| Hexi corridor is one of the regions where duststorms generate at high frequency, and the influence of duststorm has draw great attention of the government and the public. This essay mainly discuss the characteristics of temporal and spatial distribution of duststorms generated in Gansu Province, especially in Hexi Corridor, during the last 57 years; and also a regional strong duststorm case caused by Mongolian cyclone by using statistics and diagnostic analysis.The results show that, in Gansu province, duststorms and strong duststorms principally generated in Hexi Corridor, and the high frequency centers of them are mainly the same. There are three high frequency centers:Minqin, Dingxin and Dunhuang, and Minqin has obvious high numbers then Dingxin and Dunhuang whether in times or frequency. The characteristic of the seasonal and monthly distribution of duststorm and strong duststorm indicates that they mostly happen in spring. The maximum frequency of duststorm and strong duststorm appears in April, and the second largest frequency appears in March and May, and September and August are of the minimum frequency. The characteristic of interdecadal variation shows that, the number of days with duststorm and strong duststorm weather decreases fluctuatly during the last 57 years on the whole. Since the later 50's,70's was the decade when the strong dust storm and dust storm happened the mostly,90's was the least; Seeing from the trend of the change, it fluctuantly increased in 60-70's, decreased in 80-90's; After entered the new century, duststorm and strong duststorm increased during 2000 and 2002, so it has been the most frequently the duststorm happened in this period in recent two decades.In Dunhuang, which is as the center of high frequency of duststorm and strong duststorm, interdecadal characteristics and the whole trend is consistent, while in Dingxin and Minqin, there's a obviously increase in times around 2000, which made it the period of maximum value of times during the last 20 years.In average, the duration of duststorm is 70-150 minutes. The duration of duststorm doesn't show obvious descending trend under the condition that the frequency of duststorms and strong duststorms decreases obviously, especially after the year of 2000, the duration of duststorm increases while its frequency decreases.Through the study of a strong regional duststorm caused by Mongolian cyclone in spring, it can be obtained that, Mongolian cyclone is the main influencing system in this duststorm weather; The intensity and range of the duststorm has a close relationship with the development and moving of the Mongolian cyclone, and the duststorm principally generated in the regions where the barometric gradient is high or the mean wind velocity is greater than 12 m·s-1; Strong barometric gradient may lead to strong wind with the velocity of 20-30 m·s-1 and the supergeostrophic wind caused by it is important for the formation of strong wind, which is one of the mechanisms of strong duststorm generation; Surface factor fields react strongly to duststorm occurrence, temperature and dew-point temperature drop rapidly and air pressure is strengthened when duststorm happens. The time of intersection of surface temperature line and pressure line has referential meaning for the duststorm occurrence time forecasting. Strong duststorm is caused by gale, strong allobaric gradient, pumping forced by divergence of upper level and abundant sand source; negative vorticity advection and cold advection make for the development of the cold high pressure at the tail of cyclone, which results in stronger allobaric gradient between the cyclone and the high pressure, and makes for the development and maintenance of the gale and sand-dust weather.The distribution of helicity has a good instruction meaning for the development and moving of duststorm weather:the negative center of total helicity is closely related with the development and moving of cold vortex, and the cold center and its axis moves from the position of the back to the center of the cold vortex, which has instruction meaning for the development and variation of the cold vortex. Sand-dust region is located in the area of the negative total helicity where the isoline is dense or close to the line of zero; the distribution characteristic of horizontal helicity above the duststorm region is that:the upper layer of 300hPa is controlled by negative helicity, and the lower layer of 700hPa is controlled by positive helicity; the center of negative helicity area in upper layer increases obviously when duststorm occurs, and the upper and lower layers coincided with the area where duststorm generates is in the region of strong helicity gradient. |
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