Interpretation of Mesocyclone Vorticity and Divergence Structure from Single-Doppler Radar

Abstract

A kinematic mesocyclone model is developed to better approximate mesocyclone flows observed by single-Doppler radar. The model is described by two general flow regimes: an inner core region where velocity varies directly with radius from the center of the flow and an outer flow region where velocity varies inversely with radius. The new model differs from the traditional circular mesocyclone model in that the shape of the inner flow is described by an ellipse of specified eccentricity, and the vorticity and divergence structures of the inner flow region are nonuniform and described by simple functions. The effects of flow shape, vorticity and divergence structures, radar viewing angle, and radar resolution on the flow appearance and data interpretation are examined. One traditional measure of mesocyclone intensity is the shear measured between the relative peaks of incoming and outgoing Doppler velocity. In noncircular flows or flows where the vorticity structure is not uniform, shear is found to be an unreliable measure of mesocyclone intensity. A correction for shear is possible if the flow shape, internal structure, and orientation to the radar are known. Techniques to assess these characteristics from single-Doppler data are presented. The elliptical mesocyclone model is compared with observations of the 20 May 1977 Del City, Oklahoma, mesocyclone from two Doppler radars. From characteristics of the flow estimated from single-Doppler data, a simulation of the mesocyclone is produced that closely approximates the observed single-Doppler fields. The associated model fields of vorticity and divergence are comparable in structure and magnitude to the fields determined from dual-Doppler analysis.