| The boundary layer, connecting the particular land surface in Tibetan Plateau and the free atmosphere, plays an important role in the process, which influences the weather and climate in the east of China, Asian monsoon and the global general circulation. However, the spare observation data seriously restricts the research on Tibetan Plateau.The Chinese Academy of Meteorological Science set up a wind profiler (WPR1300, produced by Mitsubishi Electric Corporation) in Naqu site of the central plateau in 2003. The wind profiler worked from April 08,2004 to Sep 05, 2004. Compared with radiosonde data in the same period, the data of wind profiler is high reliable. The variation of wind profile structure in Naqu area with the process of rainy season is analyzed by using the wind profiler data and statistic method. The typical case analysis is also given. The main results are as follows:1 The wind profiler data, site data, radiosonde data and NCEP data are analyzed and the results show that, from May 20, 2004, (1) the strong westerly sharp decreases; (2) there appears obvious transition of wind direction in both upper and lower atmosphere; and (3) vapor content and precipitation rapidly increases. So, May 20, 2004 can be as the onset time of rain season in the central Tibetan plateau in 2004.2 After the rain season begins, the horizontal mean wind speed and its vertical sheer weaken very much; the diurnal variation pattern of wind speed in lower atmosphere, especially below 600m, differs from the pattern prior to rain season. Though the wind speed in lower atmosphere is highest in afternoon, there appears higher wind speed during midnight. After the rain season comes, the wind speed in lower layer doesn't rapidly increase after sunrise. At the same time, the process that momentum is transferred downward lags. The averaged wind speed profile also indicates that this transferring downward process in upper air weakens after rain season comes. The analysis of wind vector shows that, after the onset of rain season, the component of southerly clearly increases, the degree to the variation of day-to-day wind direction enhances, and the wind direction in the high level is stable. With the process of rain season, the parameter of wind consistency (Wc) markedly diminishes, and the day-to-day wind direction varies much more. These change show that the structure of wind profile becomes more complicated and the effect of momentum sink in the boundary layer weakens. The analysis also shows that the Ekman characteristic that the wind directions veer with height is very remarkable. The analysis of Wc parameter also indicates that the daily mean wind direction between 2000m and 2100m is stable and doesn't change much more day by day before and after the onset of rain season. Combined with Ekman characteristic, we think that the thickness of boundary layer is about 2000m to 2100m in Naqu area in 2004, which is lower than that in most previous researches. The frequency of wind direction shows that the southerly increases and day-to-day wind direction becomes more complicated during rain season. With the evolution of rain season, the range in which wind direction is complicated expends from boundary layer to free atmosphere. The statistic results of radiosonde data in 500hPa and 400hPa level have the similar conclusion as that ofwind profiler.3 Before the rain season, the mixed layer rapidly develops after sunrise in clear day, and the strong convective height can extend over 3000m. After the onset of rain season, strong mixing layer delays developing in daytime, but the strong convective activity can last more time. The analysis of signal noise ratio (snr) of wind profiler obtains the same results. There often exist strong low-level southwest jet and wind vertical sheer before storm weather comes in Naqu area, and the two factors above may be important causes of local storm in rain season.4 On the basis of data analysis, we discuss the seasonal variation of structure characteristic of wind profile in Naqu area, and analyze the influence of... |
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