The Lower Structure of Dust Devils

Abstract

Measurements from an extensive field program in the desert near Tucson, Ariz., are used to formulate a quantitative dust devil model. The model is based on measurements of temperature, pressure and wind velocity taken from a mobile instrumented tower near and within dust devils at levels of 6, 17 and 30 ft. Three dust devil penetrations are analyzed with respect to the temperature, pressure and wind velocity profiles contained in a plane defined by the dust devil axis (or local vertical) and the direction of motion. These profiles corroborate earlier preliminary measurements by Sinclair and thus firmly establish the existence of the warm, low-pressure core of the dust devil. Maximum temperature and pressure variations within the dust devil varied from 4 to 8C and 2.5 to 4.5 mb. The most significant result of the measurement program was the discovery of a downcurrent or a region of markedly reduced vertical motion along and surrounding the sloping dust devil axis. Immediately outside the downcurrent, the vertical velocity reaches positive peak values and then diminishes rapidly with respect to increasing radius. Superimposed on this motion is a strong rotary and radial motion which results in a combined flow pattern similar to that of a helical vortex. The measurements are synthesized into a dust devil model. The observational model represents a two-cell structure which also appears to exist in other atmospheric vortices, such as the tornado and hurricane, and some laboratory and theoretical vortex models.