The impact of evaporation on polarimetric characteristics of rain: theoretical model and practical implications

Soon the National Weather Service WSR-88D radar network will be undergoing an upgrade to allow dual-polarization capabilities. Therefore, it is imperative to understand and identify microphysical processes using the polarimetric variables. Though melting and size sorting of hydrometeors has been investigated, there has been relatively little focus devoted to the impact of evaporation on the polarimetric characteristics of rainfall. In this study, a simple explicit bin microphysics one-dimensional model is constructed to quantify the impact of evaporation in different thermodynamic conditions and with varying rainfall rates and initial drop size distributions aloft.

The modeling results indicate that the amount of evaporation that occurs in the subcloud layer is strongly dependent on the initial shape of the drop size distribution (DSD) aloft. Though conventional single-polarization radar measurements cannot provide information regarding the DSD shape, polarimetric variables (especially differential reflectivity Z_DR) provide sufficient information to better estimate the rate of evaporation and its effect on vertical profiles of all the polarimetric variables as well as rainfall rate. In addition to quantifying the effects of evaporation, we offer a simple method of estimating the amount of evaporation that occurs in a given environment based on polarimetric radar measurements of reflectivity factor at horizontal polarization Z_H and differential reflectivity Z_DR aloft. Such a technique may be useful to operational meteorologists and hydrologists in estimating the amount of precipitation reaching the surface, especially in regions of poor low-level radar coverage such as mountainous regions in the western United States or locations at large distances from the radar.