青藏高原绕流和爬流的气候学特征

本文利用1951～2008 年NCEP/NCAR 再分析资料, 通过绕流和爬流方程, 将高原附近表层风场分解为绕流分量和爬流分量两部分, 计算出了实际大气中的绕流和爬流运动的强度, 分别探讨它们的气候态特征。结果表明:高原主体年平均绕流场围绕高原地形并在高原西南部(32°N, 75°E)附近产生分支, 分支点下游的高原主体南部和北部分别表现为气旋性和反气旋性流型;年平均的爬流分量场沿喜马拉雅山脉辐散, 高原主体为偏南上坡风, 东北部为偏北上坡风。夏季绕流场为气旋式流型, 中心位于高原中部(35°N, 90°E)附近;秋季绕流场围绕高原地形边缘基本为一个反气旋流型。夏季, 高原主体偏南风爬流与偏北风爬流在高原南北中线附近辐合, 除夏季外, 沿高原南侧喜马拉雅山脉为爬流辐散区。高原主体和高原附近的关键区内, 绕流和爬流存在不同的季节循环特征。从绕流和爬流分解公式出发, 本文详细探讨了表面流场的绕流和爬流运动各分量对地形高度及地形梯度的依赖性:经向绕流与纬向绕流比值、经向爬流与纬向爬流分量比值为仅依赖于地形高度的定常值。年平均的绕流及爬流矢量强度随着所处地形高度的升高而逐步增强;从区域分布的角度而言, 高原附近绕流强于爬流的区域范围较广, 绕流占主导地位。地形纯动力强迫产生的爬流运动与观测资料中高原附近的垂直运动具有很高的位置对应关系, 但冬季和夏季均存在强度上的差异。

Climatological Characteristics of Flow around and Flow over the Tibetan Plateau

In this study, surface wind data from NCEP/NCAR for 1951-2008 are decomposed into flow around and flow over the Tibetan Plateau (TP) according to equations that control the surface flow near the orography, and the climatological characteristics of the flows around and over the TP are examined by calculating their zonal and meridional components. The results indicate that the annual mean flow around the TP bifurcates on the southwest part of the TP at nearly (32°N, 75°E), presenting an approximately anti-cyclonic pattern in the north and a cyclonic pattern in the south downstream; in contrast, the annual mean flow over the TP diverges along the Himalayas, with southerly upslope winds in the main body and northerly upslope winds in the northeastern region. In summer, the flow around the TP manifests a cyclonic pattern, with its center located at nearly (35°N, 90°E) on the central TP; in autumn, it presents mainly an anti-cyclonic pattern along the edges. The flow over the TP converges approximately at the north-south midline of the TP in summer, but it diverges along the Himalayas in other seasons. The flows around and over the TP were also found to have different seasonal evolution characteristics in the main body of the TP and key areas nearby. On the basis of the decomposition equations, the relationship between the different surface flow components and the terrain height is also investigated. The ratio of the zonal and meridional components of flow around and flow over the TP were found to remain constant, suggesting that they rely only on the topography gradient of the locations. The annual mean intensities of flow around and over the TP all increase with increasing altitude, and the flow around prevails in terms of the regional distribution, in that there are more regions in which flow around the TP is stronger than flow over the TP. The vertical motions forced mechanically by the TP correspond strongly with those in the observational data by position, but a discrepancy exists in the magnitude in both summer and winter.