Atmospheric Frontal Dynamics

在天气比例尺天气图上，中纬度地区的天气锋面用延长线表示，将具有不同性质的气团分开。然而，近距离观察显示，典型的锋面是一个过渡区，在横锋方向覆盖数十公里，而在沿锋方向延伸数千公里。在大尺度上，宽阔和辐合的风型使对比鲜明的气流在锋面区碰撞，使锋面区变得狭窄和加强。从仪器塔台和飞机的观测显示，该区域内有丰富的微观结构，其特征是湍流和大的垂直运动。小规模过程的作用是混合性质，消散和分散该区的动能，从而使锋面变宽和减弱。Blumen建议研究这些过程在不同尺度上的相对作用。由于目前还没有公认的区域内复杂环流的概念模型，Blumen的方法是假设一组简单的（数学上的）动力学模型，代表两个尺度，在几个系数的值上变化。将寻求解决方案，使锋面区与其周围的大规模流动状态相匹配。这样的解决方案提供了该区域的关键特征的预测，即能量耗散率，能量如何在锋面区域的较小尺度之间分布，以及该区域的宽度。这些预报将与1992年STORM- FEST野外试验期间收集的观测结果进行比较。本研究的意义在于提高大尺度天气预报和气候模拟模式的质量。目前可用的模式没有考虑锋面区内的消散，因为小尺度没有得到解决。目前的方法旨在提供这一重要过程的一些表示，以便在未来的模型中使用。

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.