Abstract
A phenomenological model is developed wherein vortices are introduced at random into a virtual arena with specified distributions of diameter, core pressure drop, longevity, and translation speed, and the pressure history at a fixed station is generated using an analytic model of vortex structure. Only a subset of the vortices present are detected as temporary pressure drops, and the observed peak pressure-drop distribution has a shallower slope than the vortex-core pressure drops. Field studies indicate a detection rate of about two vortex events per day under favorable conditions for a threshold of 0.2 mb (1 mb = 1 hPa): this encounter rate and the observed falloff of events with increasing pressure drop can be reproduced in the model with approximately 300 vortices per square kilometer per day—rather more than the highest visual dust devil counts of approximately 100 devils per square kilometer per day. This difference can be reconciled if dust lifting typically only occurs in the field above a threshold core pressure drop of about 0.3 mb, consistent with observed laboratory pressure thresholds. The vortex population modeled to reproduce field results is concordant with recent high-resolution large-eddy simulations, which produce some thousands of 0.04–0.1-mb vortices per square kilometer per day, suggesting that these accurately reproduce the character of the strongly heated desert boundary layer. The amplitude and duration statistics of observed pressure drops suggest large dust devils may preferentially be associated with low winds.
Large-Eddy Simulations of Dust Devils and Convective Vortices
