Role of Dynamical and Thermodynamical processes in shaping Diurnal Cycle of Rainfall over the Western Ghat of India

Diurnal Variability is one of the most fundamental modes of the global climate system arising from solar radiation variations. The precipitation over the Indian subcontinent region has significant diurnal variation as per the topographical settings of the landmass. Among the various regions with the maximum diurnal amplitude (MDA) of precipitation over India, MDA over Western Ghat (WG) is perfectly aligned along the coastal boundary.

The peculiarity of the WG region is the position of the coastally aligned mountain range stretching from Gujarat to Kerala with an average elevation of 1200m and during monsoon season with the presence of speedy low-level jet (LLJ), this range acts as a barrier to anchor precipitation over them. The MDA over this region tends to be positioned slightly inland, on the windward side of the hills and is closely associated with the geographic locations of mountain peaks over WG. Here we have attempted to understand the phase and amplitude of diurnal precipitation variation on two distinct physical regimes predominantly governed either by dynamics (DR) or thermodynamics (TR). Based on the speed of the (LLJ) we have identified 370 and 458 days from the monsoon season of 21 years, with dominant physical processes being dynamical and thermodynamical respectively. We found a substantial enhancement of MDA over the entire span of the WG region encompassing the region over the sea, coast, and land during TR which is in stark contrast to DR where MDA is concentrated mostly over the coastal side northern and central western Ghat region. This difference is also visible in the diurnal phase with gradual (abrupt) changes in TR (DR). During TR the weakened LLJ leads to the local thermodynamics to dominate, and very strong land and sea breezes are initiated, along with an unstable hot and humid boundary layer making favorable conditions for diurnal precipitation to take place. This is entirely different in DR wherein the stronger LLJ does not allow to establish a stronger temperature gradient between land and ocean leading to a lessening of diurnal rain. The storm-top height indicates the presence of low-level congestus (deep congestus) clouds during DR (TR). Thus, the diurnal rain, along with cloud types and involved microphysics is totally different in these two physical regimes. This study will be very useful for identifying the errors in the diurnal rain simulated by models segregated by dynamical or thermodynamical processes separately.