Rain Microphysics Retrieval with a Polarimetric WSR-88D

In the spring of 2005 a study was begun to determine the utility of a prototype polarimetric WSR-88D for retrieving drop-size distributions and microphysical properties of rainstorms. NCAR's video disdrometer was positioned 28 km south-southwest of the polarimetric WSR-88D (KOUN) operated by NSSL. Although the period of data collection (mid April through June) was one of the driest on record, a dataset of polarimetric radar and disdrometer observations was acquired from a number of mesoscale convective systems that passed through the area. The disdrometer data have served to verify the reflectivity and differential reflectivity measurements from the radar and drop-size distribution (DSD) attributes derived from them. Instrument offsets were mitigated by forcing the radar and disdrometer data to agree in the mean. Trends of radar reflectivity and differential reflectivity exhibit excellent agreement. Another study objective is to examine the impact of disdrometer sampling on derived DSD properties. This issue was addressed by a side-by-side comparison between the NCAR and NSSL units. Results of the disdrometer-disdrometer and disdrometer-radar inter-comparisons will be shown.

The disdrometer measurements are being used to refine a polarimetric radar-based DSD retrieval algorithm. The procedure retrieves the governing parameters of an assumed gamma drop-size distribution from measurements of radar reflectivity and differential reflectivity and an empirical relation between the distribution shape (μ) and slope (Λ) parameters. Preliminary results indicate that drops are more broadly distributed in Oklahoma than found for an earlier study in Florida.

Using the refined μ-Λ relationship and radar measurements, DSD attributes are retrieved for a squall line with an enhanced trailing stratiform rain region that was observed on 13 May. Drop median volume diameters (D₀s) greater than 3 mm determined for the leading convection are attributed to melting hail and graupel. D₀s of nearly 2 mm determined for the trailing stratiform rain are clearly a consequence of significant melting layer aggregation. The transition region between these two precipitation regimes was characterized by small drops. Modification of the national network of WSR-88Ds for polarimetric measurements will present new opportunities for improving the understanding of microphysical processes in storms and their parameterization in numerical models.