DUST DEVIL FLUX VARIABILITY DUE TO CHANGES IN DESERT SURFACE CONDITIONS

Dust devil vortices are a major contributor to the erosion and transport of fine soil particles in the arid regions of Earth and Mars. In several field campaigns conducted in Eldorado Valley, Nevada, from 1995 to 2002, chase vehicles were used to directly sample dozens of individual whirlwinds for their wind dynamics and sediment loads. Concurrently, surface characterization measurements were performed to understand those geologic and aerodynamic factors that acted to enhance or suppress dust devil formation. Similar work was carried out on volcanic landscapes in the Peruvian Andes.

Over the course of several minutes, large (>20 m diameter) vortices are found to loft several metric tons of sand, silt and fines to heights in excess of 1 km. The more impressive columns have particle loads greater than 120 milligrams per cubic meter. Such erosive productivity is directly linked to the surfaces they form over or travel across.

In years with a higher occurrence of summer thunderstorms and winter rainfall, fresh sand and silt is delivered to the alluvial plains surrounding the central playa. Dust devils were more abundant on those gently sloping surfaces. Such "wet" years, however, also promoted growth of grasses which in turn created frictional surfaces that slowed winds to speeds below the surfaces' erosion thresholds. Dust devils were scarce over those "meadows".

More recently, the test valley has experienced sustained dry conditions. The grass stubble of the "meadows" has been flattened and dust devils are now common there. Conversely, on the alluvial plains, the continued stabilization and depletion of the sand and fines, without any newly delivered input, has resulted in a reduction of dust vortex activity. Throughout all these changes, dust devil abundance on the dry lakebed has remained high. This presentation will examine several key surface characteristics in detail and report on the findings of the latest chase program.