Numerical Simulation And Formation Mechanism Studies Of The Sandstorm Processes In North China

The frequence of sandstorm in the spring of 2006 attained the highest level since 2000 years.This paper researched the dynamical and thermodynamical reasons for the occurrence of the sandstorm in the spring of 2006 at the climatic aspect by using NCEP data with 2.5°×2.5°.The result showed that the abnormal pressure between the North China and Mongolia was the dynamical background for the occurrence of the sandstorms.The intensity" gradient of pressure engendered the great wind in the west of Mongolia come into being.On the other hand,the fore-period higher temperature was the thermodynamical background.In order to research the mechanism of the sandstorm more detailed,this paper selected three sandstorms which occured in the spring and winter of 2006.Then the weather system of sandstorm was analysized by using NCEP data with 1°×1°and observed data.After successful simulations using the model GRAPES_SDM,this paper studied the dynamical and thermodynamical structure and the mechanism of three sandstorms.The result showed that:(1) the transfer path of sand was influced by the high weather system and the plateau topographty.Because the low-pressure trough was blocked by the high-pressure ridge,so that it moved slowly and the sand transfer path tend to the north and the sandstorm can keep more long time.However, because the the low-pressure trough lied ahead the high-pressure ridge that it moved more quickly and could only keep a short time.In the other hand,the intensity of sandstorm have important influence on the vertical transfer height of sand.(2)The extremum centre of helicity showed the abnormal dynamical structure of atmosphere.The switch between the positive and the negative helicity showed the adjustment of the dynamical structure of atmosphere and the change of the weather system.The stabilization of low lev had a good relation with the occurrence of sandstorm.Before the sandstorm occurred, the stabilization of the atmosphere decreased because of the wind shear, while at the end of the sandstorm the stabilization increased.The dynamical and the thermodynamical structure had reciprocity effect;(3)The high and low jet and the great surface wind were the straight factor to the occurrence of the sandstorms .The locality and the intensity of low jet and surface strong wind were restricted by the high jet. The down-sliding high jet flow along the steep isentropic surface, which was caused by the enhance of PV gradient in high level, resulted in the happening of SVD and down-transmission of high momentum because of the conservation of PV(potential vorticity) and the slanting of the isentropic surface. This caused the emergence of the low jet flow, which was reason for the emission of the sand dust storm.