Interpreting the diurnal cycle of clouds and precipitation in the ARM GoAmazon observations: Shallow to deep convection transition

Abstract:

This research aims at understanding the physical processes associated with the diurnal cycle of clouds and precipitation using observations and modeling of the Green Ocean Amazon (GoAmazon) field campaign data. Through a careful regime classification, we investigate the major differences in clouds and precipitation characteristics among different convective regimes, and most important environmental factors that govern the shallow-to-deep convection transition process. On shallow cumulus days, low clouds are at a significantly higher altitude than on days with deeper convection. As greater sensible heat flux on shallow days drives greater boundary layer growth, which entrains drier air above the PBL that lowers the relative humidity in the PBL and leads to a higher cloud base. On days with deeper convection, the peak timing of surface precipitation is more in-line with the timing of surface fluxes when there are antecedent triggers, and it takes longer for deep convection to develop with little/no pre-existing disturbances in the free troposphere. An idealized plume model is employed to explore the relative importance of different convection-controlling factors. Initial cloud base vertical velocity and buoyancy are important in helping parcels ascend to the level of free convection. After parcels become positively buoyant, the entrainment of environmental air and lower free troposphere humidity play a more important role in determining the ultimate fates of those cloudy parcels. Different cloud types exhibit different entrainment rates, which can be tied to various cloud size distribution at the cloud base. It is also interesting to note that congestus and deep regime exhibit distinct cloud tops, but their boundary and environmental conditions are similar, except for the noticeable difference in the vertical wind shear profile between the two regimes, indicating that wind shear might limit the vertical extent of convective development.