Contrasts in the raindrop size distributions of pre-monsoon polluted and non-polluted rainy days over Rourkela, India

Raindrop size distribution (RSD) is crucial for accurate quantitative precipitation estimation and forecasting and is fundamental to precipitation microphysics. This study examines the microphysical characteristics of RSD at Rourkela, Odisha, under polluted and non-polluted conditions. This investigation utilized pre-monsoon in-situ (disdrometer) and reanalysis (ERA5) data for 2018–2021, to identify the pollution impact on precipitation over Rourkela. The polluted rainfall day has been determined based on the air quality index (AQI) value provided by the Central Pollution Control Board (CPCB), Government of India, over the Rourkela region. Classifying the rainfall in polluted and non-polluted days into stratiform and convective types showed that convective rainfall had a higher raindrop concentration and higher mean diameter (D_m) in both polluted and non-polluted days. Convective rainfall has a lower normalized intercept parameter (log₁₀N_w) in polluted rainfall days. The RSD empirical relations (Z-R, μ – λ, D_m-R, N_w-R) also showed a noteworthy difference between the polluted and non-polluted rainfall days. The results disclosed that non-polluted rainfall has higher concentration of small-diameter raindrops, whereas polluted day rain has higher concentrations of midsize and large-diameter raindrops.