Diurnal Relation Between CAPE, CINE and Rainfall

One of the most important modes of the variability of Indian summer monsoon rainfall is the diurnal cycle. It acts as an appropriate test bed for various aspects of parameterizations in climate models. The physical understanding of the diurnal cycle of precipitation is currently limited by the inability to observe and model the cycle itself. The major cause of ambiguities associated with the missing physical processes and feedbacks is due to the inclusion of cumulus parameterization which establish a systematic bias in the representation of convection that leads to unrealistic simulation of diurnal cycle of convection. The convective parameterization schemes used in various numerical models use convective available potential energy (CAPE) or its rate of change as a parameter to diagnose precipitation. But how CAPE is related to rainfall is still not clear. Here, we quantified the phase relationship between CAPE and rainfall diurnal cycle over the Indian region using Integrated Multi-Satellite Retrievals for GPM (IMERG) and ERA5 reanalysis data. On a diurnal timescale, rainfall and CAPE values are highest in Bay of Bengal (BB) followed by Foothills (FT) and Central India (CI). The convective inhibition energy (CINE) values are least for BB. Bimodal peak of CAPE diurnal cycle is found in CI and FT but not for BB. The first peak in the CAPE diurnal cycle in FT and CI is because of temperature diurnal cycle and the second peak is because of humidity diurnal cycle. For BB, the diurnal cycle of humidity is more dominant for CAPE diurnal cycle. In land (ocean) regions, CAPE leads (lags) rainfall on a diurnal timescale. When CAPE leads (lags) rainfall, there is favorable (unfavorable) thermodynamic condition for rainfall to occur in all regions except south Bay of Bengal. The impact of such phase relationship between CAPE and rainfall is enormous and can add a new paradigm in simulation of diurnal cycle of rainfall.