Diagnosing the Electrical Structure of Dusty Gust Fronts in the Sahel

Dusty gust fronts (‘haboobs') are commonplace in the Sahel when cold air produced by moist convection spreads across sparsely vegetated terrain. Previous studies in Niger and elsewhere have demonstrated that saltating sand, dust devils and haboobs are strongly electrified. The present study is concerned with the vertical electrical structure of the haboobs, and ultimately with the physical mechanism for their electrification. The gust front position is followed with 5-minute surface scans with the MIT C-band Doppler radar. To elucidate the vertical electrical structure on a scale commensurate with the saltation layer, field measurements at four levels (0.4, 2.5, 5, 10 m) of a 10-meter tower were undertaken with special cylindrical field mills designed to reject the common-mode signal associated with particle impacts. To elucidate the electrical structure on a scale commensurate with the depth of the dusty gust front, measurements were made at two locations near the ground separated by ~500 meters. The assumed presence of a uniform field in undisturbed (fair weather) conditions is used to remove the effect of the tower supporting the sensors on the electric field at their different heights. In disturbed (gust front) conditions, the corrected vertical fields are first-differenced to estimate space charge density versus height from Poisson's equation. Preliminary results on one event on July 29, 2010 show evidence for enhanced positive charge prior to the gust front arrival, with pronounced negative charge overhead during the dust onset. The records at two surface sensors close to the surface and ~400 meters apart are remarkably well matched. Estimates of space charge density versus height show evidence for positive charge at the lowest height (< 2.5 meters) and negative charge at higher levels, with a persistent structure lasting ~5 minutes when the main dust cloud is overhead. Various electrostatic models are considered to account for the observations.