The rapid onset of the South Asian summer monsoon (SASM) is featured as an abrupt increase of rainfall and atmospheric column-integrated moist static energy (MSE). Using convection-permitting simulations, a novel cloud tracking technique, and an MSE budget, the relative roles of mesoscale convective systems (MCSs) and non-MCS deep convection in the SASM onset energetics are studied. The analysis shows MCSs greatly contribute to the sudden increases of rainfall, diabatic MSE sources, and the subsequent enhanced export of column-integrated MSE during the rapid onset phase. A “diabatic sources-energy export” plane analysis delineates the different roles of MCSs and non-MCS deep convection. Although deep convection always leads to a net energy export, MCSs dominate the convection-associated positive feedbacks that accelerate the energy cycle of the SASM onset. Mesoscale organization of deep convection enhances the diabatic forgings during the SASM onset. Sensitivity experiment shows that the results are insensitive to the model resolutions.
The results presented in this study indicate a successive energy cycle associated with MCSs in the SASM onset: at the early stage of the onset, the enhancement of shallow convection/congestus destabilizes the SASM by importing energy; the rapid increase of MCSs right after the peak of the energy import by shallow convection/congestus produces enhanced diabatic forcings and advective energy export; this is followed 1-2 days later by the weakening of the energy import associated with shallow convection/congestus. Given the significant roles of MCSs in modulating the positive convective feedbacks and the monsoon onset energetics, whether or not a model can capture the energy cycle related to the synergistic roles of shallow convection/congestus and MCSs might be crucial to accurately predicting the timing and intensity of individual SASM onset, and how SASM onset responds to the changing large-scale forcings.