Diurnal Relation between CAPE and Precipitation over Indian Region

Accurate representation of the diurnal cycle of convection remains a persistent challenge in numerical weather and climate models. Previous studies have highlighted the importance of the phase relationship between convective available potential energy (CAPE) and precipitation in improving the simulation of rainfall timing and intensity. This study investigates the spatial and temporal characteristics of this phase relationship during the Indian summer monsoon, emphasizing the occurrence, intensity, and underlying causes of lead (days when CAPE maxima leads precipitation maxima) and lag (days when CAPE maxima lags precipitation maxima) days using 22 years of half-hourly IMERG precipitation data alongside temperature and humidity profiles from ERA5. Spatial maps reveal that precipitation exhibits greater variability in its diurnal phase than CAPE, with CAPE maxima generally preceding rainfall peaks except over the Bay of Bengal (BB), the Himalayan foothills, and parts of the Arabian Sea and Pakistan. Over Central India (CI), CAPE leads precipitation by about 3.5 hours, whereas over BB, it lags by approximately 9.5 hours. CAPE over CI shows a bimodal structure driven by both temperature and humidity variations, while over BB it displays a single, humidity-controlled peak. Across the monsoon season, about 72% of days are lead days and 25% are lag days. Despite their lower frequency, lag days often produce comparable or greater rainfall intensity, contributing 10–30% of the total seasonal precipitation, compared to 60–80% from lead days. The diurnal phase shift between CAPE and precipitation is primarily governed by changes in precipitation timing rather than CAPE evolution. Enhanced early-morning convection on lag days is linked to strong negative surface pressure anomalies and associated mid-tropospheric moistening, highlighting a distinct thermodynamic control on rainfall phase variability over the monsoon region.