A Potential Vorticity Analysis on the Occurrence Mechanism of a Strong Sandstorm in Northern China on April 28-29,2011

The occurrence mechanism of a strong sandstorm happened in northern China during April 28-29,2011 was analyzed with PV theory using the observed data and the NCEP recalculated data.The results showed that,the strong 500 hPa tilted trough was the main weather system that caused the sandstorm,while the low level warm high over Qinghai Plateau and Xinjiang province provided an instable condition beneficial to the sandstorm occurrence.This storm mainly occurred behind of the cold frontal surface.The surface three-hour air pressure variation had an obvious indication to the moving route of the sandstorm.The values of the CAPE increased before the occurrence of the sandstorm.This was beneficial to formaninstable condition.The 500 hPa PV had an obvious indication to the weather system and the 0.5 PVU contour could be regarded as a threshold of the sandstorm occurrence.The sandstorm occurred with the expansion of the cold air in high latitude to low latitude.The slowing-down of the high-value PV system to the low level enlarged the slope of the isentropic surface but did not change the PV conservation.This may result in the developing of the cyclonic circulation on the ground surface and the growing of wind,and finally induced the outbreak of the strong sandstorm.