Analysis on Formation of a Severe Sandstorm Weather

A severe sandstorm weather befell in the south of Hebei Province on 27 March 2006.The authors use routine meteorological data and reanalysis grid data of NCEP/NCAR and the model products of MM5 to analyze the circulation background and the physical mechanism of this weather phenomenon.The results show that the severe sand storm process is formed by the upper air trough and the northwest gale behind the clod front.The enhancement of upper jet and the forming of mesoscale counter-circulation lead to the momentum downward-transporting from high level to the ground.The variation of the positive helicity center in low level has indicating significance for forecasting sandstorm weather.The weather of sandstorm always occurred in the areas at south side of the positive helicity center.There was secondary largest helicity center over the sandstorm area,and the intensity of sandstorm increased along with the increase of the helicity center value.