Probing a Nonprecipitating Cold Front with Doppler Weather Radar

Abstract

The utility of Doppler radar to study boundary-layer kinematics of a weak nonprecipitating cold front in Oklahoma on 16 October 1987 was examined with measurements from two radars. Diagnosis was impeded by operation at low antenna elevation angles, short radar ranges, and low signal-to-noise ratios. Further, kinematic parameters computed by single-radar velocity-azimuth-display (VAD) technique for meteorological wavelengths <125 km were significantly smoothed (more than 50% attenuated). Meteorological scales ≥5 km were well resolved (less than 50% attenuated) in wind fields synthesized from dual-Doppler radar observations, but derived parameters were particularly sensitive to the vertical extrapolation of radial velocity measurements in the presence of strong vertical wind shear. Nonetheless, radar-derived wind flows depicted a sequence of events consistent with other instrumentation. In the vicinity of the front, mean-flow divergence, vertical velocity, and deformation, computed from single-radar measurements for an analysis domain of 30-km radius, were ?4?10?5 s?1, 3 cm s?1, and 16?10?5 s?1, respectively. Agreement between the radars attested to the accuracy of the measurements. Local peak absolute values of divergence, deformation, and vertical vorticity, determined from dual-Doppler analysis, were 200?300 (?10?5) s?1. Extrema were concentrated along the frontal zone where signals were strong, and had dimensions of ?10 km.