Abstract
A field study directed primarily to the pressure fields produced by thunderstorms, plumes, dust devils and aircraft has been conducted during several summers at locations in northeastern Colorado and near Stapleton International Airport, Denver. A 60 m tripartite array of anemometers plus sensors of pressure, vertical wind and vorticity were employed in the study of plumes and small vortices. The transient or short-lived phenomena studied required high-sensitivity microbarographs designed for gravity-wave measurements in the Brunt-Väisälä band of the subsonic spectrum. As a supplement to usual anemometry a vorticity meter well suited to microscale work was used to observe small vortices.
Convective plumes were observed to occur in an unstable atmosphere with variable winds. One sees not only a strong updraft and accompanying negative pressure pulse but also a small peripheral downdraft with a corresponding positive pressure from the descending air, mainly at the plume's trailing edge. A second type of convective element, consisting of a plume-like column with vorticity, gives rise to a negative-positive pressure oscillation and related variations in wind speed and direction. Observations on two dust devils show some characteristics similar to the small convective vortices just mentioned, but with the principal exception that there is a descending flow in the core whose sense of rotation is opposite to that of the peripheral region of updraft. Pressure and vorticity effects in thunderstorm gust fronts were observed. The smooth, exponentially rising pressure curve generated by the gust may sometimes be interrupted by a sharp negative pressure spike indicative of shear and cross-stream circulation.
Field studies of pressure profiles of wake vortices from aircraft at take-off and landing show similarities with those from naturally occurring vortices even though the axes of the wake vortices are initially almost horizontal. Vortex sound was sometimes heard during the recording of the subsonic pressure oscillations. These subsonic pressure amplitudes were of the order of 0.1 mb or less, and the time from peak to trough ranged from a few seconds to as much as 30, depending primarily on the dimensions of the vortex flow and the ambient wind. Results show that under favorable atmospheric conditions a high-sensitivity pressure sensor in combination with suitable wind sensors can be used to determine the radius and orbital velocity of vortex flows near the ground where a potential hazard to flight may exist.
