青藏高原那曲地区风廓线结构的观测研究

【作者】 李剑东；

【导师】 卞林根； 陆龙骅；

【作者基本信息】 中国气象科学研究院 ， 大气物理与大气环境， 2005， 硕士

【摘要】 青藏高原对我国东部地区的天气和气候、亚洲季风、乃至全球大气环流和天气气候都有重要影响,而高原大气边界层作为连接高原独特下垫面和自由大气的桥梁,在上述影响过程中扮演了重要作用。高原大气边界层观测资料的匮乏严重制约着青藏高原天气与气候研究。 中国气象科学研究院于2003年在高原中部那曲地区安装了日本三菱公司的WPR1300型风廓线仪,并于2004年4-9月进行了连续的大气边界层结构探测。风廓线仪探测资料与同期探空资料比较表明,风廓线仪资料具有较高的可靠性。本文利用此次探测资料,通过统计分析和典型个例分析相结合的方法,研究了雨季到来前后那曲地区大气边界层的风廓线结构特征。主要结果如下: 1.风廓线仪、地面和高空及NCEP资料综合分析表明:2004年5月20日之后,那曲地区高空强西风减弱,高、低空风向出现明显转变,大气中水汽含量和地面降水量激增。因此,可将5月20日确定为高原中部雨季开始时间。 2.雨季到来后,水平风速及其垂直切变显著减小:大气低层,尤其是600m以下水平风速的日变化趋势从雨季前的低(午夜)—高(午后)—低(日落)型变为高(午夜)—低(日出)—更高(午后)—低(日落)型;同时,日出后大气低层风速迅速增大的时间也有所推后,高空自由大气动量下传时间推后且下传作用减弱。偏南风分量明显增多,大气低层风向的日际变化加大,但高层风向比较稳定,日际变化不大;随着雨季进一步发展,高、低空风向稳定性检验参数Wc(Wind consistency)显著减小,风向日际变化更大,边界层风廓线结构变得更为复杂,而且大气边界层动量汇效应也有所减弱。大气低层风向随高度顺时针转动的Ekman螺线效应显著。Wc分析表明,雨季前后40天2000-2100m之间日平均风向稳定,日际变化小。结合Ekman理论,指出2004年那曲地区的大气边界层高度为2000-2100m,此高度低于以往多数研究结果.风向频数统计也表明,雨季到来后偏南风显著增多,且逐日风向趋于复杂;随着雨季进程,风向的复杂程度由近地层向大气边界层以上的自由大气层扩展。那曲探空500hPa和400hPa的风向频数统计结果证实了风廓线仪所得结果。 3.雨季前,晴天日出以后混合层有较迅速的发展,强对流混合区超过3000m.而雨季到来后,除强对流天气过程外,日出后强混合层发展时间有所推后,但对流活动持续时间延长。风廓线仪信噪比SNR(Signal-to-noise ratio)分析结果也有类似结论。那曲强雷暴天气发生前,低空常出现强西南或东南风急流,且存在强水平风速垂直切变,两者可能是那曲雷暴天气频发的重要原因之一。 4.在资料分析基础上,本文还讨论了那曲地区大气边界层风廓线结构特征的季节变化,并分析了高原雨季对边界层风廓线结构的影响。 5.风廓线仪资料适用于天气学和气候分析,而且分析所得的低空急流、水平风速垂直切变和SNR对于强降水过程有很好的预报指示意义,同时SNR资料也可用于研究晴天边界层对流活动。更多还原

【Abstract】 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更多还原