Stably Stratified Flow in a Shallow Valley
Stably Stratified Flow in a Shallow Valley
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DOI：
10.1007/s10546-016-0191-4
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发表时间：
2016
影响因子：
4.3
通讯作者：
L. Mahrt
中科院分区：
文献类型：
--
作者：
L. Mahrt
作为案例研究，我们基于 20 个声波风速计网络和一座 20 米高、有八层声波风速计的中央塔，对一个深度约 12 米、宽度数百米的小山谷上方和内部的夜间分层流进行了案例研究。为了组织数据分析并与现有文献进行比较，从概念上定义了平缓地形上的分层流的几种状态。在我们的案例研究中，傍晚时浅山谷内形成边缘冷池，但几个小时后产生更大的环境风速，对应于分层地形跟随流，其中山谷外的气流下降到谷底。顺势流持续约10小时，然后在晚间大规模流崩溃时过渡到间歇性边缘冷池。在这10小时期间，分层地形跟随流具有三层结构的特征，包括谷底几米深的薄表面边界层、对应于较大规模流的较深边界层、以及具有显着风向切变和可能产生背风湍流的中间过渡层，即使在我们研究的平缓地形中也可能产生背风湍流。山谷中的水流经常受到振荡的调节，典型周期为 10 分钟。湍流强度较小、持续时间为数十分钟的冷事件在整个地形跟随期间穿过观测域。对这些事件之一进行了详细检查。
Stratified nocturnal flow above and within a small valley of approximately 12-m depth and a few hundred metres width is examined as a case study, based on a network of 20 sonic anemometers and a central 20-m tower with eight levels of sonic anemometers. Several regimes of stratified flow over gentle topography are conceptually defined for organizing the data analysis and comparing with the existing literature. In our case study, a marginal cold pool forms within the shallow valley in the early evening but yields to larger ambient wind speeds after a few hours, corresponding to stratified terrain-following flow where the flow outside the valley descends to the valley floor. The terrain-following flow lasts about 10 h and then undergoes transition to an intermittent marginal cold pool towards the end of the night when the larger-scale flow collapses. During this 10-h period, the stratified terrain-following flow is characterized by a three-layer structure, consisting of a thin surface boundary layer of a few metres depth on the valley floor, a deeper boundary layer corresponding to the larger-scale flow, and an intermediate transition layer with significant wind-directional shear and possible advection of lee turbulence that is generated even for the gentle topography of our study. The flow in the valley is often modulated by oscillations with a typical period of 10 min. Cold events with smaller turbulent intensity and duration of tens of minutes move through the observational domain throughout the terrain-following period. One of these events is examined in detail.