Vertical Velocities in a Thunderstorm Gust Front and Outflow

Abstract

Continuous vertically pointing measurements of a thunderstorm outflow, including its gust front, were obtained with a Doppler radar near New Salem, North Dakota. The measurements provide a high-resolution depiction of the vertical structure of reflectivity and vertical velocity within the gust front, the outflow, and the parent storm. Earlier gust front remote sensing studies have used Doppler observations obtained with low-elevation-angle scans to accurately measure the horizontal flow pattern from which vertical velocities were subsequently estimated by integrating the continuity equation. In contrast, the New Salem case provides direct, rather than derived, Doppler measurement of vertical velocities with better vertical resolution and vastly superior temporal resolution. The gust front?s vertical structure is in general agreement with earlier observations and numerical simulations, except that the transition from strong upward to strong downward motion was more abrupt. The maximum updraft, of almost 10 m s?1, was measured in the gust front at 1.35 km above ground level and was followed by equally strong downward motion only 1 min later at a slightly higher altitude.The observations support the earlier use of the continuity method for deriving the basic pattern of vertical motions in density currents from quasi-horizontal scan data.