Estimation of raindrop size distribution over large spatial and temporal scales has been a long standing goal of polarimetric radar. Algorithms to estimate the parameters of a gamma raindrop size distribution model from polarimetric radar observations of reflectivity, differential reflectivity and specific differential phase are developed. Differential reflectivity is the most closely related measurement to a parameter of the drop size distribution, namely, the drop median diameter (D0). The estimator for D0 as well as other parameters are evaluated in the presence of radar measurement errors. This parameterization alogorithm is developed at S , C and X band frequencies. It is shown at S band that the drop median diameter can be estimated to an accuracy of 10% whereas the equivalent intercept parameter (Nw) can be estimated to an accuracy of 6% in the logarithmic scale. Similar error structure of the algorithms are also presented for C and X bands. The estimators for the raindrop size distribution parameters are also evaluated using disdrometer data based simulations. The disdrometer-based evaluations confirm the accuracy of the algorithms developed herein. The algorithms developed in this paper are also applied to data from Polarimetric radar.