Seasonal variation of raindrop size distribution over a coastal station Thumba: A quantitative analysis

Abstract

Seasonal variation of raindrop size distribution (DSD) is examined using long-term (2007–2015) observations made with Joss-Waldvogel disdrometer at Thumba (8.53°N, 76.87°E), a tropical coastal station. Data sets were classified based on the rain rate and studied the DSD variations for pre-monsoon (PRE) season, post-monsoon (POST) season and summer monsoon (MON) season. The result shows that the larger drop concentration is higher in PRE season compared to other two seasons for the same rain rate. The values of median volume diameter (Dm) are found to be lowest (highest) in MON (PRE) seasons. The gamma parameters (Λ, μ) and normalized DSD scaling parameter for concentration (Nw) are also observed to have a clear-cut seasonal variation. There is a clear trend in the slope (Λ) and Nw parameters with rain rates. Although shape parameter (μ) did not show a linear trend as such, there exist a pattern to its evolution with rain rates that is similar across seasons. The coefficients of A and b in Z-R relations also showed seasonal variations. The plausible causative mechanisms for the observed surface seasonal DSD variations are discussed using satellite and radiosonde observations. The differences in cloud structure and cloud microphysics are found to have a direct influence on the rain DSD spectra at the surface. The present results envisages that the change in monsoon circulation and the origin of cloud system (i.e. maritime or continental) play a vital role in characterizing the rain DSD, while the microphysical and dynamical processes occurring during the descent of raindrops will modify the same.