梅雨锋上MCS的发展、传播以及与低层“湿度锋”相关联的CISK惯性重力波

Evolution and Propagation of MCSs over Meiyu Fronts and Inertia-Gravitational Wave-CISK Related to "Low-level Moisture Frontal Zone"

采用一个包含简单积云对流参数化的线性模式讨论在长江流域相当正压性质的梅雨锋上,贯穿整个对流层的深厚惯性重力波的发生、传播和频散性质。结果表明,与积云对流加热场有最密切关系的低层湿度条件及其水平分布不均匀性质都对扰动的波速和稳定度有重要影响,最有利出现不稳定发展的地方是在“低层湿度锋区”南界附近,因此这里是中尺度对流系统(MCS)的活跃地带。关于频散性质的分析表明,能允许群速度大于相速度的低层湿度条件及其水平分布的特征值范围比较宽。而在上述稳定度接近中性的条件下最有利于出现沿“湿度锋区”轴向的上、下游效应。这个结果也许能很好地解释:在梅雨锋上,恰恰是当一个旺盛的MCS达到顶点而开始趋向衰减时,在它的上、下游很快发展起新的MCS,新、旧MCS一起形成“云团波串”这样一个重要而有趣的观测事实。

The so-called “Meiyu front” over the Changjiang River basin is actually an equivalent-barotropic zone in which the horizontal thermal gradient is negligibly weak but the humidity gradient is very strong particularly in the lower troposphere. In the Meiyu season most of the severe MCSs （mesoscale convective systems） with torrential rain may form, develop, move and newly emerge up and down-stream along such “low-level moisture frontal zone”. What kind of wave disturbance most relevant to these activities of MCSs is likely the deep mesoscale inertia-gravitational waves in company with the macroscopic cumulus cloud ensemble heating, which will be named “inertia-gravitational wave CISK” hereafter. A three-dimensional linear model in a dimensionless vertical coordinate [pseudo-height z=-In（p/p0 ）]with a simple parameterized cumulus heating expression is most suitable to be used to discuss the deep mesoscale disturbances analytically. In this model some assumptions based on the characters of the thermal, humidity and wind field near the Meiyu front over the Changjiang River basin are made to investigate the behaviors and the weather influences of the inertia-gravitational wave CISK around a “low-level moisture frontal zone” on the MCSs activities. The results show that the low-level condition of the humidity and its horizontally nonhomogeneous characters directly related to the cumulus heating field have an important influence on the wave speed and stability of the disturbances. The areas where the instable evolution occurs most possibly are around the south border of the “low-level moisture frontal zone”. This leads to that the MCSs are likely most active in this area. So, when the dispersion of the wave disturbances and the possibility of the occurrence of “up and down-stream development” are discussed, more attention must be paid to those wave disturbances taking place near the south border of the moisture frontal zone and propagating basically along this frontal zone. In these situations, it have been found the range of the characteristic values indicating the low-level humidity condition and its horizontal distribution, within which the envelope velocity is permitted to exceed the phase velocity and then the MCS will develop over both up and down-stream sides along the axis of the “low-level moisture frontal zone”. The range of these values is quite wide, which means that the MCSs may be easy to develop up and downstream. Particularly, it is most favorable when the stability conditions mentioned above is nearly neutral. Actually the range of the characteristic values is wide on the stable side and narrow on the instable side. This result may be used to explain the following important and interesting observational phenomena： along the Meiyu front, once a vigorous MCS has developed to attain its peak and then tend to weaken, some new MCSs will rise rapidly on both sides to its “upper and lower reaches” to form a “cloud cluster wave train” with the existing former MCS. It does not mean that the instability and dispersion of the inertia-gravitational wave-CISK are the only mechanism of the genesis of MCSs on both “up and down-stream” sides. Other mechanisms, including the “dry” inertialgravitational wave in some stratification atmospheric layer, may also lead to the MCS development.