Thermodynamic Controls on the Scaling Between Precipitation and Cloud Radiative Impacts

Controls on the coupling between cloud and precipitation processes represent an important link in understanding and defining climate sensitivity. Here we use A-Train observations to examine cloud impact parameters that describe the relationship between precipitation and cloud radiative effects and test their sensitivity to dynamic and thermodynamic regimes. The surface radiative cooling impact, R_c, represents the ratio of the surface shortwave cloud radiative effect to latent heating from precipitation. The atmospheric radiative heating impact, R_h, represents the ratio of the atmospheric cloud radiative effect to latent heating from precipitation. Individual cloud objects are identified and their mean cloud impact parameters are estimated from the CloudSat/CALIPSO joint radar-lidar cloud and radiation products and CloudSat and AMSR-E precipitation. Sampling by environmental regime shows largest sensitivity to thermodynamic regime, with column water vapor exhibiting the greatest control on the coupling between precipitation and cloud radiative effects. As clouds become deeper and larger with increasing water vapor, the radiative effects increase faster than the latent heating from precipitation. We speculate that this is caused by two effects. First, the latent heating increases with increasing water vapor are likely driven by stratiform precipitation, which does not increase as fast as it would if driven by convective precipitation. Second, because cloud object size dramatically increases with water vapor, the relative importance of the increasing anvil cloud area drives a systematic increase in the cloud greenhouse effect, which outpaces the latent heating from precipitation. The role of the characteristics of the non-precipitating cloud area in modulating the relationship between precipitation and cloud radiative effects will also be investigated.