Raindrop size distribution (RSD) characteristic variations between two southern Indian stations [Gadanki (13.5° N, 79.2° E) Kadapa (14.47° N, 78.82° E)] using ground based parsivel disdrometer data are studied. Number concentration of mid and large drops is more over Gadanki when compared to Kadapa precipitation. The mean value of mass weighted mean diameter (D<sub>m</sub>) is higher in Gadanki than Kadapa precipitation. Both monthly and diurnal variations of D<sub>m</sub> show higher values of D<sub>m</sub> over Gadanki than Kadapa. After classifying the precipitations systems into stratiform and convective, Gadanki has higher (lower) D<sub>m</sub> than Kadapa in stratiform (convective).
We have been working on rainfall observation project at tropical site in India, in order to study tropical storm structure and raindrop size distribution (DSD) characteristics for improving the current PR rainfall retrieval algorithm, and for making the ground validation of TRMM PR observation. At Gadanki (53-MHz VHF MST radar, an L-band lower atmospheric wind profiler (LAWP), a disdrometer and an optical rain gauge (ORG) are set up to obtain more knowledge on vertical properties of DSD and rain structure during monsoon season. Measurements of drop size distribution (DSD) with disdrometer have been providing the information to study basic DSD characteristics in tropical India. We have found a clear seasonal dependence in Reflectivity (Z) - Rainfall (R) relations (i.e. DSD characteristics) in India. Our results indicate that there are about 3-times differences peak-to-peak in estimates of rain rate using a single Z-R relation. It seems that this type of seasonal dependence should be taken into account to improve the accuracy of the PR algorithm. Drop size distribution characteristics were retrieved in moderate/heavy precipitation using VHF wind profiler. The retrieved drop size parameters were compared to corresponding disdrometer data and found that there is reasonably good agreement between the measurements, lending credence to the profiler retrievals of DSD parameters. Preliminary study on the ground validation of TRMM PR shows fairly good agreement between the disdrometer and TRMM precipitation radar measurements.