Abstract
This study demonstrates how the variability of the VAD (Velocity Azimuth Display) pattern obtained with a Doppler radar may be used to map qualitatively the turbulent regions in a snowstorm. The synoptic situation is presented; time-height plots of reflectivity, wind direction, wind speed, and vertical wind shear are discussed; and the association between characteristic features of the wind field and regions where turbulence is detected are investigated. The turbulent regions correspond to zones where the vertical wind shear exceeds 10−2 sec−1. It is demonstrated that the velocity fluctuations in the turbulent zones are strongly oriented and that the largest variability in the velocity field lies in the direction parallel to the wind shear vector. These findings are consistent with a turbulence geometry where roll instabilities extend horizontally across the shear direction with the overturning causing velocity perturbations mainly in the vertical and in the shear direction. It is suggested that this is a manifestation of Kelvin-Helmholtz shear instability waves. Typical values of the variance of velocity fluctuations in a turbulent region are 6 m2 sec−2, in a non-turbulent region 0.4 m2 see−2.
