Idealized Model Simulations to Determine Impacts of Storm-Relative Winds on Differential Reflectivity and Specific Differential Phase Fields

Hydrometeors of varying sizes have different fall speeds; for example, larger raindrops have greater fall speeds than smaller raindrops. The resultant differential sedimentation leads to differences in residence time in a sorting layer where drops can be advected by the storm-relative winds. The resulting size sorting has an effect on the polarimetric radar variables including specific differential phase K_DP and differential reflectivity Z_DR. This study uses a simple numerical model of raindrop size sorting to analyze and further elucidate the relationship between the storm-relative winds and the Z_DR and K_DP fields. Increased mean storm-relative winds lead to increased Z_DR magnitudes and decreased K_DP magnitudes. The separation distance between Z_DR and K_DP maxima is proportional to the magnitude of the mean storm-relative wind and the orientation of a vector from Z_DR to K_DP maxima is aligned with the mean storm-relative wind over the sorting layer. Further, it is shown that larger values of storm-relative helicity are associated with greater separation distances and separation orientations approaching orthogonal to the shear vector over the sorting layer.