在青藏高原大地形及其邻近区域, 低层等压面的资料是从对流层中上层通过外插法插值得到的, 不能代表高原地区近地面的实际天气系统。因此, 在常用的等压面分析方法中, 如何较准确地分析高原近地面的天气系统是个难题。本文引入有限区域矢量场分解的平均调和—余弦算法, 基于σ面坐标及资料, 引入一个满足准地转近似的新变量, 其作用相当于等压坐标中的重力位势, 称为等σ面上的相当重力位势, 在等σ面上给出相当重力位势分布图后, 可直接在等σ面上就能分析出天气系统。在方法介绍基础上, 本文以2008年7月20日08时到21日14时 (北京时) 青藏高原上一次高原低涡东移的个例为例, 对等σ面上的相当重力位势对天气系统和天气形势的描述能力进行考察。结果表明: 在美国国家环境预测中心/美国国家大气研究中心 (NCEP/NCAR) 海平面气压分析场上, 高原附近有一些长期存在的气压异常偏低系统, 高原上也存在很多面积较小气压却异常高 (或低) 的天气系统, 这些系统都是由于外插时受高原地形影响而计算出来的误差, 不是高原地区近地面天气系统的正确反映, 因而无法正确描述近地面高原涡东移出高原并与四川盆地附近西南涡耦合后加强的过程。而运用相当重力位势变量来表示高原近地面的天气形势后, 能够清晰反映高原近地面上此次高原涡东移南压引起低层西南涡加强的过程, 可把高原大地形上的天气分析与下游地区天气形势分析更好地衔接起来, 在天气分析方面具有明显的好处。
Due to the topographic effect of the Tibetan Plateau and its vicinity, some small but strong high and low pressure systems often occur on the isobaric levels of the lower atmosphere by extrapolation, and the real systems cannot be reflected clearly. Therefore, it is still a difficult problem to analyze and diagnose the vortexes near the surface of the Tibetan Plateau with current isobaric analysis methods. In recent years, there has been significant progress in the numerical models, but the problem of weather analyses near the surface of Tibetan Plateau is not yet solved well. One of the reasons is that the pattern of geopotential height contour on the terrain following σ coordinate commonly used in models is similar to the topography and cannot represent the synoptic systems.In the article, a new variable, which is referred to equivalent isobaric geopotential (øe) and satisfies quasi-geostrophic relation, is introduced based on the averaged harmoic-cosine vecter decomposition in a limited region. It can be used in the same way as the geopotential used on p coordinate. As an example, a eastward moving Tibetan Plateau vortex is analyzed. The results show some small but low and high systems always exist over the Tibetan Plateau due to extrapolation in the sea level pressure field, but at σ=0.995 level, the process of the eastward moving Tibetan Plateau vortex coupling with the southeast vortex at lower levels can be depicted well by the distribution of øe, and these artificial systems in the Tibetan Plateau disappear. Therefore, the weather system near the surface of Tibetan Plateau can be analyzed well with the analysis method of equivalent isobaric geopotential height. On the other side, the distribution of øe can link the synoptic pattern over the Tibetan Plateau with that in its lower reaches, which is favorable to reveal how the systems over the plateau impact the weather in the lower reaches.