Characteristics Of Water Vapor Over East Part Of Tibetan Plateau And Its Relationship With Precipitation In Yangtze River

Using the sounding and ground-based observational data in the east and nearby area of Tibetan Plateau and ECMWF reanalysis data from 1981 to 2002 in summer, with many diagnostic methods, first, we analyzed the water vapor transportation characteristics in summer in the east and nearby area of Tibetan Plateau. Second, we studied the interannual characteristics of water vapor transportation anomaly and its corresponding general circulation. Finally, we discussed the relationship between water vapor transportation and precipitation in Yangtze River, and deeply analyzed the influence of water vapor change in the eastern Tibetan Plateau on precipitation in Yangtze River. The results showed as follows:(1) The water vapor transportation characteristics vary in the vertical direction that transporting forms and water vapor source are different in each layer and water vapor flow capacity gradually decreases with the height increasing. The vertically integrated values of water vapor transportation can reflect the main characteristics of different isotonic levels. The eastward water vapor transportation has significant contributions to the latitudinal water vapor transportation over Yangtze River. The main areas of water vapor convergence are Yangtze River, southeastern corner and northwestern part of Tibetan Plateau, and Sichuan Basin. The largest amount of water vapor convergence is Sichuan Basin. The areas of water vapor convergence are generally the rainy centers and rivers.(2) It is different between water vapor transported by transient eddies and stationary disturbance that the former is mainly the longitudinal transportation. The largest transient eddies transportation occurs in Yangtze River, and water vapor is transported northward from Yangtze River. Almost all water vapor northward in the north of Yangtze River is transported by transient eddies. The atmospheric water vapor distribution is basically zonal, and the isotonic levels are distinct from each other, for which the orographic lifting of Tibetan Plateau is very important to the horizontal distribution of atmospheric humidity. The direction of water vapor transported by transient eddies is same as the humidity gradient direction, and is determined only by the latter, while the numerical value depends on the humidity gradient and synoptic-scale disturbance. Furthermore, this kind of transportation has obvious interannual variation.(3) Complex EOF analysis indicates that water vapor transportation anomalies are mainly due to the anomalies of lower-layer water vapor transportation, and middle-high layer anomalies of water vapor transportation are mostly exhibited in the Tibetan Plateau. The more the eigenvector accounts for the total variance, the more consistent of the spatial distribution and intensities of anomalies. The three belts of water vapor transportation in the southeast part of Tibetan Plateau vary at the same phrase. Divergence (corresponding to precipitation decreasing) or convergence (corresponding to precipitation increasing) anomaly of water vapor in Yangtze River is the most remarkable mode. When the West Pacific Subtropical High Pressure (WPSHP) is strong and shifts southward and westward, Indian Monsoon Low Pressure (IMLP) is weak, Ural and Okhotsk are occupied by High Pressure Ridge, and Baikal is Low Pressure trough, it is easy to lead to anomalous water vapor convergence in Yangtze River. When WPSHP is weak and shifts northward and eastward, IMLP is strong, from Baikal to Middle Siberian there exist High Pressure Ridges, and Japan Sea is little High Pressure Ridge, it is unfavorable for water vapor convergence and could lead to anomalous water divergence in Yangtze River.(4) The water vapor transportation over east part of Tibetan Plateau has an important effect on precipitation in the middle and upper course of Yangtze River, and the effect is not only related to the intensity of itself, but also to the position of West Pacific Subtropical High Pressure(WPSHP). RegCM3 is efficient to simulate the main position, intensity and intra-seasonal variability of summer precipitation in 1993 and 1983. When water vapor over Tibetan Plateau flows eastward, it extends to Yangtze River along with the periphery of WPSHP. It is obvious that the precipitation region depends on the position of WPSHP. When the precipitation reduces, the latent heat flux also decreases, in addition to the influence on radiation caused by water vapor decreasing, the thermal field will be changed and could generate a cyclonic circulation on the left of WPSHP which will alter the water vapor transportation and lead to anomalous precipitation.