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Sensitivity of a Large Ensemble of Tropical Convective Systems to Changes in...

Eitzen, Z. A., and K. Xu (2008), Sensitivity of a Large Ensemble of Tropical Convective Systems to Changes in the Thermodynamic and Dynamic Forcings, J. Atmos. Sci., 65, 1773-1794, doi:10.1175/2007JAS2446.1.

A two-dimensional cloud-resolving model (CRM) is used to perform five sets of simulations of 68 deep convective cloud objects identified with Clouds and the Earth’s Radiant Energy System (CERES) data to examine their sensitivity to changes in thermodynamic and dynamic forcings. The control set of simulations uses observed sea surface temperatures (SSTs) and is forced by advective cooling and moistening tendencies derived from a large-scale model analysis matched to the time and location of each cloud object. Cloud properties, such as albedo, effective cloud height, cloud ice and snow path, and cloud radiative forcing (CRF), are analyzed in terms of their frequency distributions rather than their mean values.

Two sets of simulations, Fϩ50% and FϪ50%, use advective tendencies that are 50% greater and 50% smaller than the control tendencies, respectively. The increased cooling and moistening tendencies cause more widespread convection in the Fϩ50% set of simulations, resulting in clouds that are optically thicker and higher than those produced by the control and FϪ50% sets of simulations. The magnitudes of both longwave and shortwave CRF are skewed toward higher values with the increase in advective forcing. These significant changes in overall cloud properties are associated with a substantial increase in deep convective cloud fraction (from 0.13 for the FϪ50% simulations to 0.34 for the Fϩ50% simulations) and changes in the properties of non–deep convective clouds, rather than with changes in the properties of deep convective clouds.

Two other sets of simulations, SSTϩ2K and SSTϪ2K, use SSTs that are 2 K higher and 2 K lower than those observed, respectively. The updrafts in the SSTϩ2K simulations tend to be slightly stronger than those of the control and SSTϪ2K simulations, which may cause the SSTϩ2K cloud tops to be higher. The changes in cloud properties, though smaller than those due to changes in the dynamic forcings, occur in both deep convective and non–deep convective cloud categories. The overall changes in some cloud properties are moderately significant when the SST is changed by 4 K. The changes in the domain-averaged shortwave and longwave CRFs are larger in the dynamic forcing sensitivity sets than in the SST sensitivity sets. The cloud feedback effects estimated from the SSTϪ2K and SSTϩ2K sets are comparable to prior studies.

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Research Program: 
Interdisciplinary Science Program (IDS)
Modeling Analysis and Prediction Program (MAP)