Effects of aerosols on hail formation and polarimetric radar signatures of hail storms

Description

Effects of aerosols on formation of hail is investigated using the Hebrew University Cloud model with spectral bin microphysics. The model solves the equation system for size distribution functions for liquid drops, three types of ice crystals, snow, graupel, hail and freezing drops. Recent improvements of the model include detailed description of time dependent freezing. Freezing drops are described using corresponding size distributions designed on mass grid containing with 43 bins. The freezing drops consist of interior water covered by ice. Time dependent freezing is calculated by solving equations of heat balance that takes into account effects of accretion of supercooled drops as well as ice crystals. The possibility of both dry growth (when surface is icy) and wet growth (surface is covered by water film) are taken into account. It was assumed that freezing drops growing by dry growth collect supercooled water drops. In case of wet growth, freezing drops were allowed to collide with other hydrometeors, such as cloud-ice, with a collection efficiency of unity. The processes of wet and dry growth of hail are considered in detail by solveing corresponding thermodynamical equations in hail particles. A mid-latitude hail storm similar to that observed over northwest Kansas on 29 June 2000 during the Severe Thunderstorm and Electrification and Precipitation Study (STEPS) has been simulated under different concentrations of cloud condensational nuclei (CCN). It was shown that mass of hail is the largest in case of low CCN concentrations. However, the size of hail particles does not exceed 1-2 cm. The structure of the storm developed in case of high CCN concentration is much closer to observations: radar reflectivity reaches 70 dBz, and the size of large hailstones reaches 5-6 cm. Polarimetric parameters of the simulated storm are calculated using a polarimetric radar observation operator for a cloud model with spectral microphysics. It is shown that high Zdr columns arise only in polluted clouds. There is a clear correlation between height of Zdr columns and the value of the differential reflectivity, on the one hand, and the hail size, on the other hand. In clouds developing in clean air the height of Zdr columns typically does not exceed the altitude of freezing level. The physical mechanisms leading to so strong aerosol effects on microphysical structure and polarimetric signatures are discussed.