Role of Thermodynamics and Dynamics in the Diurnal Cycle, Propagation, and Progression of Convective Storms in the Eastern Flank of the Indian Monsoon Trough

Abstract

Abstract This study investigates the diurnal cycle, propagation, and progression of convective storms (CSs) on the eastern edge of India’s monsoon trough (MT) using 9 years of S-band radar measurements with satellite and reanalysis datasets. CSs initiate over ocean during midnight–early morning hours and propagate onshore in succeeding hours. CSs exhibit two semidiurnal peaks, one during afternoon hours over inland areas and another during midnight–early morning hours in oceanic/coastal locations. The deep and intense afternoon peak over inland regions is attributed to land surface heating and associated destabilization. The weak and shallower but organized midnight–morning peak and propagation of CSs toward the coast are attributed to the nocturnal land breeze and its interaction with prevailing onshore flow. The observed lead–lag of a few hours in the diurnal cycle of different cumulus modes correspond to the transition of congestus into deep and then, often, into overshooting modes. Moisture budget analysis showed atmospheric regulation of this transition through thermodynamic (congestus moistening) and dynamic processes (vertical advection). Theoretical time scales were invoked to estimate the relative role of vertical advective versus congestus moistening for promoting the afternoon transition from congestus to deeper modes. Comparing the time scales for congestus moistening (18–46 h) and dynamics (3 h) with the actual transition time scales (2–4 h) reveal that congestus moistening is too slow to explain the observed lead–lag in CS modes. Though both thermodynamic and dynamic processes moisten the midlevel prior to deep/overshooting convection, vertical advection is the dominant dynamic process for the observed congestus–deep–overshooting transition. Significance Statement Tropical rainfall is usually linked with convection in the morning and afternoon hours. We look at the basic physical processes that lead to those convective activities peaks. The afternoon peak is linked to maximum heating, resulting in an unstable environment, whereas the morning peak is linked to the interaction of large-scale monsoon flow with a land breeze. Furthermore, daily solar heating visually shows a shallow-to-deep progression of convection. The moist midlevel environment was shown to precede such convective development in a day. The large-scale monsoon flow is a dominant cause of this moistening. The monsoon dynamic flow takes roughly 2–3 h to sufficiently moist shallow storms into deep storms, whereas the local thermodynamic moistening process takes about 18–46 h.