Raindrop size distribution of easterly and westerly monsoon precipitation observed over Palau islands in the Western Pacific Ocean

This paper explores the characteristics of raindrop spectra in terms of raindrop size distribution (RSD) using 4 years of Joss-Waldvogel disdrometer data over Palau islands (7^o 20' N, 134^o 28' E) in Western Tropical Pacific ocean. The RSD characteristics are studied in two seasons (easterly monsoon-EM and westerly monsoon-WM) using three (stratiform, deep convection, and shallow convection) rain types identified from collocated 1290-MHz wind profiler radar (WPR). In addition to the ground-based sensors observations, TRMM and MODIS satellite-derived rain parameters and atmospheric parameters are utilized to study RSD characteristics. RSD characteristics stratified on the basis of rainrate show that the mean values of raindrop concentrations of small (medium and larger) drops are same (more) in WM compared to EM season. Normalized gamma distribution of RSD shows that the mean value of mass-weighted mean diameter, D_m (normalized intercept parameter, log₁₀N_w), is higher (lower) in WM than the EM season. In addition, the mean value of D_m (log₁₀N_w) is higher (lower) in deep convective precipitation as compared to the other two types of precipitation (stratiform and shallow convection) in both monsoon periods. In conjunction with the remote sensing data (MODIS & TRMM), RSD shows that the presences of cold clouds which extend to deeper altitudes are responsible for the higher D_m during WM season. The immediate significance of the present work is that (1) it contributes to our understanding of seasonal variations of RSD and distribution of different rain types, and (2) it provides information which is useful for quantitative estimation of rainfall from weather radar observations.