南海夏季风爆发前后华南地区大气结构和能量收支的变化

本文计算、分析了1981年南海夏季风爆发前后华南地区平均的大气动力学和热力学结构,水汽和能量收支情况。分析表明,在季风爆发前后这些物理量场,特别是垂直运动场,都经历了一次明显的变化。南海夏季风的建立,是以副热带高压从南海东撒,越赤道气流北进为特征。它与以“爆发性涡旋”为开始,降水与西南气流同时开始的印度夏季风有所不同。另外,南海夏季风建立过程,主要变化发生在中、上层大气之中。这说明夏季风的建立,主要是大气环流季节性调整的结果,海陆差异需要有合适的大气环流配合才能产生明显的季风。对大气能量收支的分析表明,夏季风爆发前,华南地区主要是动能向总位能转换。所产生的总位能部分辐散到边界之外,部分消耗在对流层上层非绝热冷却过程之中,因而在对流层下部总位能才有稍明显的增长。夏季风爆发后,整个能量循环几乎相反。总位能大量转换为动能,以维持南海夏季风的增长。而整层大气中的非绝热加热以及总位能的辐合又补充了大气中总位能的损耗。潜热释放是夏季风爆发后大气的主要非绝热加热过程,它是南海夏季风维持,发展的主要能源。

THE VARIATION OF THE ATMOSPHERIC STRUCTURE AND ENERGETIC BUDGET OVER THE SOUTHERN CHINA BEFORE AND AFTER THE ONSET OF SUMMER MONSOONS

In this paper, the variation of the atmospheric dynamic,thermodynamic structures and the moisture,energetic budget over the southern China before and after the onset of summer monsoons was investigated. The results show that the establishment of monsoons over the South China Sea is an adjustment process of the general circulation, which happens mainly in the middle and upper troposphere. The main features are: the southern Asian high is shifted northwestward into the Indo-China Peninsula in the upper troposphere, whereas the subtropical high over the South China Sea stretches westward for a few days and then withdraws eastward in the middle-lower layer. The eastward retreat initializes the SW monsoon flow all over the South China Sea. As a result of this change, strong rising motion takes the place of sinking motion. The amount and direction of vapour transport change before and after the onset, however, significant increase in vapour convergence and heavy rainfall occurs only after the onset. All these features are obviousely different from those of the Indian monsoon onset process, which is characterized by the monsoon onset vortex, and the simultaneous advance and feedback mechanism of the SW monsoon and its rainfall. The change of energy budget is also distinctive. Before the onset, a large amount of kinetic energy is converted into the total potential energy. Then part of it is exported outside the region and the rest is consumed by the diabatic cooling in the upper troposphere. After the onset, the energy cycle is almost reversed. The diabatic heating in the troposphere generates a large amount of total potential energy, and the convergence term of potential energy flux also makes a positive contribution. This increase of potential energy is converted into the kinetic energy to maintain and speed up the summer monsoon over the South China Sea. It is also shown that the change of wet hydrostatic energy after the onset is mainly determined by the change of latent heat. In other words,the latent heat released by the convective process is the most important factor to the maintenance of the Indian monsoon and the East Asian monsoon.