An application of the horizontal helicity to forecasting sandstorm.

To more accurately forecast the strength and scope of a sandstorm over northern China, the daily 08:00 and 20:00 BT wind grid data at the upper levels and upper weather maps, as well as every 3hour surface weather maps are used to calculate the horizontal helicities from near surface up to 500 hPa from March to June during 2002-2010. The main results show that the bigger the helicity’s negative value is, the stronger the convergent rising movement and the wind velocity are, and thus the stronger the forming sandstorm is. The helicity’s negative center is often the strongest near the Hexi Corridor of Gansu Province with the sandstorm often happening over the downstream region or near the Helicity’s negative center within 24 hours. Over the sanddust key region (40°-48°N，84°-120°E), when the helicity center value ≤-600 m2/s2, the strong sandstorm whose visibility is less than 500 m within 6 hours will happen in the center or its downstream region with the negative value center fitted well with the sandstorm’s occurring areas downstream. By contrasted and analyzed among the northern China typical largescale strong sandstorm cases and between the sandstorm and nonsandstorm weathers in east of the Hexi Corridor of Gansu Province, it is found that the horizontal helicity has strong daily variation which is stronger in daytime than in night, and shows a strong ability of forecasting cold front sandstorm from winter to spring over the arid regions of northern China