The Turning Causes of Trough to Ridge in a Typical Black Sandstorm Process Occurred in North-west China

On April 24-27,2010,a strong sandstorm and gale process occurred in Hexi Corridor,Gansu,developed to black sandstorm and affected most areas in north-west China,brought about serious economic loss.In order to have a thorough understand over the causes of that disastrous weather process,we analyzed and compared the weather circulation changes,the high vertical wind fields,and the horizontal relative helicities of this black sandstorm process.The daily surface to upper wind grid data at 08:00 and 20:00 and the upper weather maps and three-hourly surface weather maps were used to calculate the horizontal helicities from near ground surface up to 500 hPa from March to May during 2002-2011.The results showed that the strong cold air at 700-500 hPa,the violent unstable atmospheric layer at 700 hPa,and the helicity's negative value less than-1 000 m2·s-2 at 20:00 on April 24 were dynastic factors leading to the outbreak of the black sandstorm.The existence of low-level jet bigger or equal 15 m·s-1 at 700 hPa at 08:00 on April 25-27 was a critical factor for the windstorm to form and last.