Sources of Gravity Wave Activity Seen in the Vertical Velocities Observed by the Flatland VHF Radar

Abstract

Observations of vertical velocity made with the Flatland VHF radar located in the extremely flat terrain near Champaign, Illinois, are used to study sources of enhanced variance. The variance is used as an indicator of gravity wave activity. In contrast to sites in or near mountains where lee wave activity often masks signals due to other sources, at Flatland we find that all episodes of enhanced variance are correlated with synoptic or mesoscale weather events, such as the passage of fronts or jet streams and convection. Case studies are used to characterize the sources of variance in the data, with specific examples from the spring of 1987. Also, summaries from data collected over the entire period March 1987 through May 1988 are presented. It is found that largest variances of vertical velocity are associated with low stability in the lower troposphere; most often indicated by clouds and convection and less frequently due to a dynamic feature such as strong winds or a front. 11 is found that wave activity is about 50% greater in the troposphere and lower stratosphere in the cloudy skies ahead of midlatitude storm systems than in the clear skies above the stable air behind storms.