Thermodynamic Controls on Convective Instability and Lightning Activity Over India: A CAPE – CIN Composite Framework

ABSTRACT Understanding the thermodynamic controls governing convective instability and their linkage with lightning activity is essential for interpreting regional climate variability over the Indian subcontinent. This study develops an integrated framework using ERA5 reanalysis (1940–2025), radiosonde observations, and World Wide Lightning Location Network (WWLLN) lightning data. Lightning analyses are restricted to 2010–2019 to ensure temporal consistency with the WWLLN observational record, whereas long‐term climatological and trend analyses are based on the full ERA5 period. Convective environments are characterised using convective available potential energy (CAPE), convective inhibition (CIN), and two composite diagnostics: the Normalised Convective Instability Index (NCII), representing the combined influence of buoyancy and inhibition, and the Diurnal Instability Change Index (Δ I ), representing diurnal instability evolution. Seasonal climatology reveals a transition from a pre‐monsoon ‘stored instability’ regime, characterised by high CAPE (~552 J kg −1 ) and strong CIN (~405 J kg −1 ), to a monsoon ‘released instability’ regime, where reduced inhibition (~177 J kg −1 ) promotes convective development. Validation against radiosonde observations from 29 stations demonstrates good agreement for CAPE ( r ≈ 0.81, p < 0.001) and CIN ( r ≈ 0.79, p < 0.001), supporting the application of ERA5 for regional climatological analyses. NCII exhibits strong but spatially variable correlations with lightning density ( r ≈ 0.27–0.78, p < 0.05), with the strongest relationships over eastern and north‐eastern India, whereas Δ I shows weak and seasonally inconsistent correlations, indicating that it is better interpreted as a secondary indicator of diurnal modulation rather than a direct predictor of lightning activity. Long‐term analyses reveal modest but statistically significant ( p < 0.05) increases in CAPE of up to ~2.5 J kg −1 year −1 during the pre‐monsoon over northern India, while CIN trends remain generally weak. Overall, the proposed NCII–Δ I framework provides a physically consistent basis for diagnosing convective instability–lightning coupling, with NCII emerging as a robust indicator of lightning potential and Δ I offering complementary insight into the diurnal evolution of atmospheric instability under a changing climate.