Atmospheric Frontal Dynamics

在天气尺度天气图上，中纬度地区的天气锋用延长线表示，将气团分开，性质截然不同。然而，近距离观察显示，典型的锋面是一个过渡区，横锋方向覆盖数十公里，而沿锋方向延伸数千公里。在大尺度上，宽阔而辐合的风型使反差气流在锋区发生碰撞，有缩小和加强的趋势。从仪表塔和飞机上的观测显示，该区域内存在丰富的微结构，以湍流和巨大的垂直运动为特征。小尺度过程起到混合性质的作用，并将动能从区域中耗散开，从而使锋面变宽和减弱。布鲁门建议研究这些过程的相对作用，作用范围大相径庭。由于目前还没有公认的区域内复杂环流的概念模型，布鲁门的方法是假设一组简单的(数学上的)动力学模型，代表两种尺度，几个系数的值不同。将寻求使锋区与其周围的大尺度流态相匹配的解决方案。这些解决方案提供了对该区域的主要特征的预测，即能量耗散率、能量如何在锋区内的较小尺度之间分配以及该区域的宽度。这些预测将与1992年暴风雨现场实验期间收集的观测数据进行比较。这项研究的意义与大规模天气预报和气候模拟模式质量的提高有关。目前已有的模式没有考虑锋区内的耗散，因为小尺度没有得到分辨。本办法的目的是提供这一重要过程的某种表述，以便在未来的模式中使用。

Weather fronts in middle latitudes are represented as extended lines, seperating air masses with contrasting properties, on synoptic scale weather maps. A close-up view, however, shows the typical front as a transition zone, covering tens of kilometers in the cross-front direction, while extending for thousands of kilometers in the along-front direction. On the large scale, broad and converging wind patterns bring streams of contrasting air into collision at the frontal zone, tending to narrow and intensify it. Observations from instrumented towers and aircraft reveal a rich microsture within the zone, characterized by turbulence and large vertical motions. The small-scale processes act to mix properties, and to dissipate and disperse kinetic energy away from the zone, and thus tend to broaden and weaken the front. Blumen proposes to study the relative roles of these processes, acting on widely differing scales. Since there is, at present, no accepted conceptual model of the complex circulation within the zone, Blumen's approach is to postulate a simple (mathematically) set of dynamical models, representing both scales, varying in the values of a few coeffi- cients. Solutions will be sought that match the frontal zone to its ambient large scale flow regime. Such solutions provide predictions of key features of the zone, viz., energy dissipation rates, how energy is distributed among the smaller scales within the frontal zone, and the width of the zone. These predictions will be compared with observations collected during the STORM- FEST field experiment in 1992. The significance of this study is related to improvements in the quality of large-scale weather forecasting and climate simulation models. Currently available models do not take account of dissipation within frontal zones because the small scales are not resolved. The present approach is intended to provide some representation of this important process that could be used in future models.