Moisture build-up and thermodynamic processes in precipitation regimes during the southwest monsoon over a tropical coastal region

Abstract

The thermodynamical and microphysical characteristics of precipitation systems in shallow, Bright Band (BB), and Non-Bright Band (NBB) categories over a coastal site in Thiruvananthapuram, India were investigated. Radar reflectivity and fall velocity profiles, and surface rain rates were utilized to categorize the precipitation systems in June-July 2022. Shallow and NBB events exhibit a significant increase in humidity within the 4–8 km layer around 10–20 min prior to the onset of precipitation. In contrast, BB events maintain a deeper humid layer extending from the surface up to 8 km, with high liquid water content observed at least 30 min before precipitation, supporting widespread precipitation over the coastal region. In both NBB and shallow events, a sharp increase in liquid water path, integrated water vapor, and convective available potential energy (CAPE) occurred ∼10 min before rainfall onset. The substantially higher magnitudes of moisture and instability parameters in NBB systems indicate convective development, while the relatively lower values in shallow systems reflect weak instability and limited vertical growth, resulting in short-lived, low-intensity precipitation. BB systems maintained elevated and steady moisture with minimal variation in CAPE, supporting prolonged stratiform rain. The study shows that shallow systems are primarily influenced by low-level moisture, whereas BB and NBB systems rely on deeper mid- and upper-tropospheric moisture to sustain precipitation. CAPE influences rain rates and raindrop size distributions, with NBB events pronounced response, marked by intense rainfall and broader drop spectra.