Mesoscale Wind Structure Revealed by Doppler Radar

Abstract

Doppler wind soundings were taken in the lower 4.5 km of the atmosphere at 12 minute intervals during a seven hour period in a snowstorm over eastern Massachusetts. A time-height cross section of the wind revealed numerous small scale, short period changes in the wind structure. Initially there was a region of strong wind shear near 4 km, above the warm front zone. Periodic break-downs in the wind shear appeared to result in a downward transfer of momentum. In this way, winds at lower levels increased by as much as 10 m sec?1 within a half hour period. The series of three breakdowns occurred at about hourly intervals at successively lower levels, until by the end of the period the wind speed at all levels had increased by about a factor of two, and the wind shear zone was confined to the lowest few hundred meters. A time-height cross section of vertical motions indicated that each breakdown was preceded by a down-draft, followed by a turbulent region of successive updrafts and downdrafts of 2 to 4 m sec?1. These turbulent regions may be responsible for much of the short period change in structure of ?uniform? precipitation. A comparative analysis, using hourly rawinsondes during a rainstorms with an analogous wind structure also revealed similar breakdowns, although the absence of resolution precluded delineation of the smaller scale turbulence which the Doppler observations so clearly reveal.