A Synoptic Climatology Of The Bimodal Precipitation In West China

Based on the station-based precipitation data, hourly precipitation grid precipitation data combined by Chinese station observations and CMORPH products covering the years 1951-2012 compiled by the National Climate Center of Chinese Meteorological Administration, daily grid precipitation data over global land and the observed daily and hourly precipitation data during 1961-2012 from Shaanxi Meteorological Bureau, as well as the NCEP/NCAR,NCEP/DOE AMIP-II and high-resolution NCEP GFS reanalysis data, the climatological features of the bimodal-distributed precipitation over the West China, various time parameters of the rainy seasons, the characteristics of atmospheric circulation for the years and stages with anomalous rainfall and a typical case of West China autumn rainfall are analyzed by using composite analysis, K-means clustering analysis, wavelet analysis, linear regression and polynomial fitting in this study.In this paper, the study area(West China) mainly includes the most part in southwest China, southeastern Northwest China and western Central China(26o–37oN、93o–111oE). Due to the unique geographic location, the precipitation over the West China, simultaneously influenced by the Indian and East Asian summer monsoon, is characterized by a bimodal pattern for the annual cycle but different from Yunnan and Hainan province bimodal type precipitation, which is more complicated than the single-peak monsoon precipitation over the eastern East Asia.Divided by 111°E, there is one precipitation peak over the East China and two peaks over the West China. The duration of rainy season over the West China is one month longer than that over the East China. The rainy season of West China has an obvious multi-stage character, and is divided into seven different stages according to the threshold of 1mm/d, 2mm/d(marking the rainy season onset and retreat), and 4mm/d(the value to distinguish the peak phase and the break phase). The stages as follows: phase 1st is rainy season preparation(30th March to 30 th April), phase 2nd is beginning of rainy season(1st May to 11 th June), phase 3rd is summer rainfall peak period(18th June to 26 th July), phase 4th is climatic monsoon break period(27th July to 13 th August), phase 5th is autumn rainfall peak period(14th August to 16 th September), phase 6th is the last period of rainy season(17th September to 13 th October), phase 7th is the period after rainy season(14th October to 28 th October). Climatically, the rainy season over the West China onsets at 1st May and retreats at 13 rd October including the summer rainfall peak period and autumn rainfall peak period, corresponding to the third phase and the fifth phase respectively. The date for the summer precipitation peak is 6th July and the daily precipitation amount is 5.4 mm/d, while the date for the autumn precipitation peak is 31 st August and the daily precipitation is 4.4mm/d. The climatic monsoon break(CMB) occurs in 4th August, which usually corresponds to the drought period in summer before or after CMB. The monsoon precipitation is divided by the monsoon break into two parts, summer and autumn precipitation, respectively.Corresponding to the multi-stage characteristics to rainy season in West China, the atmospheric circulation in each stage has its unique situation and allocation. It is under the control of summer monsoon system from early May to the first dedak of October according to the potential pseudo-equivalent temperature and vertical velocity. At the first stage, in the low level there are two cyclonic circulation symmetrically existing on both sides of the equator over Indian Ocean; in the mid-troposphere the western pacific subtropical high(WPSH) presents zonal distribution feature, its ridge line lies south of 20oN, there is very little rain in this stage because of under the controling of westerlies. At the second stage, the cross-equatorial flow was established, the ridge line of WPSH fracture over Indian Ocean, the rainfall is increasing in West China. At the third stage, the cross-equatorial flow enhance obviously and northward extend bring a lot of water vapor for West China through southwest monsoon,the WPSH strengthens with its ridge line moving northward by around 5o, and the westerlie trough and ridge control West China, these weather system converges over West China leading to heavy precipitation in summer West China. At the fourth stage, in the low level, the cross-equatorial flow turns to eastward converging into WPSH in right angle; at mid-troposphere the WPSH is weak while moves northeastward, with the ridge line lies just over West China, and westerlies jump northward, these systems diverge and lead to CMB phenomenon in West China. At the fifth stage, the cold air affects the north China at low level, the WPSH extends westward obviously with its west point to less than 110oE, the low level somali cross-equatorial flow converge into southeasterly of south part of WPSH providing an abundant supply of water vapor for West China. The westerlies retreat to West China again at high level, there is a convergence situation over West China leading to a heavy rainy season again, called autumn peak rain season. At the sixth stage, the low level cross-equatorial flow transport water vapor indirectly through monsoon trough for West China while it mainly retreat to the ocean. The cold high pressure forms in mainland China, and West China is under the control of south wind of it’s back part. The WPSH retreat southward to area between coastal of southern China and Hainan province at mid-troposphere with it’s west ridge point extending westward to 100oE, all these condition favors the increase of precipitation in West China. At the seventh stage, East Asian summer monsoon convert to winter monsoon as the rapidly developing of continental cold high, and lead to the end of rainy season. This shows that the concentrated rainfall come into being at the summer(autumn) peak rainy period because of northward jumping(southward retreating) of WPSH, and staying steadily over West China latitude of westerlies jet stream, and the water vapor transporting from the southeast wind by low level cross-equatorial flow and south flank of WPSH.The bimodal-type precipitation process generally exhibits distinct features in the temperature and humidity fields corresponding to the different stage of rainy season of West China. Based on the evolutions of pseudo-equivalent potential temperature and vertical motion, it is found that the summer monsoon in West China starts in early May and ends in early-mid October. The high frequency of cold air activity in West China favors the increase in relative humidity and condensation of water vapor, which therefore enhances the precipitation rate over this region. Moreover, the front induced by the cold air activity is one of the key trigger conditions for the precipitation. In summer, the water vapor supply for the precipitation in West China is mainly dominated by the southwesterly flow, which is primarily controlled by the cross-equatorial flow. In contrast, the water vapor supply is mainly dominated by the southeasterly flow associated with the WPSH in autumn. The WPSH is the most important large-scale system in influencing the precipitation in West China, and plays a crucial role in modulating the bimodal-type precipitation event over this region.Autumn rainfall of West China exhibits obvious interannual variability. Composite analysis indicates that the atmospheric circulation is significantly different between the above-normal and below-normal rainfall years in this region. For the former case, an ?-pattern of the temperature anomalies at 850 hPa is observed in Asia. Consequently, the cold air masses from the east and the west sides of this pattern result in an occluded front and induced above-normal rainfall in West China. For the latter case, however, the cold air activity is relatively weak and zonal flow dominates over Asia, leading to below-normal rainfall in West China. Additionally, the heat source in the mid-upper troposphere during the above-normal-rainfall years is remarkably larger than that in the years with less rainfall. Also, positive temperature anomaly occurs over the region downstream the Tibetan Plateau during the above-normal-rainfall years, favoring the accumulation of energy over West China; and vice versa. Moreover, the southerly flow in the low troposphere, originating in the Indian Ocean and with warm and moist air masses, is intensified over the region from the eastern Tibetan Plateau to West China during the heavy-rainfall years. At 500 hPa, there exists a deepened trough over the Lake Balkhash and a strengthened and westward-shifted subtropical high over the northwest Pacific, and vice versa. The area of the high-level jet axis on the 100 hPa level is increased and the intensity is strengthened during the years with more rainfall. The rainfall of West China is affected strongly by the WPSH at the 3rd, 4th, 5th, 7th stage, and with different situations. For the summer rainfall, the WPSH shifts westward and the intensity over the south to the Yangtze River is stronger than normal years. During the rainy season break, the locations of subtropical high in the years with more rain, normal rain and less rain arrange from the southwest to northeast, and the area become smaller, there exists difference both in zonal and meridional directions. For the autumn rainfall, the subtropical high shifts westward during its southward retreat to the south to the Yangtze River. The west ridge point is approximately at 100°E when the precipitation is heavier; while it is at 120°E when the precipitation is lighter. After the rainy season, the ridge line of the subtropical high retreat southward to the south to 20°N and the high cell present a belt pattern. Different from the summer and autumn rainfall period, when the precipitation is smaller the high cell shifts westward blocking the water vapor supply from low latitudes and as the precipitation decrease the high cell is even fractured. Furthermore, there exists significant 6–8a period of autumn rainfall in West China by period analysing.The typical case of autumn rainfall in West China can be illustrated the evolution of the atmospheric circulation and the corresponding relation with the intensity of rainfall in detail. An example of the typical autumn rain over the West China is in September 2011. During that time, the atmospheric circulation background over Guanzhong area, which delivers plenty of water vapors and facilitates for the rainfall, includes the strong subtropical high over northwestern Pacific, westerly trough and ridge, the formation and accumulation of surface cold air over middle and high latitudes, the anticyclone over the North China, the monsoon trough over northwestern Pacific, the typhoon and cross-equatorial flow to the east of East China Sea. Different from the summer rainfall, the westward input of water vapors is closely linked with the start, retreat and intensity of autumn precipitation, thus, the easterly wind or easterly low level jet over Jianghuai area can be an index for the occurrence of heavy rainfall over Guanzhong region. The occurrences of the autumn typhoons and their paths and intensities significantly affect the Guanzhong rainfall. When the paths of typhoons are to the north and east, the typhoon intensity is weaker and the precipitation is lighter. When the subtropical high is stronger and westward than normal, the typhoon paths are to the west and their intensity are stronger and the corresponding precipitation is heavier.