Wind Structure in a Supercell Thunderstorm as Measured by a UHF Wind Profiler

Abstract

A mesocyclone associated with a tornadic supercell passed almost directly over the Lamont, Oklahoma, 404-MHz wind profiler on 24 May 1998. The archived spectral moment data, including 6-min resolution radial velocity measurements from the vertical and two oblique beams of the profiler, provided a rare opportunity to construct a detailed time?height section of the three-dimensional wind structure of a severe convective storm near and in its mesocyclone. Supplemental information used to create this cross section was provided by nearby operational Next Generation Weather Radar/Weather Surveillance Radar (NEXRAD/WSR-88D) radars, as well as visual and surface observations. The most difficult challenge in analyzing the profiler data was the removal of hydrometeor fall-speed contamination in order to determine the true air motions. Since the profiler beams saturate under heavy precipitation conditions, the use of the returned signal power in the profiler beams to estimate precipitation fall speeds was not reliable. To overcome this difficulty, radar reflectivity data from four surrounding NEXRAD/WSR-88D radars were objectively analyzed for the region scanned by the profiler. The components of vertical velocity due to precipitation were then estimated by developing empirical statistical models between the objectively analyzed reflectivity factors and profiler-measured vertical velocities located well outside the storm updraft. This technique was successful in that it removed fully 80% of the explainable variance associated with the precipitation fall speeds; the corresponding estimated expected error, on the order of 1 m s?1, was fully one magnitude smaller than the scale of the measured vertical velocities within the updraft. Although the recovery of the horizontal winds was less successful than the recovery of the vertical velocities, horizontal winds were recoverable in regions where the horizontal vertical velocity gradient was reduced, and where corroborated by observations from nearby operational NEXRAD radars. The resulting time?height cross section of wind exhibited a region of strong updraft near and in the mesocyclone and downdraft flanking it. Weak downdrafts were also present at low levels surrounding the region of strongest updraft. The updraft peak velocity of 50 m s?1 represents one of the largest vertical velocities ever measured in a convective storm. Evidence is presented of significant modification of the horizontal wind structure away from the updraft and of storm-generated buoyancy waves in the surrounding environment.