Diurnal variation of rain drop size distribution over the Western Ghats in India

Abstract

Joss-Waldvogel Disdrometer (JWD) measurements at the High-Altitude Cloud Physics Laboratory (HACPL: 17.56°N, 73.4°E, above 1373 m MSL), Mahabaleshwar were investigated for determining the diurnality of the drop size distribution (DSD) associated with the precipitation characteristics over the Western Ghats. Here, we examined the JWD data from 2015 to 2019 collected during the Indian Summer Monsoon (ISM) season. The number concentration of rain droplets of various diameters is considerably varying with the rain rate (R) and type of precipitating cloud. With increasing the value of R, rain droplets having larger diameter concentration significantly increase, and distribution tail moves to the biggest droplets. The average of reflectivity (Z), R, liquid water content (LWC), mass-weighted mean diameter (Dm), and normalized intercept parameter (log10Nw) have higher values from the heavy rainfall (Rhigh ≥10 mm/hr) in comparison to the low rainfall (Rlow < 10 mm/hr) for seasonal rainfall. The gamma distribution of DSD shows significant difference during the low and heavy precipitation on different time period (e.g. 00-06, 06-12, 12-18, 18-24 LST). The number of rain events contributing to the total accumulated rain varies with time. The maximum number of rain events occurred during 12-18 LST, with 23.6 % rain events of low rainfall and 4.9% of heavy rainfall. The bimodality is observed in the diurnal variation of Dm, R, and Z, with the largest peak recorded in the late afternoon (13-16 LST) and the second crest in the early morning hours (05 LST). At the same time, the log10Nw value drops down, indicating the lowest concentration of rain droplets. Several factors, like the land-sea breeze, complex mountainous terrain, and amount of surface reaching solar radiation may control the formation of precipitating clouds at the microphysical scale on the windward side of the Western Ghats, and may also responsible for the observed diurnality in DSD.