Polarimetric Radar Observation of Evaporation and the Implications for Rainfall Precipitation Estimation

Abstract:

This study presents polarimetric radar observation of evaporation and investigates the impacts of evaporation on rain rate at the surface. Assuming that raindrops are horizontally-oriented oblates, an evaporation model is constructed to examine the sensitivity of initial rain drop size distribution (DSD) to the surface polarimetric observables and the related surface rain rate. Due to the evaporation process, radar reflectivity (Z) decreases toward the surface. However, the differential reflectivity (Zdr) increases since the depletion of small raindrops enhances the mean size in DSDs, while a decrease of Zdr is also found due to temperature gradient of the atmosphere. Two cases of evaporation processes observed by a polarimetric radar and micro rain radar are shown to offer a detail insight into the evaporation process and the related DSD evolution. The first event where an initial Zdr below the melting layer is observed above 1.5 dB undergoes relatively weak evaporation and can be observed by the decrease of small raindrop concentration toward the surface, since the relatively larger mean size of the DSD leads to a slow evaporation process. In the other precipitation event with a low initial Zdr below 0.5 dB, all raindrops evaporate before reaching the surface. Subsidiary cooling effects brought up by evaporation illustrate that small raindrops are efficient cooling producer. Based on the simulation and observation results, a retrieval scheme which relates the Zdr aloft to the surface rain rate is proposed in this study to mitigate the uncertainties in rainfall retrievals during evaporation.