Possible effect of Kelvin-Helmholtz instability of VHF radar observations of the mean vertical wind

Mean vertical velocities (w) over bar are of great interest in meteorology, and vertically directed VHF radars have been used to measure w directly. Recently it has been pointed out that in a gravity wave with upward energy propagation the radar reflectivity and the vertical velocity are negatively correlated, giving rise to a downward bias for the observed (w) over bar. In the present paper, it is shown that in regions of Kelvin-Helmholtz instability (KHI) the correlation between refractivity-surface tilting angles and the (apparent) vertical velocity observed with a vertically pointing VHF radar can lead to a bias <(Delta w)over bar> with magnitudes of several tens of centimeters per second. This ''KHI bias'' is upward in the shear zone above the horizontal wind speed maximum of a jet stream and downward below. Theoretical vertical profiles of the predicted KHI bias exhibit considerable similarity with (w) over bar profiles observed during jet stream events by other authors, suggesting that a major part of the observed (w) over bar might be attributable to the presumed KHI bias.