The influence of atmospheric moisture budget on the tropical deep convection and cloud radiative forcing

This study uses measurements of multiple Tropical Rainfall Measuring Mission (TRMM) sensors, namely Clouds and the Earth's Radiant Energy System (CERES), TRMM Microwave Imager (TMI) and Visible and InfraRed Scanner (VIRS), from January through August 1998 to estimate area coverage, effective size, rainfall efficiency, ice water path (IWP), and radiative fluxes of tropical deep convective systems (DCS). This study finds that both DCS precipitation and rainfall efficiency increase with SST. The increasing rate of the efficiency with SST is ~2%/K, indicating that DCSs more effectively remove the moisture transported into the upper troposphere when the temperature gets warmer. Despite increasing rainfall efficiency, the cloud area coverage rises with SST at a rate of ~7%/K in the warm tropical seas. There, the boundary layer moisture supply for deep convection and the moisture transported to the upper troposphere for cirrus-anvil cloud formation increase by ~6.3%/K and ~4.0%/K, respectively. There is little evidence that greater rainfall rates caused by increasing SST would reduce the cluster size and detrainment of tropical DCS as suggested by some previously hypothesized dehydration scenarios for warmer climates. The changes in cloud formation efficiency, along with the increased transport of moisture available for cloud formation, likely contribute to the large rate of increasing DCS area coverage. Besides the area coverage, cloud IWP also increases with SST. These changes of DCS properties could produce a net about -2 W/m2 radiative feedback for the earth's climate system due to strong reflection of DCS on solar radiation according to CERES measurements. This study partially supports the thermostat hypothesis but indicates a smaller magnitude of the negative feedback. These results have great potential in testing current cloud resolving models and convection parameterizations of general circulation models.