Drop size distributions of tropical rainover south India

Abstract

Drop size distributions (DSD) associated with tropical rainfall at Cuddalore in the south-eastern part of India have been measured by a Joss-Waldvogel disdrometer (RD–80 model) during September to November 2002. The rainfall data corrected for instrumental error, matches very well with rainfall rates measured by a self recording raingauge, at the same site. For further analysis of the DSD, the rainfall events were separated into convective and stratiform rainfall by an algorithm based on variation of DSD parameters. One rain event in the form of a squall line of 15 September 2002, was analysed in greater detail to investigate the validity of the classification scheme as well as to study the variation of the DSD parameters during the course of a rain event. It was observed that, the algorithm was robust and had quite good correspondence with other independent rainfall separation algorithms. During the rain event, at low rainrates, the convective phase of the rainfall event was marked by DSD spectra that have greater population of small droplets as compared to stratiform DSDs at the same rainrates. At higher rainrates, the convective regime is characterised by narrow spectra centred at higher diameters. At the transition region between convective and stratiform spectra, mixed large and small drop spectra are observed. Similar variation was also observed in the averaged drop spectra. In addition, the averaged spectra also reveal an equilibrium distribution of the drop population in DSDs at higher rainrates (>39 /hr) for diameter range (>1.91 mm) corresponding to nearly constant values of the slope of the distribution, the intercept and the mean mass diameter. The value of the shape parameter, which for small rainrates varies the same as the slope parameter, starts to increase with increasing rainrate as the other two parameters of the gamma distribution approach a constant value corresponding to equilibrium shape. The value of the intercept parameter is highest for low to moderate convective rainfall and decreases as the rainrate increases.