Comparison of measured and radar - equivalent gamma DSDs to investi gate the effect of gamma raindrop size distribution assumption on the rain rates

The knowledge of rain drop size distribution (DSD) has a wide range of applications in earth sciences such as precipitation physics, hydrology, and agricultural and soil sciences. DSD is notoriously important in developing retrieval algorithms in precipitation remote sensing . In radar meteorology all the radar rainfall algorithms are, at different extent, sensitive to DSD variability. Although literature reports different parametric forms to model the measured DSD, to date, the commonly most used distribution is the three- parameter gamma (Ulbrich, 1983). The goal of this study is to evaluate the error, in terms of rain rate, due to this assumption. A methodology was set up to compare the rain rates of a disdrometer-measured DSD and a simulated gamma DSD equivalent in terms of radar measurements (reflectivity factor, Zh, differential reflectivity, Zdr,and specific differential phase shift, Kdp). The differences, expressed in terms of normalized standard error (NSE) and normalized bias (NB) computed between the two rain rates will provide an indication about the influence of the gamma assumption on rain rate estimation. The influence of other factors , such as the raindrop shape model, the integration time interval, the radar frequency, and the influence of disdrometer sampling error are also investigated.