The anatomy and physics of ZDR columns

Description

Polarimetric radar observations of deep convective storms frequently reveal columnar enhancements of differential reflectivity (ZDR). Such “ZDR columns” can extend upwards more than 3 km above the environmental 0 deg C level, indicative of supercooled liquid drops being lofted by the updraft. Previous observational and modeling studies of ZDR columns are reviewed. In order to address remaining questions, the Hebrew University Cloud Model (HUCM), an advanced spectral bin microphysical model, is coupled with a polarimetric radar operator to simulate the formation, life cycle, and demise of ZDR columns in a deep convective continental storm. In doing so, we clarify the mechanisms by which ZDR columns are produced, including the formation of large raindrops in the updraft. The internal hydrometeor structure of ZDR columns is quantified, and we describe the life cycle of ZDR columns from early formation, growth to maturity, and demise. In addition, we explore the practical applications of ZDR columns, including their relation to updraft strength and their prognostic value for increases in low-level precipitation and hail production. Further, it is shown that the height of ZDR columns and the magnitudes of ZDR within the storm dramatically depend on the aerosol (CCN) concentration. At low CCN concentrations typical of maritime air, ZDR values are much lower than in clouds developing in polluted conditions. The maximum hail size in clean air also is substantially lower, and thus a significant fraction of hail melts prior to reaching the surface. The mechanisms of such dependences are discussed.