The Characteristic Of Precipitation Over The Tibetan Plateau And Its Association With Water Vapor

Based on high resolution (0.5 degrees) data of the daily precipitation over the Tibetan Plateau (hereafter TP) from 1961 to 2010, the spatial and temporal characteristics of precipitation over the TP are studied. Applying monadic linear regression model, the cubic polynomial curve fitting and other statistical methods, the trend and distribution of annual and seasonal precipitation over the TP are analyzed and discussed during the studied period. Combined with the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim hereafter), the distribution of average water vapor transport flux and the water vapour transported by the large-scale circulation in summer over the TP is calculated and studied. The conclusions are as follows:(1) The distribution of average annual precipitation over the TP is uneven and has spatial differences, decreasing from the southeastern to northwestern TP. At the same time, the precipitation exists seasonal differences, with largest/smallest amount in summer/winter, followed by spring and autumn. The distribution of precipitation in each season is alike to the annual average precipitation contributed mostly by summer. The trend in the east/west of 102°E is quite different in the TP. The trend of seasonal average precipitation is obvious, dominated by region and time. Regions with larger average precipitation have larger trend magnitudes. (2) From the average water vapor transport, four moisture paths influencing the precipitation over the TP are found, which are defined as the paths of westerly, Arabian Sea, the northern side of Bay of Bengal and South China Sea, respectively. Compared with the rest paths, the influence of the westerly path is weakest, which only has an role in the northwest TP from Shiquanhe to the south of Tarim Basin; The northern side of Bay of Bengal path has an effect in the central-southeastern TP; the path of the South China Sea dominates the southeastern and central-south of TP such as Naqu, Linzhi, Changdu, Yushu and other regions; Arabian Sea path has negative correlations with the rest three paths, especially with the northern side of Bay of Bengal path (R=-0.65, P<0.05). Furthermore, Arabian Sea path indirectly modulates the central-southwestern TP by adjusting the intensity of both the northern side of Bay of Bengal path and South China Sea path. These results suggest that both the water vapor transport path and pattern of precipitation over the TP is coherently connected. (3) The TP is divided into five boundaries by a reference to the 3000 m contour, and the flux of horizontally and vertically integrated atmospheric water vapour over the TP with its surrounding has been calculated. The time-averaged vertically integrated water vapour fluxes enter the TP through the southern, southwestern, western and northern boundaries. And water vapour transport is not simply input or output through one boundary. The time-averaged fluxes of water vapour on each layer show that water vapour transport to the outside/inside TP comes from different level (above/below 600 hPa) of the eastern boundary and the main exchange of water vapour occurred below the level of 500 hPa, which cannot reach the higher layer of the southern and northern boundary. There is an outflow region from 76°E to 83°E on the vertical section of northern boundary. It may be related to the topography blocking effect of northwestern TP. (4) In the same area, the influence of mid-latitude westerly shows the opposite effect of the South Asian Monsoon. In years of strong/weak mid-latitude westerly or weak/strong South Asian Monsoon, the water vapour transport from northern boundary becomes stronger/weaker and from southern boundary becomes weaker/stronger.