Impact of Cumulus Microphysics and Entrainment Specification on Tropical Cloud and Radiation in GFDL AM2

Clouds and precipitation simulated by climate models still have a large room for improvement. Geophysical Fluid Dynamics Laboratory (GFDL) High-Resolution Atmospheric Model (HiRAM) has much larger ice water path (IWP, ~ 5 times) and stratiform precipitation fraction (~ 10 times) than its Atmospheric Model version 2 (AM2) over the Tropics. It is found that such differences are mainly due to the replacement of the relaxed Arakawa Schubert (RAS) scheme in AM2 by the modified University of Washington (UW) shallow convection used in HiRAM. The focus of the study is to investigate the sensitivity of simulated cloud, precipitation, and radiation to the two key parameters (precipitation efficiency and entrainment specification) in RAS, and interpret the difference between AM2 and HiRAM. With more deep plumes inhibited, the convective heating and moistening decrease, and the upper troposphere becomes colder and drier. With reduced precipitation efficiency, more convective condensate is detrained and stratiform precipitation increases. Both precipitation efficiency and entrainment specification change the vertical heating profiles and precipitation partitioning, but via different mechanisms. Using offline radiation calculations, convection scheme-induced tropical radiation variation is investigated. Increased longwave trapping by increased upper level ice clouds is partially compensated by a dry and cold bias in the upper troposphere. However, top of atmosphere absorbed shortwave reduction is proportional to increased IWP, but the reduction is not as large as that computed using offline radiation calculation assuming similar increase of IWP. The reason is that the increased IWP associated with large-scale precipitation does not peak around noon with the maximum solar radiation as that associated with convective precipitation. The study highlights the importance of convective parameterization in regulating tropical clouds and radiation.