Predicting duststorm evolution with the vorticity theory

Abstract

The relationship between duststorms and vorticity in Mongolia and northern China was investigated from March 19–31, 2001, using the three-dimensional, nonhydrostatic, mesoscale model (MM5). At the 500-hPa level, the area of maximum negative relative vorticity, or strong upward motion of air, coincided with the area of duststorm generation in Neimongo of northwestern China when relative humidity was less than 40% and wind speed was greater than 10 m s − 1 . The transport of dust arising from the source region always followed the negative vorticity area downwind. This region of duststorm generation was the same as the layer of unstable atmospheric air (negative potential vorticity layer) near the ground surface in the vertical distribution of baroclinic potential vorticity, which was a function of diabatic heating and frictional terms with respect to time. During the day, dust parcels were lifted to about 700 hPa (about 3 km) where the potential temperature gradient with pressure (∂ θ/∂ p) was zero. The height of these dust parcels was confined to the 700-hPa level where stability extended to the stratosphere. The convective boundary layer (negative potential vorticity value) extended to about 1 km, and initially dust particles floated from the ground surface to the mixed layer of about 1.5 km above the convective boundary layer where they remained. The westerly wind drove the particles downwind. At night, a shallow stable boundary layer near the ground surface (inversion layer, large positive potential vorticity) developed, and the particles inside the stable layer merged near the ground and moved downwind. The dust particles in the mixed layer still moved downwind, and dry deposition occurred from the top of the stable layer to the ground surface.