-resolving scales to continental scales. Furthermore, simulation studies must be closely constrained by observational analysis of the organizing properties of convection. Prediction of tropical and warm-season higher-latitude convection, and the response of the synoptic-scale and planetary-scale flow is vital for increasing our ability to anticipate significant weather events more than a day in advance. It is also vital for credible representations by models of regional climate.

Much of the work in the previous year has been related to the Short-Term Explicit Prediction (STEP) program or the Water Cycle Across Scales program. The focus of water cycle research is the global comparisons of regional rainfall coherence. Warm season precipitation cycles were examined for Africa and Europe, using digital infrared imagery from 1999 to 2003. Daily propagation of organized precipitation in tropical Africa can be interrupted by large-scale changes such as equatorial wave passages, while in the subtropics and midlatitudes, propagation is associated with westerly wind shear and warm, moist, low-level inflow. Propagating episodes were also observed over the Bay of Bengal and the Tibetan Plateau using the TRMM Real Time Multi-Satellite Precipitation Analysis products. It was found that free-tropospheric circulations (short-waves) were not phase-locked to the topography and the diurnal cycle upstream from convection over the U.S., but were substantially modified by convection and hence became phase-locked downstream.

Numerous simulations of convection episodes over the Central U.S. have been conducted, emphasizing the impact of land surface treatment, cloud microphysics, and aerosol properties on warm-season organized convection. Quasi-idealized simulations reveal that time averaged conditions (without transient tropospheric features) are sufficient to generate long-lived propagating rainfall systems in the lee of the Rockies. Other simulations of propagating convection include cloud-system-resolving simulations of tropical large-scale organized convection coupled with gravity/Kelvin waves using a tropical channel model, and cloud-resolving simulations of convective cloud systems observed during the South China Sea Monsoon Experiment (SCSMEX) using the WRF model. A cautionary note about "convection-permitting" simulations that use grid spacing greater than about 1 km was noted wherein convection cells are anomalously large at coarser grid spacing, leading to delayed development of the convective systems; insufficient production of the stratiform region; and over-aggressive transport of moisture into the lower stratosphere.

Next year, simulations of convection episodes in various parts of the world (E. Asia, U.S., and Africa will continue or commence). New physical parameterizations will be tested, ranging from new convection schemes that attempt to improve propagation signals, to cloud physics and boundary-layer schemes aimed at a more realistic representation of convective system structure. New observations will be obtained during the Taiwan Island Mesoscale Rainfall Experiment (TIMREX) that allow comparisons with other orographic rainfall experiments, and comparisons between marine (TIMREX) and more continental orographic control of convection.

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Atmosphere/ocean interactions

Air-sea interaction occurs over a wide spectrum of scales ranging from millimeters (spray droplets and air bubbles) to hundreds of kilometers (synoptic scale storms) and even larger (global climate). A goal of marine surface layer research is to identify and quantify coupling mechanisms that connect the atmospheric and oceanic boundary layers (the ABL and OBL) and surface waves. Some of the specific problems of interest in the ABL include the effects of wave age, swell, surface roughness, and wind-wave misalignment. In the OBL, waves induce Langmuir circulations and break which generate mean currents and turbulence. Wave influences on the OBL are of particular importance under high wind conditions. Turbulence resolving simulations and in particular large-eddy simulation (LES) and its ability to perform systematic process studies plays an important role in air-sea interaction research. LES has provided new insights into the couplings between imposed waves and turbulence.

Winds and waves in marine boundary layers are often in an unsettled state as fast-running swell generated by distant storms propagates into local regions and modifies the overlying turbulent fields. A large-eddy simulation (LES) model with the capability to resolve a moving sinusoidal wave at its lower boundary is being used to investigate this low-wind/fast-wave regime. Surprisingly, LES predicts significant momentum transfer from the ocean to the atmosphere under conditions of light winds wind above swell. In certain circumstances the wave field is capable of generating a low-level jet near the water surface. This regime is not adequately described by common parameterizations of the air-wave interface. An analysis of the momentum flux from recent field campaigns Coupled Boundary Layers Air-Sea Transfer (CBLAST) (see figure 1) and the Ocean Horizontal Array Turbulence Study (OHATS) validates the LES predictions and illustrates that the wave field modifies the drag of the ocean in fundamentally different ways than a rough land surface. In the upcoming year, we will continue our analysis of the CBLAST and OHATS databases and extend our analysis to the OBL. In the latter we will focus on the interactions between waves and currents under high winds. This work is supported by NSF and the Office of Naval Research.

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Long-term Climate Change In The Thermosphere

Simulations using the NCAR global mean upper atmospheric model were published in GRL by Qian et al. [2007]. A long-term decrease of thermospheric neutral density, on the order of 1.7% per decade for the past several decades, which is in agreement with results from satellite drag data analysis. This long-term decrease of neutral density is consistent with the hypothesis by Roble and Dickinson [1989] predicting cooling and contraction of the upper atmosphere due to increased greenhouse gas forcing. In the figure, Panel (a) is CO₂ concentration measured at Mona Loa observatory from 1970 to 2000; Panel (b) is solar activity index F_10.7 in red and its 81-day average in blue. When the model was driven by these time-varying CO₂ and F_10.7, it gave global mean neutral density at 400 km as shown in panel (c). It is apparent that variation in this neutral density is dominated by solar-cycle variation. This solar-cycle variation needs to be removed in order to extract the relatively very small long-term change. Therefore, another model run was conducted in which the CO₂ concentration was fixed at 1970 level but using same solar forcing as shown in panel (b). The ratio of neutral density from the two model run was then calculated and shown in panel (d) in red. The blue is its linear regression, which gives an average decrease rate of neutral density of 1.7%/decade. Panel (d) also shows that the density decrease is greater at solar minimum than solar maximum, which is also in agreement with results from satellite drag data analysis.

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Climate change - probabilistic climate change, and solar forcing of climate

Climate Change: probabilistic climate change, and solar forcing of climate

NCAR was one of the first centers to study anthropogenic climate change with global coupled climate models starting in the late-1970s. Consequently, the earliest climate change experiments done at that time were pioneering at a national and international level. Few groups were doing climate change modeling as it was considered to be a sidelight to other more highly regarded modeling problems. NCAR climate change modeling (funded by DOE and NSF) was prominent in the DOE State-of-the-Art climate change assessments in the late 1980s, and in the first IPCC assessment in 1990 and the 1992 IPCC update since only four groups in the world (including NCAR) had functioning global coupled climate models that were being used for climate change projections.

Since then, climate change modeling has become a very prominent activity at NCAR, most recently through the Community Climate System Model effort, and is now a headline activity for ESSL. As climate change modeling evolves to include more complexity, we are moving toward an earth system model-type activity. This crosses division boundaries in ESSL and requires close cooperation with the other science divisions since such earth system models will include not only the basic atmosphere-ocean-land surface-sea ice global coupled model, but also components of chemistry, aerosols, dynamic vegetation and carbon cycle.

In addition to multiple papers describing results from climate change modeling (Han et al., 2006; Hu et al., 2007a,b; Santer et al., 2006, 2007), CGD scientists and collaborators have been involved with research that has directly influenced and characterized national and international assessment activities. For example, one of the new aspects of climate change projections is quantifying uncertainty with regards to probabilistic climate change. This is possible for the first time due to the large number of multi-model ensemble members available in the CMIP3 multi-model dataset archived at PCMDI (including two models from NCAR, the Parallel Climate Model and the CCSM3). In a ground-breaking study, Furrer et al. (2007) used a multivariate Bayesian analysis to show probabilistic climate change results that were featured in the IPCC Fourth Assessment report (Meehl et al., 2007). In one depiction of probabilistic climate change, temperature changes are shown that exceed 80% probability by the end of the 21st century relative to the end of the 20th century for northern winter and summer (Fig. 1). In another depiction, probabilities that temperature change exceeds 2C by the end of the 21st century compared to the end of the 20th century are shown (Fig. 2). Such analyses move beyond the typical simple differences that are commonly shown for future climate change, and provide probabilistic information that better illustrates the geographical distribution of possible future climate change.

One of the outstanding mysteries of climate science has been how solar variability affects the earth's climate. For many years associations of the solar cycle with various climate parameters have been noted, but what was missing was a plausible mechanism that linked the two. Recent work has shown a distinguishable and significant signal in Pacific climate as a response to solar forcing (van Loon et al 2007). Taking the peak years of the 11 year solar cycle and looking at the patterns of climate anomalies in the Pacific for those years, the response resembles that of a low amplitude La Nina, with anomalously cold water in the equatorial Pacific, and enhanced precipitation in the climatological precipitation maxima in the Pacific, the ITCZ and SPCZ. This pattern was first suggested in a model analysis of the increase of solar forcing in the first half of the 20th century (Meehl et al., 2003). In that study, a mechanism was proposed that could explain such a coupled response in the Pacific. More recently, the mechanism has been defined for the 11 year solar cycle in much the same way as the lower frequency response (Meehl et al., 2007). Though the globally averaged solar forcing at peak years in the solar cycle is small, locally in cloud free areas of the subtropics, that forcing can be considerably larger (on the order of nearly 2 Wm^-2). This extra energy is absorbed by the ocean, and the trade winds remove that extra heat in the form of enhanced evaporation. That moisture is then carried by the trade winds to the convergence zones in the Pacific, strengthening the precipitation and associated vertical motion, feeding back on the trade winds to strengthen them as well. That result in greater upwelling and colder water in the equatorial Pacific in observations and two models (Meehl et al., 2007). Thus, for the first time, a pattern of coupled air-sea climate response has been identified in observations, a mechanism has been proposed to explain it, and similar patterns are seen in two global coupled climate models.

Future research priorities in climate change modeling include addressing the effects of aerosols on the climate system response (e.g. carbon aerosols), further studies of extremes (e.g. El Nino and extremes, attributing changes of extremes to human activities), and understanding the relative contributions of inherent decadal variability and forced response in 20th century climate change.

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Land-atmosphere coupling

Over the last forty years, atmospheric research has shown that land-atmosphere coupling is of critical importance for weather and climate prediction. Trees are a dominant feature on planet Earth. Forests cover some 30% of the world's land area, accommodate two-thirds of life on Earth, and are responsible for 90% of the biomass on solid ground. Due to its diverse nature, a tall canopy's influence on turbulent exchange is extremely complex, e.g., because of their spatial distribution, seasonal variability, flexibility, porosity, etc. Within the layer of the atmosphere directly influenced by the canopy, turbulence exhibits dramatically varying properties depending on the detailed structure of the roughness elements and cannot be described by traditional similarity relationships. Where vegetation covers the surface, it becomes the important momentum sink and a key player in distributing sources and sinks of moisture, heat and trace atmospheric constituents. Parameterization of turbulent transport in and above tall canopies remains somewhat elusive but is essential for accurate weather and climate prediction. Large-eddy simulation (LES) is an important tool for studying the coupling between microscale and mesoscale motions. LES can also incorporate the influence of vegetation on momentum, energy, and scalars. Because observing three-dimensional and time-dependent fields of all quantities of interest is difficult, LES has become a direct link between currently observable quantities and larger-scale models which are forced to parameterize all of the turbulence.

LES needs to be validated and improved to deal with complex flows, especially for surface layers where dependence on the subfilter-scale (SFS) model increases. To address this issue, NCAR, in collaboration with several university groups, have carried out three pioneering observational studies to improve subfilter-scale parameterizations over flat terrain with short sparse vegetation (Horizontal Array Turbulence Study, HATS, over the ocean (Ocean HATS; OHATS), and, during spring 2007, within and just above a tree canopy (Canopy HATS, http://www.eol.ucar.edu/rtf/projects/CHATS/isff/). These studies provide an observational basis for testing and improving closure approximations used in LES and they have substantially increased our confidence in parameterizations developed using LES as their basis.

At this point, the character of within-canopy SFS motions is not known, nor the role that the eddies shed in the lee of the plant elements play, nor how these wake-scale motions affect scalar and momentum transport. Therefore, the objectives of CHATS are to: 1) measure SFS variables in a complex environment linking the biosphere, geosphere, and the atmosphere, and 2) study the fundamental interaction between vegetation and atmospheric turbulence. CHATS will allow for validation of currently utilized SFS models and improvement of parameterizations representing this critical regime.

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Emission in ventories and application

Trace gas and aerosol emissions into the atmosphere are the major drivers of the chemical composition of the atmosphere. There is widespread concern about the effect of human activities on these emissions and their impact on atmospheric chemical composition. Changes in human activities are the underlying cause of the current increase in pollutant levels on regional and global scales. In some cases, changes in trace gas emissions are due to obvious pollutant sources including many technological sources. Other sources, including biomass burning and biogenic sources, have a natural component but are strongly influenced by humans. In order to understand these increases and to predict future changes, ACD/TIIMES scientists are quantifying emissions from various sources and improving our understanding of the natural and human influenced processes that control emissions.

ACD/TIIMES scientists have completed a new version of the Model of Emissions of Gases and Aerosols from Nature (MEGAN), which is a modeling system for estimating the net emission of gases and aerosols from terrestrial ecosystems into the atmosphere. It is driven by landcover, weather, and atmospheric chemical composition. MEGAN is a global model with a base resolution of ~ 1 km. A stand-alone version of MEGAN is now available on the NCAR community data portal during the past year and has already been downloaded by more than 30 users. MEGAN has also been incorporated as an on-line component of several regional and global models including MOZART, CCSM-CLM, GEOS-CHEM and WRF-CHEM. The number of compounds included in MEGAN has been extended to over 130 chemical species. U.S. and Canadian landcover has been improved using databases that include 30 m resolution data based on satellite imagery. Landcover has also been substantially improved for several U.S. urban areas.

ACD/TIIMES scientists have also continued to improve a North American wildfire emission model and have used the model to produce fire emission estimates for the NCAR MIRAGE field campaign. The model estimates daily emissions from fires for all of North America at a 1km resolution. Mercury emissions were included in the fire emissions model and have provided the first, state- and monthly resolved mercury emission estimates from fires (see figure). ACD/TIIMES scientists teamed with Jason Neff (U. Colorado) to quantify monthly, state-level CO2 emissions from fires and discuss the potential policy implications of these emissions. This has included the incorporation of mercury emissions into the model (see figure).

FY2008 work will include continued improvements of MEGAN and the wildfire model and enhanced support for the communities using these models. The emission models will be used in regional chemical transport modeling studies to investigate the radiative impact of aerosols from fires and biogenic sources, interactions between direct particulate fire emissions and secondary organic aerosol formation, and mercury deposition. This work is funded by NSF/NCAR and EPA.

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Water Cycle

As part of the Water Cycle Program, which involves scientists across ESSL and other national lab and university colleagues, various aspects of the water cycle in observations and climate models has been examined. The focus is on precipitation, atmospheric water vapor, and land surface water fluxes, with the goal to improve our understanding and thus modeling and prediction of atmospheric moist convection, precipitation processes, and land surface hydrology on a broad range of time scales. The diurnal cycle of warm-season precipitation over the U.S. and other parts of the world has been employed as a means to systematically examine precipitation characteristics (onset, diurnal timing, frequency, intensity, duration, amount, type, etc.) in data and models, thus allowing a diagnosis of deficiencies in weather and climate models.

The Water Cycle Program also interacts with other ESSL programs such as the Biogeoscience Program. Furthermore, data sets and model evaluation work produced under this project are helpful for improving the Community Climate System Model (CCSM) and other climate models. The project also leverages other NSF, NASA, and NOAA-funded studies related to the water cycle.

Recent work under this project includes 1) quantifying the hydrological effect of volcanic eruption of Mt. Pinatubo (Figure) and its implications for geo-engineering solutions to global warming; 2) creation of a global data set of continuous monthly river outflow for community use and quantifying decadal and long-term changes in continental freshwater discharge; 3) quantifying the various components (precipitation, evapotranspiration, soil moisture, stream flow, atmospheric water vapor flux, etc.) of the water cycle and their changes over the past 57 years and under global warming, such as recent hydroclimatic trends over the Mississippi river, and 4) analyses of satellite-observed and model-simulated precipitation and other hydrologic fields. These studies have resulted in a number of refereed publications (see Aiguo Dai and Kevin Trenberth). For example, the work on the Pinatubo's effect on land precipitation, stream flow (Figure), and drought and its implications for similar geo-engineering solutions to global warming is a pioneer study which has attracted considerable attention. The study shows that large decreases in land precipitation and record-low stream flow occurred during a 12-month period following the June 1991 eruption of Mt. Pinatubo, which resulted in wide-spread drought during that period. The results suggest that major unintended adverse effects, such as reduced water resources and increased drought, may occur from proposed geo-engineering solutions to mitigate global warming through emulating volcanic eruptions by injecting large amounts of aerosols into the Stratosphere.

Other major findings from recent ESSL work include: a). evapotranspiration has increased over the Mississippi river basin from 1948-2004 mainly due to increased precipitation while the decreases in surface solar radiation associated with increased cloudiness were compensated by decreases in both net long-wave radiation and sensible heat flux; b). continental freshwater discharge into global oceans show a slight decrease from 1949-2004, in contrast to the notion that continental discharge has increased as the global hydrological cycle intensifies under global warming; and c). large increases in the discharge into the Arctic Ocean during 1949-2004 are not accompanied by increases in precipitation; instead, increased runoff resulting from melting of soil ice in Eurasia may be a significant contributor. Future plans call for further work to improve cumulus parameterizations in ESSL models, more comprehensive analyses of potential impacts of global warming on the water cycle, including increased drought over land.

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Parameterization

Representation of cloud microphysical processes in models of various complexity (from small-scale to global) is a challenging aspect of numerical weather prediction and climate modeling. This is mostly because of the disparity between scales at which cloud microphysical processes operate (i.e., millimeters and centimeters) and scales resolved by models and observations. With the advent of convection-permitting numerical weather prediction using the Weather Research and Forecasting (WRF) model and application of the superparameterization approach to climate modeling (Grabowski and Smolarkiewicz~1998; Randall et al.~2003) representation of cloud microphysics emerges as the next "key problem", similarly to the "convection parameterization problem" in the past. The superparameterization approach to climate modeling is the focus of the NSF's Science and Technology Center for Multiscale Modeling of Atmospheric Processes (CMMAP) at Colorado State University. Several NCAR scientists are members of the CMMAP team and are actively involved in the CMMAP research.

Much of the research to improve cloud microphysical schemes is driven by impacts of atmospheric aerosols that are involved in cloud formation processes (cloud condensation nuclei and ice-forming nuclei) on weather and climate. These are referred to as the indirect aerosol effects. Their uncertain role in climate was highlighted by the 2007 report of the Intergovernmental Panel on Climate Change (IPCC). With help from NOAA and NSF funding (the latter both directly to NCAR and through NCAR's involvement in CMMAP), MMM's Wojciech Grabowski and Hugh Morrison developed a new comprehensive two-moment bulk microphysics scheme to represent warm-rain and ice processes. The new scheme is to be used in research concerning indirect aerosol effects using large-eddy simulation (LES) models and cloud-resolving models (CRMs), as well as in climate models applying superparameterization. The warm-rain scheme builds upon the two-moment scheme of Morrison et al. (2005). The scheme has been validated against a detailed (bin) microphysics scheme in Morrison and Grabowski (2007a) and it is currently being used in LES simulations of shallow tropical convection (Morrison and Grabowski 2007b). The new two-moment bulk ice scheme applies a novel strategy to represent ice processes developed by Morrison and Grabowski~(2007c). The key feature of the new ice scheme is that it allows representing in a natural gradual transition from small to large ice particles due to growth by water vapor deposition and aggregation, and from unrimed crystals to graupel due to riming. In traditional approaches, these processes are treated by separating ice particles into predefined categories (such as cloud ice, snow, and graupel) using fairly arbitrary mass or size thresholds and applying poorly-justified conversion rates. Figure 1 illustrates capability of the new ice scheme to simulate gradual increase of the mass-weighted terminal fallspeed of the ice field as a function of crystal rimed mass fraction. The figure shows that the fallspeed for unrimed ice particles is between 0.5 abd 1.0 m/s, whereas heavily rimed ice particles (i.e., grauped) fall with significantly larger terminal velocities (1 to 3 m/s).

The new microphysics scheme will be used in LES and CRM simulations of various cloud types (shallow and deep; ice-free, mixed-phase, and totally glaciated; convective and stratiform), with the emphasis on the indirect aerosol effects. Studies concerning shallow tropical cumulus clouds are already progressing. The scheme will also be included into a suite of microphysics parameterizations in the WRF model. Finally, through the involvement with CMMAP, the scheme will be used in climate studies applying superparameterization approach.

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Role of the oceans in climate

Covering 71% of the Earth, the oceans absorb the majority of the solar energy reaching the surface, and are the dominant source of water vapor to the atmosphere. The heat capacity of the upper three meters of the oceans exceeds that of the entire atmosphere, and the oceans contain approximately 50 times greater inventory of CO2 than the atmosphere. Ocean currents accomplish roughly equivalent energy transport from the tropics to higher latitudes as does the atmosphere. Through this capacity for storing and transporting energy, water, and radiatively active gases, the oceans act to moderate, modulate, and initiate climate variability and climate change. A comprehensive understanding of, and the ability to predict, the behavior of the climate system must therefore be based on an understanding of the physical, chemical, and biological processes operating in the oceans and their interaction with other components of the Earth system.

Through research to develop an understanding of ocean processes, and using this understanding in improving their representation in ocean models, ESSL ocean scientists support a broad spectrum of ESSL scientific objectives. These include: prediction of the Earth's energy, water and biogeochemical cycles, and understanding natural and human influenced climate variability, including high impact variations such as sea level rise. In turn, the ESSL objective of understanding two-way scale interactions within the Earth system is central to improving our understanding of how ocean circulation features such as coastal upwelling zones, western boundary currents, and meso-scale eddies are affected by and affect the basin- to global-scale ocean circulation.

Significant increases in computational resources together with improved physics and greater confidence in CCSM climate models with modest horizontal resolution has allowed ocean model developments to be evaluated in fully coupled models for their effects on the climate system as a whole. In a number of cases, these coupled model results have been much more profound and widespread than anticipated from consideration of effects on the ocean in isolation. The key factor is for the ocean model changes to produce small, but persistent, changes in near surface ocean temperatures, then for the coupled model to react to these signals in such a way as to amplify the temperature changes.

In a complementary way, global atmospheric modeling efforts within ESSL are examining the sensitivity of physical processes in the atmospheric component model to small errors in Sea Surface Temperature (SST). Scientists are conducting experiments in which coupled SST biases are imposed on uncoupled atmospheric configurations on a regional basis to better understand the way in which parameterized physics responds to the anomaly for range of amplitudes in the bias. This work has illustrated a large simulation sensitivity, particularly at low latitudes, pointing to the strong coupling that exists between the ocean and the atmosphere.

A concerted effort was made to lower the ocean model viscosity as much as possible, with the primary goal of simulating realistic tropical instability waves. A particularly significant and unexpected consequence was a dramatic improvement is the sea-ice distribution in the Labrador Sea, as shown in the accompanying figure. The far too great ice extent seen in the high viscosity control case was a major factor holding up adoption of the finite volume dynamical core in the atmosphere and despite extensive efforts focused on the atmospheric model the problem remained, until the ocean viscosity was reduced. As might be expected, the currents in the lower panel with low viscosity are stronger than with higher viscosity (upper panel), and they advect more warm water into the Labrador Sea, especially along the west coast of Greenland. This warm water tends to melt the sea ice and initiate the positive ice-albedo feedback, where the reduced ice lowers effective albedo, which allows more absorption of solar radiation, which warms the ocean and leads to more melting. Another interesting effect of lower viscosity is to decrease the transport of the Antarctic Circumpolar Current through Drake Passage, when an increase was expected. Lower viscosity also improved the modeled structure of currents and temperatures in the Kuroshio extension, with positive impacts on the atmospheric circulation over the North Pacific.

A number of additional ocean model developments are being evaluated in a coupled climate context. Among the more promising are: enhanced tidal mixing in the vicinity of the Indonesian Throughflow, which effects SST a little and improves regional convection and precipitation a lot; improvements to the parameterization of ocean mesoscale eddy effects, which changes the amplitude of ENSO variability; lower vertical diffusivity, which improves ocean temperature in upwelling regions, which in turn improves the direction of near coastal winds, which leads to an additional improvement in the upwelling. Two other developments are planned to be evaluated in the coupled climate model context. The first is increased ocean vertical resolution. The second is the nesting of very high horizontal resolution in select coastal regions where the largest biases in ocean temperatures are found in most coupled climate model systems.

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Structure and evolution of clear and cloudy atmospheric boundary layers

Marine stratocumulus and trade-wind boundary layer research are important elements of the NCAR program. Of particular interest to ESSL/MMM scientists is the investigation of the structure and evolution of the marine stratocumulus regime off the southern California coast. Globally, these persistent clouds significantly cool the Earth because they radiate at a temperature similar to the surface, and yet reflect a much larger fraction of solar radiation than clear ocean regions. A key variable needed to understand their evolution is the