Blocking Highs In Key Regions Of Eurasia And Their Relations With Meteorological Disaster In China In Winter

Since 1940s, blocking highs have attracted much attention from meteorologists. Blocking high persists in the westerly in mid-high latitudes as a large-scale circumfluent system, its onset and decay are often accompanied with big circumfluent adjustment, even in the large hemisphere scale, resulting in widely abnormal weather and climate. Thus, under the global warming condition, it is practical and scientific to study blocking high, not even for a cogitation of inherent connection between abnormal general circulation and frequent extreme weather and climate events, but also for a better medium-range weather forecasting. Because of this background, in this paper, most of the existing blocking indices have been classified into four types, and then a comparison and analysis of these four objective methods are studied, by studying the blocking highs associated with the weather disaster in early 2008 over China for instance and combining related international studies. Based on the comparison, as its nature with dynamics and thermodynamics and ability to search blockings, the PV-θblocking index is chosen to study blocking highs in the Eurasia, as well as the relationship between blocking highs in the Eurasia and the ENSO, NAO, EA and PNA.Especially, the influence of the blocking highs in the three key regions of the Eurasia on the winter meteorological disasters is studied in China, including cold waves, the cryogenic freezing rain and snow weather, consecutive rain, and drought.The main results are as follows:(1) Existing blocking indices have been classified into four:the departure method, the T&M method, the PV-θindex, and the circumfluent type method. Using the blocking highs associated with the weather disaster in early 2008 over China for instance, a comparison and analysis of these four objective methods are studied. Results show that every method has its own merits and flaws. Comparatively, the departure method and the T&M method are easy to use and the PV-θindex and the circumfluent type method may have a better ability to define blocking episodes. Totally, every method for identifying blocking provided here can be used feasibly by operational departments and can be chosen to be compatible according to their need. However, the PV-θindex represents a new breakthrough for blocking indices which have previously used only geopotential height and/or the character of circulation. Furthermore, the ability of PV-θindex to quantify the occurrence of blocking could also satisfy the need for applying a consistent definition.(2) The PV-θblocking index is used to search the blocking highs of the Eurasia during 1950-2008. The results are shown as follows:the annual blocking frequency of the Ural is more than in the Baikal and the Okhotsk in the three regions of the Eurasia. The seasonal variations of the blocking highs of the three regions are different from each other. In the Ural, the annual blocking frequency is more in the winter, spring and summer than in the autumn; In the Baikal, the annual blocking frequency is more in autumn, summer and winter and less in spring. In the Okhotsk, the annual blocking frequency is more in summer and winter and less in spring and autumn. In the background of global warming, during 1950～2008, the blocking frequency/days increase linearly and significantly in Ural and Baikal, the trend of the blocking frequency/days in the Okhotsk is also ascending, but not so significant. The period analysis by using Morlet wavelet method shows that in recent 59 years, there are periods of 2a and 4a for the blocking activity in the Ural. The blocking frequency changes with period of 4～5a in the Baikal and 6～8a in the Okhotsk.(3) The blocking highs in the three key regions of the Eurasia are dominated by the ocean-atmosphere variability associated with ENSO directly or indirectly. Blockings in the three key regions are suppressed (enhanced) during the winter of EN (LN) years; in summer, LN during the developing stage can enhance blockings in Okhotsk. Relatively speaking, the influence of ENSO on the blockings in the three key regions is all unapparent during the developing EN year, the decaying EN and LN year. (4) The blocking highs in the three key regions of the Eurasia are related directly or indirectly to the NAO,EA and PNA. NAO in the negative phase could enhance the blockings in the Ural (the Ural and Baikal) in the winter (spring), but have less influence in the summer and autumn; with no significant influence as the NAO, EA in the negative phase could suppress the blockings in the three key regions in the winter, enhance the blockings in the Ural and eastern Baikal with negative phase in the autumn, and have less influence in the spring and summer; PNA could enhance the blockings in the Ural and Baikal with negative phase in the winter, enhance the blockings in the Ural and western Baikal with positive phase in the autumn, and have less influence in the spring and summer.(5) The influence of the blocking highs in the three key regions of the Eurasia on the winter disasters is studied in China, and the results show that, above 40% of the total cold waves were caused by blockings highs in the winter, indicating blocking highs have much influence on the cold waves, mainly by two aspects with the long time persistent steady posture and the cold air as the ridge extending to the north. As a important factors for the disaster with cryogenic freezing rain and snow weather in early 2008, blocking highs in the Eurasia are mainly abnormal in three aspects with more (appreciably less) blocking days in the Ural and Baikal (Okhotsk), stronger (weaker) blockings in the Ural (Baikal and Okhotsk), and concentrated blockings in the Ural and western Baikal, compared to the blocking activity during the same period from 1950 to 2008. Blocking highs in the Ural and western Baikal influenced the consecutive rain of south China during January 10th～February 2nd, mainly by long time persisting to bring more small trough and cold air supplied to the cold waves. The composite analysis shows, blocking highs is enhanced in the west of the Ural (Baikal) in winter with (without) arid disaster, statistical result of blockings in the Ural and Baikal also shows corresponding episodes to the drought in the winter, however result also indicates that the factors influencing the winter drought are very complex, maybe include atmosphere, ocean, Qinghai-Tibet Plateau and so on.