Effects of aerosols on size and structure of hail and on the polarimetric radar signatures

A mid-latitude hail storm that produced hailstones with diameter up to 5-6 cm is simulated using a new version of the Hebrew University Cloud model (HUCM) with spectral bin microphysics. The HUCM describes time dependent melting of snow, graupel and hail, as well as freezing of raindrops and freezing of liquid water within snow, graupel and hail. The model describes process of wet and dry growth of hail. To simulate such fine processes the calculation of size distribution functions for liquid water fractions in these hydrometeors are implemented. Simulations have been performed under different aerosol loading. Polarimetric radar signatures are calculated to investigate a relationship between the polarimetric signatures and hail size and hail shaft intensity. It is shown that hail forms in clean and polluted clouds using different mechanisms. While in clean air hail forms largely due to freezing of raindrops ascending above freezing level, in the storm developing under high concentration of cloud condensation nuclei hail forms and grows by accretion of supercooled cloud droplets. As a result, mass of hail is larger in the storm developing in clean air, while hail size is substantially larger in polluted air. Since small sized hail melts during its fall below melting level, hail precipitation at the surface is much higher in the polluted case. It is shown that high values of differential reflectivity (Zdr columns)arise in updrafts due to process of recirculation of raindrops and hail. Zdr columns can serve as be a good predictor of big hail only in polluted case. In clean case no Zdr columns above freezing level arise.