Freezing of Raindrops in Deep Convective Updrafts: A Microphysical and Polarimetric Model

Abstract

Abstract Polarimetric radar observations of convective storms routinely reveal positive differential reflectivity ZDR extending above the 0°C level, indicative of the presence of supercooled liquid particles lofted by the storm’s updraft. The summit of such “ZDR columns” is marked by a zone of enhanced linear depolarization ratio LDR or decreased copolar cross-correlation coefficient ρhv and a sharp decrease in ZDR that together mark a particle freezing zone. To better understand the relation between changes in the storm updraft and the observed polarimetric variables, it is necessary to first understand the physics governing this freezing process and the impact of freezing on the polarimetric variables. A simplified, one-dimensional explicit bin microphysics model of stochastic drop nucleation by an immersed foreign particle and subsequent deterministic freezing is developed and coupled with an electromagnetic scattering model to explore the impact of the freezing process on the polarimetric radar variables. As expected, the height of the ZDR column is closely related to the updraft strength and initial drop size distribution. Additionally, the treatment of the stochastic nucleation process can also affect the depth of the freezing zone, underscoring the need to accurately depict this process in parameterizations. Representation of stochastic nucleation and deterministic freezing for each drop size bin yields better agreement between observations and the modeled vertical profiles of the surface reflectivity factor ZH and ZDR than bulk microphysics schemes. Further improvements in the representation of the LDR cap, the observed ZDR gradient in the freezing zone, and the magnitude of the ρhv minimum may require inclusion of accretion, which was not included in this model.