Relationship Between Hail Microphysics, Aerosols, and Parmaeters of ZDR Columns

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Description

A hail storm with hailstones up to several cm in diameter is simulated at different aerosol concentrations using a new version of Hebrew University Cloud Model (HUCM) with spectral bin microphysics. The model includes detailed description of wet and dry growth regimes of freezing drops, graupel, and hail. Using a polarimetric radar operator, the field of differential reflectivity, ZDR, is calculated. It is shown that in the presence of spontaneous breakup of raindrops, ZDR columns develop during the wet growth of hail arising from intense hail riming in the updrafts of deep convective clouds growing commonly in polluted air. The heights and volumes of ZDR columns were shown to increase with the increase in aerosol concentration. It is shown that big hail forms only in polluted clouds with a high amount of supercooled liquid. In updrafts of polluted clouds, hail grows by wet growth; this wet hail increases the height of ZDR columns. In the case of low aerosol concentration, the amount of supercooled liquid is low, and hail and freezing drops begin growing in a dry growth regime 100-200 m above the 0° C isotherm. Since ZDR from dry hail is generally lower than that from wet hail, the height of ZDR columns in clean air is substantially lower than that in the polluted air. Owing to high values of calculated correlation coefficients, the parameters of ZDR columns can serve as a predictor and measure of vertical velocity and hail characteristics. As shown in the study, the structure of the ZDR fields contains information about aerosol concentration and supercooled mass water contents. These results indicate the importance of detailed descriptions of the process of wet growth to simulate ZDR columns and other polarimetric characteristics.