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Improving climate modelsUnderstanding of the Earth system is a prerequisite to predicting its behavior, the latter being however of a more direct use to many components of society. In that context, the priorities within the laboratory deal with improving climate models, exploring new approaches to prediction across scales and global and local weather prediction. The NCAR highlight deals with the WRF/ARW. Furthermore, the laboratory presents two highlights on CCSM and on the international experimental endeavor THORPEX. CCSM (link to under community models) [Highlight]
- CGD
CCSM and IPCC
Historical context and rationaleThe development and continuous improvement of a comprehensive climate modeling system that is at the forefront of international efforts to understand and predict the behavior of the Earth’s climate is a high priority of NCAR research. This includes the Community Climate System Model (CCSM) as well as its component models. The CCSM, run on some of the world’s most powerful supercomputers, simulates the many interconnected events that drive Earth’s climate. These include changes in the atmosphere and oceans, the ebb and flow of sea ice, and the subtle impacts of forests and rivers. CCSM is unique among powerful models. Primarily supported by the National Science Foundation (NSF) and the Department of Energy (DOE), with additional support from the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA), it belongs to the entire community of climate scientists, rather than to a single institution. Hundreds of specialists from across the United States and overseas collaborate on improvements to CCSM. The model’s underlying computer code is freely available on the Web. As a result, scientists throughout the world can use CCSM for their climate experiments. The CCSM project was started in 1994, although climate modeling at NCAR has a much longer history than this stretching back to about 1980. The first version of CCSM was unveiled in 1998, and the most recent version, CCSM-3, was released in 2004. CCSM-3 represents a major advance over earlier versions of the model because it contains far more information about Earth’s physical processes. For example, it tracks the flow of major rivers that empty into the oceans and influence currents such as the Gulf Stream, and it now resolves five different thickness categries of sea ice within each grid cell, such as the thickness and the melt rate. Moreover, the finer scale of its resolution allows scientists to capture significantly greater detail about ocean currents and the mixing of salt and fresh water. CCSM is constantly being updated and improved, CCSM4 is most likely to be released in 2009. In addition to remaining at the forefront of international modeling efforts, the scientific goals of the CCSM project are as follows:
Accomplishments in FY 2006A major project that has involved ESSL staff has been development of the Fourth Assessment Report (AR4) by Working Group One (WG I) of the Intergovernmental Panel on Climate Change (IPCC). ESSL scientists have served as convening lead authors, lead authors, and many as contributing authors. ESSL scientists have also reviewed chapters for WG I as well as WG II, and they have contributed to the Technical summary and Summary for Policy Makers. The research of ESSL is prominently featured in the IPCC AR4. In addition, the CCSM project played a major role in the IPCC AR4 through the completion and analysis of an extensive series of emission scenario experiments. The suite of CCSM-3 experiments is the most extensive and highest-resolution multimember ensemble of any of the international global coupled models run for the IPCC AR4. The resulting large data sets are freely available to the climate research and education community via the Earth System Grid (ESG), and a subset of the data is archived at the DOE Program for Climate Model Diagnosis and Intercomparison (PCMDI). The CCSM data are part of the Climate Model Evaluation Project (CMEP), which includes over 200 researchers from around the world who analyzed the multi-model data set for the IPCC AR4. A major accomplishment in FY06 was that ESSL scientists, with external collaborators, played a role not only in CMEP, but in describing, documenting and analyzing the CCSM-3 for the broader climate research community. In particular, results from the CCSM-3 have been documented in two special issues of leading journals. The software engineering aspects of CCSM3, including its performance and portability, were documented in a special issue of the Journal for High Performance Computing Applications. In addition, 26 original research papers from nearly 100 authors, nearly half of whom come from outside of NCAR, appeared in a special issue of the Journal of Climate. The papers include an overview of CCSM-3; descriptions of the climate state for each component model; documentation of the responses of the model to past, present, and future forcing states; an evaluation of the major coupled modes of variability; and a documentation of the climate sensitivity of the model. In general, the climate for present-day conditions produced by CCSM-3 has greater fidelity to the observed climate than do simulations from previous generations of CCSM. Another accomplishment of the past year was the production of a coupled integration that is as good as that from the CCSM-3, but using the Finite Volume (FV) dynamical core in the atmosphere component. Previous attempts using the FV core had produced a too cold Arctic climate and too much sea ice, especially in the Labrador Sea. This was corrected by changing the horizontal viscosity parameterization in the ocean component, which produced a much more realistic, warmer climate in the central Labrador Sea. The sea ice distributions from the new FV version and the released CCSM-3 version are very comparable (Figure 1). The new coupled run using the FV core was run out for 410 years, and this version of the CCSM will now form the basis for future model development. Using the FV core is very important when chemistry is added to the model, for example, because of its conservation properties. A third accomplishment has been made by the CCSM Software Engineering Group (CSEG). A single executable version of the multiple-executable (MPMD), concurrent CCSM has been produced and validated. The single executable implementation is easier to port and debug than the MPMD version, and it will also be able to run on machines that do not currently support MPMD implementations, such as the NCAR IBM BlueGene/L. CSEG has ported CCSM to the Oak Ridge National Laboratory (ORNL) Cray X1 and Cray XT3 and the NCAR IBM Linux Cluster. Current work is underway to port CCSM to the NCAR IBM Bluegene/L. CSEG has also put in place numerous infrastructure improvements. A new testing framework has been implemented to ensure system robustness as new science is incorporated across model components. A new model run database is being created that will permit CCSM runs to be easily reproduced, searched and documented. And a user-friendly performance utility has been created that provides users with the capability of easily determining the optimal load-balanced configuration for their production runs. In FY06, the CCSM data models were completely rewritten to ensure uniform functionality and code re-use across all data model components. In addition, new slab-ocean functionality was added to the data ocean model. The new data models have been an integral part of the land component inter-comparison and the biogeochemistry spin-up experiments. Significant progress has also been made in the creation of a sequential, single-executable CCSM. The goal is to create a sequential system that contains backwards compatibility with the current concurrent system, provides "plug and play" capability of data and active components and produces the same climate as the current concurrent system. Plans for FY 2007One of the higher-priority short-term activities of the CCSM program is a concerted effort to address systematic model biases in the tropics on seasonal and longer time scales, such as the appearance of a double Inter-Tropical Convergence Zone (Figure 2) and warm sea surface temperatures (SST) under the stratocumulus regions off the west coasts of North America, South America and Africa. Several hypothesis-driven activities are under way in collaboration with colleagues outside of NCAR to address such biases, which are common in other global models as well. In addition, new collaborative efforts have started within ESSL to examine, in climate simulations with embedded regional models, the importance of explicitly resolving mesoscale and microscale processes that govern weather and local climate but that may also have significant impacts on the large-scale circulation. The reduction of such biases becomes even more important as the complexity of CCSM increases. Several of the most pressing scientific questions regarding the climate system and its response to natural and anthropogenic forcing require that physical models be extended to include the interactions of climate with biogeochemistry, atmospheric chemistry, ecosystems, glaciers and ice sheets, and anthropogenic environmental change. While the ultimate goal is a comprehensive Earth System Model (ESM), practical considerations suggest that there will be a multitude of versions with different capabilities. The CCSM project will work towards developing a first-generation coupled chemistry-climate model in the next two to three years. A project of this scope will necessarily involve scientific partnerships across ESSL, NCAR, and the external CCSM community. This model could be used to study the complex interactions among biota, chemical processes, and physical climate for paleoclimate studies or scenarios for future climate change. It could also be used to study variations of the chemistry of the present-day atmosphere driven by external forcing from solar variability and major internal natural modes of variability, such as the El Niño–Southern Oscillation. The current long term plan of the CCSM project is to develop and freeze the next version of the model, CCSM-4, by the end of 2008. In addition to several other improvements, this version will most likely have new components for the carbon cycle and interactive atmospheric chemistry. This will enable a whole new range of scientific questions to be asked of, and answered by, the CCSM. In addition, the CCSM-4 will be the model used to contribute to the next IPCC report.
Chemistry-climate coupling: Past and future
Application of atmospheric chemistry to paleoclimate studies is still very much a developing field. Lamarque et al (Modeling the response to changes in tropospheric methane concentration: Application to the Permian-Triassic boundary, Paleoceanography, 2006) performed a set of simulations (with a version of WACCM (Whole Atmosphere Community Climate Model) relevant to the Permian-Triassic boundary (about 250 Ma). It is frequently assumed that during this time large amounts of methane (most likely from methane clathrates at the bottom of the ocean) were released. In the simulations, the amount of methane that reaches the atmosphere was varied over a range of possible values. Methane at concentrations larger than 1000 times pre-industrial levels result in a collapse of the total ozone column (Figure 1). This leads to a very large increase in the amount of UV radiation that reaches the ground. Lamarque et al. postulate that this could be a mechanism for the mass extinction that occurred at the Permian-Triassic boundary .
The impact of climate change on U.S. surface ozone levels has been investigated by ACD scientists. They simulated two 10 year periods using the global chemical transport model MOZART-2 (Model of Ozone and Related chemical Tracers version 2): 1990–2000 and 2090–2100. In each case, MOZART-2 is driven by meteorology from the National Center for Atmospheric Research (NCAR) coupled Climate Systems Model (CSM) 1.0 forced with the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1 scenario. During both periods the chemical emissions are fixed at 1990s levels, so that only changes in climate are allowed to impact ozone. The impact of climate change is calculated separately for background ozone and for the ozone generated through U.S. NO x emissions. The results show that the response of ozone to climate change in polluted regions is not the same as in remote regions. MOZART-2 predicts a 0–2 ppbv decrease in background ozone in the future simulation over the United States but an increase in ozone produced internally within the United States of up to 6 ppbv. The decrease in background ozone is attributed to a future decrease in the lifetime of ozone in regions of low NO x . Over the western United States the decrease in background ozone approximately cancels the increase in locally produced ozone. As a result, the main impact of future climate change on ozone is centered over the eastern United States, where future ozone increases up to 5 ppbv. The predictions show that in the future over the northeast United States, up to 12 additional days each year will exceed the maximum daily 8-hour averaged ozone limit of 80 ppbv. This is an approximate increase of 50%. Various climatic factors are identified which impact the net future increase in ozone over the United States including changes in temperature, water vapor, clouds, transport, and lightning NO x . Significant future changes are generally not found in planetary boundary layer height and precipitation. The CO emitted by the annual biomass burning in the tropical southern hemisphere (SH) is an excellent tracer of tropospheric transport due to its medium lifetime. CO is also one of the few tropospheric trace gases currently observed from satellite and this provides long-term global measurements. The 6 year CO data record from the MOPITT instrument was used to examine the inter-annual variability of the SH CO loading and show how this relates to climate conditions which determine the intensity of fire sources. The MOPITT observations show an annual austral springtime peak in the SH zonal CO loading each year with dry-season biomass burning emissions in S. America, southern Africa, the Maritime Continent, and northwestern Australia. Although fires in southern Africa and S. America typically produce the greatest amount of CO, the most significant inter-annual variation is due to varying fire activity and emissions from the Maritime Continent and northern Australia. This variation in turn correlates well with the El Nino Southern Oscillation precipitation index, Figure 2. Between 2000 and 2005, emissions were greatest in late 2002 and an inverse modeling of the MOPITT data using the MOZART chemical transport model estimates the southeast Asia regional fire source for the year August 2002 to September 2003 to be 52 Tg CO. Comparison of the MOPITT retrievals and NOAA surface network measurements indicate that the latter do not fully capture the inter-annual variability or the seasonal range of the CO zonal average concentration due to biases associated with atmospheric and geographic sampling. Climate change and regional air quality implications
Biomass burning has an important impact on climate, however, the interactions between climate change and biomass burning are not well understood. In a continuing investigation by ACD scientists, the MOZART and the CAM models have been used to investigate the radiative forcing of ozone and carbon aerosols from wildfires in Alaska and Canada. Some of these results have been incorporated into an exhaustive study of the impact of biomass burning on the boreal forest. This work shows that despite the release of large amounts of carbon dioxide, aerosols and other trace species into the atmosphere by fires, the ability of fire to change surface albedo dominates the net climactic affect of fires, leading to cooling. ACD scientists collaborated with Jack Chen, Jeremey Avise, and Brian Lamb (Wash. State U.), Cliff Mass (U. Washington) and Susan Fergusen (US Forest Service) to investigate the impact of future climate and landcover on regional air quality in the Pacific Northwest and NorthCentral U.S. The results indicate that U.S. regional air quality will change even if anthropogenic emissions remain the same. The changes are due to a combination of pollutant transport from other countries emissions, changes in wildfire emissions, and changes in biogenic emissions. A range of future biogenic emission scenarios are being investigated including various land cover change scenarios (i.e. tree plantations, agriculture, and urbanization). Dramatic impacts on regional air quality are predicted for scenarios associated with increases in tree plantations for carbon sequestration. During the MILAGRO campaign, ACD scientists, working with scientists from the University of Minnesota, the Georgia Institute of Technology, and the University of Colorado, studied the properties of particles that arise from a phenomenon known as “nucleation.” Nucleation occurs when certain key gases, such as sulfuric acid, reach a sufficiently high concentration that they cluster together to form a very small particle. These particles can be detected when their size approaches 3 nm, corresponding to a cluster of 160 sulfuric acid molecules. These events are sudden and dramatic, such as the event pictured in Figure 1d on March 16th, 2006, at a site located about 40 km northeast of Mexico City. As the figure shows, the 6-hour long event started at 3:00 PM GMT, or 9:00 AM local time, and seems to have occurred when the wind shifted from the North (shown in Figure 1b) and the sun rose (as shown in the ultraviolet radiation, or UVB, measurements in Figure 1c). Figure 1a shows measurements collected to study the impacts of the newly formed particles on climate using a “Cloud Condensation Nucleus,” or CCN, counter. The CCN counter counts the number of particles of a given size, such as for 100 nm diameter particles plotted in Figure 1a, that are capable of forming cloud droplets when exposed to a given supersaturation of water in the air. Supersaturation is a measure of the amount of excess water vapor above 100% Relative Humidity (RH) that is present in the air. The large red region in Figure 1a lines up with the particles formed from the nucleation event, meaning that 100 nm diameter particles created from nucleation can readily form cloud droplets. These results from these measurements will be used as input to computer models that will study the impact of nucleation events on clouds and climate. FY 2007 work will continue global and regional modeling studies on the impacts of climate changes on regional air quality as well as ongoing studies of aerosol nucleation processes and impacts. This work was funded by NSF/NCAR, NSF/Biocomplexity, and DOE.
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