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Understanding the effects of gravity wavesExploring atmospheric, Earth system, and solar processes, and the variability and change of these processes, are critical components to reaching NCAR's strategic goal #1, “Improve understanding of the atmosphere, the Earth system, and the Sun.” The Earth and Sun System Laboratory (ESSL), with partnerships with the universities and other agencies, has the scientific knowledge to attack the problems associated with attaining this goal. Exploration into these areas will focus on three key activities: simulating the natural Earth system variability, researching the magnetic-flux eruptions from the sun, and investigating the coupling between the upper troposphere and lower stratosphere (including gravity waves). For each of these areas, ESSL has numerous activities underway and they are highlighted below. Note the special laboratory highlights on paleoclimate which offers a remarkably fertile ground for testing climate models and the underlying parameterizations and on the recovery and usage of the HIRDLS satellite as well as the discovery of a way to predict the next solar cycle using the memory term linked to the meridional circulation in the Sun. Furthermore, this priority has two highlights at the level of NCAR, namely the research on Space Weather and that on the building of a coronal magnetometer which allows for the first time a direct measurement of the magnetic field in the solar corona. HIRDLS recovery and application [Highlight] - ACD
HIRDLS recovery and application
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, O3 , H2O, 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. NASA funded the U.S. share of the HIRDLS development. When HIRDLS was launched on the Aura spacecraft in July 2004, a plastic lamina from inside HIRDLS came loose and obstructed most of the instrument's aperture, limiting the view to the atmosphere to a small fraction of the width of the optical beam. HIRDLS scientists developed a procedure for processing the signals that could be seen through the partial aperture. These efforts demonstrated that there was recoverable atmospheric information in the signals. The next steps were to correct the measured signals to make them as close as possible to the expected radiances. The major efforts this year were to improve and refine the correction algorithms, make them more robust, and if possible develop a physical basis for them that would assure that they would work under all conditions. The NCAR part of the team hosted the next HIRDLS science team meeting, held in June. The team reviewed the correction algorithms and approaches, presented comparisons of HIRDLS results with other data, and discussed future activities. The comparisons showed that the nitric acid data had also reached an acceptable quality level. The team agreed to make the temperature, ozone, nitric acid and cloud top pressure data publicly available to those who wanted to assist in its validation and evaluation. The HIRDLS team hosted the Aura Science Team meeting, attended by over 220 people, at NCAR on September 11-15. In association with this, several working groups met to discuss data validation as well as science applications of the data. HIRDLS team members presented results based on improved processing codes. Among a number of noteworthy results, ACD scientists showed that HIRDLS could map sub-visible cirrus in the tropical upper troposphere. These showed a movement of the thickest layers from SE Asia to India to Indonesia, and from Central to South America, over the May to October time period. These cirrus layers play an important role in the earth's radiative balance as well as in dehydrating the UT/LS, and are a unique measurement by HIRDLS on Aura. Further improvements continue to be incorporated in the operational data processing code. Initially they are used to process days for which there are validation data. The goal is to improve the results as rapidly as possible, with the desire to recover all the originally expected species. This work is funded by NASA and NSF/NCAR. UTLS initative
The region of ~5 kilometers above and below the tropopause is usually referred to as the upper troposphere and lower stratosphere (UTLS). Investigation of the processes occurring in the UTLS is crucial for understanding long-term global climate change and tropospheric air quality. The interaction between chemistry and clouds in this region and the perturbations of dynamical processes to the radiatively-sensitive species on both sides of the tropopause presents a challenge to the performance of climate models. The new Gulfstream 5 (G-5) research aircraft, with its high altitude and long-range capabilities, offers new and exciting opportunities for photochemistry, cloud, aerosol, radiation, and transport research in this region. The goals of the UTLS Initiative are to plan and conduct field campaigns, using this new aircraft, to investigate the coupled dynamical, chemical and microphysical processes in the UTLS and to complement the aircraft studies with satellite observations and multiscale NCAR models. During FY06, the UTLS Initiative team participated in the first two field deployments of the G-5 and obtained initial results of observing the UTLS region from the new aircraft. The first experiment, the Stratosphere-Troposphere Analyses of Regional Transport (START05), was conducted as a component of the G-5 Progressive Science Mission in December, 2005. The behavior of the tropopause as a chemical transport boundary was observed under a variety of dynamical conditions. The experiment was led jointly by staff from the Earth and Sun Systems Laboratory - ESSL (Laura Pan, William Randel, Melvyn Shapiro, Christopher Davis, Teresa Campos, and Sue Schauffler) and staff from the Earth Observing Laboratory - EOL (William Cooper, Jorgen Jensen, Jeffrey Stith, David Rogers), along with external collaborators Ru-shan Gao (National Oceanic and Atmospheric Administration/Earth System Research Laboratory [NOAA/ESRL]), Kenneth Bowman (Texas A&M University), Jennifer Wei and Christopher Barnet (NOAA/National Earth Satellite Data and Information System [NESDIS]). These initial flights successfully demonstrated the new aircraft's capability to sample detailed structure in the region of tropopause folds. The second experiment, Terrain-induced Rotor Experiment (T-REX) was conducted during March-April, 2006. ESSL staff who participated include Pan, Ilana Pollack, Campos, Brian Ridley, Schauffler, and Randel), in collaboration with the T-REX Science Team (led by Vanda Grubisic, Desert Research Institute), investigated the chemical signature of mountain waves near the tropopause. Using a small suite of in situ tracer measurements (O3, CO and H2O vapor) and analyses of meteorological variables, transport pathways between the stratosphere and troposphere were probed during these two field experiments.
Two more extensive field experiments have been planned for FY07-09 (START08 Experiment [FY08] and Deep Convective Clouds and Chemistry (DC3) Experiment [FY09]). The focus in FY07 is to carry out the planning, proposal, and preparation for these two experiments. The START08 experiment, with an instrument payload to measure chemical and microphysical tracers will identify the transport pathways and characterize the transport boundary in the extratropical UTLS region. The DC3 experiment targets the chemical impact of the convective transport and cloud processing. Results from both experiments will contribute to the observational database for process-oriented validation and diagnoses of chemistry-climate models. Co-leading the START08 effort are Elliot Atlas (University of Miami) and Pan. The DC3 effort is co-led by Mary Barth (NCAR), Christopher Cantrell (NCAR), William Brune (Pennsylvania State University) and Steven Rutledge (Colorado State University). Gravity wavesAtmospheric Gravity Waves are important because they cause a redistribution of momentum and energy, trigger convection, and induce mixing, which changes the transport of chemical species such as ozone. The vast spatial and temporal extent of gravity waves has important implications for the atmosphere from the mesoscale to the global scale and poses a stiff challenge to improve weather and climate predictions at all ranges. Gravity waves play an important role in studying the coupling of lower and upper atmospheric regions and are therefore of tremendous inter-disciplinary interest.
Given this significance in atmospheric studies, the Institute for Integrative and Multidisciplinary Earth Studies (TIIMES) hosted a Gravity Waves Retreat between 19 June and 7 July 2006 which brought together leading experts in atmospheric gravity wave studies from the U.S. and international institutions to explore various aspects of gravity wave studies in the context of lower and upper atmosphere coupling. The retreat yielded a definitive report, Gravity Waves in Weather, Climate, and Atmospheric Chemistry: Issues and Challenges for the Community, containing several key recommendations. Among these were the formation of a working group under the Scientific Committee on Solar-Terrestrial Physics (SCOTEP), the Stratospheric Processes and their Role in Climate (SPARC) and NCAR to facilitate interactions between mesoscale modelers, gravity wave theorists and observationalists; the initiation of a collaboration between Fuqing Zhang (Texas A&M University) and the Whole Atmosphere Community Climate Model (WACCM) group (Rolando Garcia, Fabrizio Sassi, Jadwiga Richter) to develop a source spectrum parameterization for baroclinic jet-front systems for climate models. Stronger links with the convection community are envisioned (Mitch Moncrieff, Rit Carbone, M. Joan Alexander, and Richter), including a project to extend the Weather Research and Forecast (WRF) model with a radiative upper boundary condition. An important part of the NCAR mission is to understand the coupling of lower and upper atmosphere through dynamical, chemical, and radiative processes. Sudden stratospheric warming (SSW) involves dynamical changes on vastly different scales from the troposphere to the lower thermosphere, and thus provides us an opportunity to understand the coupling process. Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) temperature measurements have shown that mesospheric cooling is both weaker and occurs at lower altitudes than previous studies had indicated (Siskind et al., 2005). Ground-based observations showed that the mesospheric cooling is generally weak in the subsequent major Southern SSW (Vincent, private communication). It was hypothesized that such discrepancies stem from uncertainties in gravity wave specification and parameterization in General Circulation Models (GCMs). For this reason, Hanli Liu and Raymond Roble performed Thermosphere-Ionosphere-Mesosphere-Electrodynamic General Circulation Model (TIME-GCM) simulations of the 2002 Southern SSW with varying wave sources. From these simulations, it was found that the mesospheric cooling and thermospheric warming and the mesosphere and lower thermosphere (MLT) wind change are reduced if the eastward components of the gravity wave sources are reduced during the southern winter season. Further wave source sensitivity studies and observations, will help to constrain such gravity wave sources. UTLS dynamics, trends, and composition
ACD scientists are involved in studies of global scale chemical behavior using satellite measurements, meteorological data sets and model simulations. Recent work has 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). Satellite data was used to study interannual changes in stratospheric water vapor, focusing on a remarkable step-like decrease observed after 2001 (continuing to present; see Figure. 1). Observations show a contemporaneous cooling in temperatures near the tropical tropopause, and a decrease in tropical ozone at about the same time. The coupled decreases in temperature, water vapor and ozone are consistent with an increase in the stratospheric upwelling circulation in the tropics (the so-called Brewer-Dobson circulation). An increase in the Brewer-Dobson circulation is predicted by models to accompany future climate change, although modeled changes are much smaller than those inferred from the observations after 2001. ACD scientists authored a study demonstrating that stratospheric temperature trends derived from historical radiosonde data can have large cooling biases, associated with changes (improvements) in radiosonde instrumentation over time. The biases are demonstrated by direct comparisons between radiosondes and co-located satellite measurements (from the Microwave Sounding Unit, MSU), which often show discontinuities or jumps in the difference time series. The fact that the jumps occur at different times for different radiosonde stations, and not at all for some stations, demonstrates that the biases are a result of radiosonde inhomogeneties. These results allow substantially improved estimates of historical stratospheric temperature trends (by omitting results from radiosonde stations with the most severe problems). ACD scientists also used satellite and meteorological data sets to study constituent transport near the tropopause within the Asian monsoon anticyclone. This anticyclone dominates the global circulation in the UTLS region during NH summer, and is an important contributor to stratosphere-troposphere coupling. They used observations of ozone, carbon monoxide and water vapor from the Aura MLS instrument, demonstrating that air with tropospheric chemical characteristics (high CO and low ozone) extends above the tropopause within the anticyclone (the latter acts to confine the constituents within this region). Furthermore, time variability in constituents is closely tied to deep convection in the monsoon region. Ongoing work is aimed at understanding the three-dimensional circulations that contribute to transport in this region. In addition, ACD scientists focused on understanding the extent of ice supersaturation in the atmosphere and the associated impacts on chemistry and climate. Supersaturation is a common occurrence in the atmosphere, and appears frequently near the tropopause over the globe. A new observational climatology of supersaturation derived from AIRS satellite measurements has been developed, and the satellite data compared in detail with aircraft and balloon measurements. A formulation for supersaturation in CAM has been developed and tested, to examine the impact on climate simulations and chemistry (this parameterization will also be included in future WACCM simulations). ACD scientists compiled observations from the airborne Fourier transform spectrometer (0.06 cm -1 resolution) that span more than 20 years to produce a unique archive of observations, from the base of the stratosphere, covering a wide range of latitude and season. This archive of infrared spectra has been used along with recent advances in regression fitting to study the long-term trends in water and its isotopes. Water transfer across the tropopause and redistribution in the lower stratosphere are important factors to the chemistry, radiation and dynamics of that important atmospheric transition region. Variations in the behavior of the water isotopes can provide insight into the sources and distribution of water. A remarkable result from that study was that HDO showed an enhanced depletion with respect to H 2 16 O for low latitude upper tropospheric/lower stratospheric air. FY2007 work will involve continuation of studies of chemical and dynamical behavior in the UTLS region as well as further analysis of measurements from the Fourier transform spectrometer. This work was funded by NSF/NCAR and NASA. |
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