Comparison of Simulated Scattering Characteristics of Large Rain Drops and a Special Form of Melting Hail and Relation to C, S and X Band Radar Observations

This paper presents a comprehensive simulation study of specific differential attenuation, differential phase, and differential reflectivity caused by large rain drops and a special form of melting hail. The scattering characteristics of the two types of hydrometeors are computed using the appropriate analysis techniques and are compared against each other and discussed. The simulation results and conclusions are related to observations by C-band radars in some intense storms, reported by some previous studies.

Based on thousands of 2D-video disdrometer measurements of large raindrops, the most probable (taking into account drop oscillations) is a ‘smoothed' conical raindrop shape that can be described by a closed-form empirical formula in the Cartesian coordinates for different diameters of the equi-volumetric sphere. Since these raindrop shapes are smooth and rotationally symmetric, their polarimetric radar parameters are calculated using the fast T-matrix code.

A special form of melting hail where the initial stage of melting ice is assumed to occur around the ‘equator' as has been observed in wind-tunnel experiments is represented by an ice-sphere in the center, surrounded by a water torus, with also a thin layer of water over the top and bottom of the ice sphere. For these hydrometeors, a more advanced and general numerical technique based on the method of moments is used, because of their relatively complex shapes and material composition.

The simulation results show much higher specific differential attenuation factor values for hail particles, with the smallest hail showing similar values to the largest raindrops, except near the resonance region. Hence, the presence of hail with hailstone shapes represented by an ice sphere surrounded by a water torus can explain the higher than expected differential attenuation observed by C-band radars in some intense storms. Results also show that the melting hail particles exhibit higher specific differential phase factor than large rain drops, which too agrees with some reported radar observations. Results for the differential reflectivity, however, show hail particles giving more comparable values to large rain drops.