Microphysical Evolution of Precipitation During Convective Storm Life-Cycles and Implications for Radar QPE: Combined Radar–Disdrometer Observations from Kolkata, Eastern India

SHUBHENDU KARMAKAR , Suman Saha, Malay Pal

Through dedicated observations with a Joss–Waldvogel disdrometer at Dumdum (22.65°N, 88.43°E) and the S-band Doppler Weather Radar (DWR) (22.57°N, 88.35°E) over Kolkata, this study provides in-depth analysis of drop size distribution (DSD) evolution over each stage from initiation to decay of contrasting convective storms over Kolkata, eastern India due to its complicated terrain in monsoon period which may bring changes in raindrop processes significantly compared to classical approaches for layered clouds. This dataset consists of 1210 phase-classified disdrometer DSD records at 1-min at nine significant storm events occurring over a total of four seasons (post-monsoon, monsoon, and pre-monsoon between Oct. 2024 to Apr. 2026. Storm life-cycle phases—developing, mature, and dissipating—are defined on the basis of rain rate evolution and validated against PPI radar images.
Phase-specific Z–R relationships are derived from the combined dataset: Z = 887R1.62 (developing, R² = 0.891), Z = 840R1.66 (mature, R² = 0.857), and Z = 1021R1.60 (dissipating, R² = 0.868). The overall relationship is Z = 959R1.62 (n = 1210, R² = 0.882), with the 4-year climatological value being Z = 870R1.59 (n = 138,420). Radar–disdrometer comparison reveals a systematic calibration bias of −10.0 dBZ (r = 0.64, n = 36). Quasi-vertical profiles show seasonal variation in bright-band height from 3.2 km (winter) to 5.8 km (post-monsoon), directly explaining the observed surface DSD differences. Independent validation against 15-minute gauge observations during a pre-monsoon event on 16 April 2026 at Kolkata (22.53°N, 88.33°E) demonstrates that the mature-phase Z–R produces the best QPE (RMSE = 3.04 mm, +21% bias), while the Marshall–Palmer default overestimates by +239%. These results provide the first combined radar–disdrometer documentation of storm life-cycle microphysics from the Gangetic plains.