Estimation by Doppler Radar of Curvature, Diffluence, and Shear in Cyclonic Flow

Abstract

The potential for single-Doppler radar determination of wind field characteristics in cyclonic flow is examined. The influence of the four independent first-order derivatives of a wind field, namely curvature, diffluence, downwind shear, and crosswind shear, upon the Doppler radial velocities is studied. Simple models of wind fields containing each of the derivatives defined in natural coordinates are presented. When only one derivative is present at a time, it has been found that there are unique signatures for diffluence and downwind shear and qualitatively similar signatures for curvature and crosswind shear. With a model incorporating all four derivatives, techniques are developed for the recovery of these derivatives. A method is also presented that corrects the mean speed estimate. It is concluded that in most cases the recovery of the downwind shear, diffluence, the sum of curvature and crosswind shear, and mean wind is possible to within 5 percent of the true values. Application of these techniques to radar data collected from Hurricane Gloria is discussed. A storm strength indicator based on shearing deformation and distance of cyclone center yielded signs of the declining trend of the storm an hour or two before this trend manifested itself significantly in the wind speed as estimated by the Doppler radar, therefore suggesting potential as a forecast tool.