NUMERICAL SIMULATION OF SYNOPTIC CONDITION ON A SEVERE SAND DUST STORM


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Abstract

By using observational data and MM5, an observational analysis and numerical study was conducted on the synoptic condition of a severe dust storm that was caused by a Mongolian cyclone which occurred from 6 to 8 April 2001. Results illustrated that the cyclogenesis was due to the isentropic potential vorticity (IPV) advection in the upper troposphere and the terrain modified baroclinicity in the mid-lower troposphere. The Altai-Sayan complex of mountains blocked the lower level cold air and made the isentropic surface sharply steep. When the air slid down along the isentropic surface the increasing of baroclinicity and decreasing of stability blew up the vertical vorticity development. The formation of the dust storm was a result of a cyclonic cold front passing across the area. The occurrence of this dust storm was closely related to the strong surface wind, which was accompanied by a cold front passing, rather than the cyclogenesis, itself. Hence, the reason for the pre-front dust storm formulation was the formation of heating convection. Reasons behind the formation of a black storm (visibility lower than 50 m), which occurred in the mid-north part of Inner Mongolia, lay in several aspects. Firstly, in this area the surface wind was strong, a direct result of the downward transport in mid-lower troposphere. Secondly, the cold front passed over the effected area near sunset so the air obtained much more surface heating to form a deeper mixed layer (ML). Thirdly, cooperation between the lower level wind and the terrain made the atmosphere in this area and acquired the maximum advective contribution necessary to form a deep post-front ML. The sensitivity experiment revealed that surface heat flux was important to the frontal lifting. In addition, the forcing of surface heating was also seen as the primary forcing mechanism of frontogenesis. Meanwhile, removal of the surface heat flux made the atmospheric stratification became stable and the pre-storm ML very shallow, which weakened the strength of the dust storm.

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