Study On Summer Extreme Precipitation Event In Xinjiang Aletai Region In Last50a

As the global average surface temperature rising, the intense and frequency of the extreme precipitation events in most of the mid-latitude regions and humid tropics might increase. Due to the global warming, the climate in Xinjiang also transited from warm dry to warm wet. The precipitation mechanism in Alatai, Xinjiang, a high-latitude and arid hinterland, is very different from other regions, which is attributed to the complicated terrain, special atmospheric dynamics, long-distance away from sea as well as various sources for water vapor in this region. From the detailed study of the summer extreme rainfall events in the region, the climate change behavior would be acquired, which is in favor of understanding the response mechanism of this special terrain in the high-latitude and arid hinterland to global warming. In this paper, the change law of the summer extreme precipitation in the region was analyzed using a variety of statistical methods including EOF analysis, linear trend, variation analysis, Mann-Kendall (MK), sliding t test, Morlet wavelet transform, correlation analysis, and synthetic analysis methods, etc. on the summer daily rainfall data (June to August) of seven national basic meteorological complete station (Jimunai, Habahe, Buerjin, Aletai, Qinghe, Fuhai, and Fuyun station) from1961to2010, NCEP/NCAR data as well, and74circulation characteristics. Moreover, the atmospheric circulation in the years of the typical extreme precipitation events was compared. Based on the above analyses, the following conclusions have been obtained:1. Although the interannual variation is not obvious, the decadal variation of the summer rainfall in Aletai could be observed. The summer precipitation in most Aletai regions is not so affected by the global warming that the interannual variation could be negligible. However, for the interdecadal change, the effect of global warming is obvious, which could be reflected by the rainfall data in the years from the1980s to the early2000s. The rainfall was not so much in the early1980s. It increased in the late1980s and reached the maximum in the early1990s. From2000s, the rainfall began to fall down. Due to this kind of sustainability, the precipitation had no obvious change. Hence, Aletai had no mutation in the past few decades except for the Fuyun Station. This Station had mutation from late1980s to early1990s. The summer rainfall in this area has quasi5a annual and12-15a decadal oscillations.2. The threshold values of the summer extreme precipitation between North and Sourth Aletai are different. The threshold value of the summer extreme rainfall events in Aletai is9-13.1mm/d. The threshold value depends on the spatial distribution, which increases from northwest to southeast. The terrain and altitude also affects the threshold value. Among three different terrains, foothills region has the highest threshold value, followed by the Sawuer Mountain, and the river valley plain has the lowest value. Altitude is also close to the threshold value. They are basically in the exponential relationship, that is to say, the threshold value increases with altitude.3. Summer extreme rainfall frequency (intensity) variation in Aletai has decadal oscillation. Due to the negligible response of the summer extreme rainfall events in the high-latitude and arid hinterland Aletai to global warming, the interannual variation of the summer precipitation frequency (intensity) in most Aletai regions was not significant. And a few of the stations had an increasing interannual variation, which could be caused by either some local response or the good response of the foothills region to the global warming. Although the interannual variation is not obvious, the decadal oscillation of the summer extreme rainfall in Aletai could be observed. The decadal variations of frequency and intensity are in a good agreement. They were both low in the early1980s, then increased from the late1980s until the middle of the1990s. They got the maximum value at that time, and decreased from2000s. This trend is similar with the summer rainfall. Due to this kind of sustainability, the precipitation had no obvious change. Hence, Aletai had no mutation in the past few decades except for the Fuyun Station. This Station had mutation from late1980s to early1990s. In this region, the frequency and intensity of the summer extreme precipitation also have a good consistent interannual cycle, but a little bit different the decadal cycle.4.The main reason for the summer extreme precipitation event of Altay Prefecture is the mid-high latitude circulation:the event is closely correlated with500hPa activity frequency of cold air from high latitude, the intensity and location of west-siberia trough and the location of subtropical system. At the same time, the event is obviously connected to200hPa popar front jet, the location of subtropical jet, the continuity of westerly jet, the intensity of tropical easterly jet and the location of zero degree longitude of east wind. The atmospheric circulation characteristics like the North Atlantic North America subtropical high ridge, the North Africa subtropical high ridge and the A-dex of Tibet Plateau are main factors for the summer extreme precipitation event.