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In the area of theory and modeling, branch research efforts are focused on two complementary streams: cloud-radiative processes and climate dynamics. Plane-parallel radiation codes are developed for use in calculating radiative effects of clouds, water vapor, and the liquid water drop-size distribution; for the estimation of surface energy balance; and for radiative-convective boundary layer equilibrium calculations. The radiation codes include effects of absorption, emission, and scattering of radiation due to water vapor, clouds, aerosols, CO2, O3, O2, and minor trace gases. Using multi-level energy-balance models derived from the radiation models, branch scientists have carried out a series of sensitivity experiments to study the effect of CO2, clouds, volcanic eruptions, and SO2 emission on global climate change. Work is underway to develop a versatile radiation modular code that separates spectral information from the radiative transfer computations so that it can be used readily in general circulation models to test out different climate feedback hypotheses. The full 3D radiative transfer problem through inhomogeneous clouds is being studied by both Monte Carlo and analytic methods, using a variety of cloud structure models with parameters determined by analysis of data from the DOE ARM and NASA FIRE field programs. Results of the 3D studies are parameterized in terms of "effective" cloud parameters for use in plane-parallel GCM codes, and also provide improved algorithms for the retreival of cloud parameters from remote sensing data.
For climate dynamics studies, a hierarchy of models ranging from one-dimensional radiative-convective models to coupled atmosphere-ocean-land models with full physics is used. A one-dimensional model (or single-column model, SCM) derived from the Goddard Laboratory for Atmospheres (GLA) Global Circulation Model (GCM) has been used to study the effect of radiation/dynamic feedback using the modified Arakawa-Schubert cumulus parameterization in leading to steady state or oscillatory behavior of tropical convection. In support of TOGA, intermediate two- and three-dimensional models are used to study the fundamental linkage between diabatic heating and dynamics of the tropical atmosphere, leading to theoretical interpretations of the observed multi-scale and multi-frequency nature of hydrological processes in the tropics from intraseasonal variations (such as the Madden-Julian oscillation, super cloud clusters, twin cyclones, and westerly wind bursts) to interannual time scales (as for example in El Niño). In cooperation with Goddard's Global Modeling and Assimilation Office (GMAO) branch scientists are also involved in developing improved physical parameterizations (including cloud liquid water, cumulus heating, surface fluxes, boundary layer processes, and land-atmosphere interactions) in the GCMs. Using these GCMs (including the GEOS GCM and the fvGCM), climate simulation experiments are carried out to study global and regional climate, focusing on the mean and variability of the global water and energy cycles. Model results are compared with climatological observations as well as satellite and in situ observations obtained during special field experiments such as FIRE and TOGA/COARE. In support of TRMM, numerical experiments have also been carried out using the Goddard cumulus ensemble model to study the water budget and physics of tropical cumulus clusters and their changes as a function of surface temperature. Branch members participate in international model intercomparison projects to assess individual model performance against common references, to seek improvements in model physics, and to evaluate collective progress in climate modeling research. Contact: Santiago Gasso
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