Observations of Thermally Developed Wind Systems in Mountainous Terrain
Observations of Thermally Developed Wind Systems in Mountainous Terrain
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DOI：
10.1007/978-1-935704-25-6_2
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发表时间：
1990
期刊：
影响因子：
--
通讯作者：
C. Whiteman
中科院分区：
文献类型：
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作者：
C. Whiteman
坡谷风系统是在复杂地形地区频繁形成的局地热驱动环流。最近的研究主要集中在导致风系统的沿坡和山谷轴的温度结构上。在这些分析中使用的两种新工具包括地形放大因子和大气热收支，前者量化了地形在山谷轴线上产生整体温度梯度的作用，后者确定了导致温度结构变化的关键物理过程。这两种工具都处于早期开发阶段，主要应用于夜间稳态，并带来了新的概念和理解。最近在奥地利的Inn山谷和科罗拉多的几个山谷的气候证据支持这样一个概念，即山谷风是由水平压力梯度驱动的，而水平压力梯度是由沿着山谷长度变化的温度结构在流体静力学上建立起来的。地形放大因子在评估河谷风系统的强度方面似乎是有用的。山谷大气热收支的几个组成部分已证明难以测量，并且正在经历巨大的不平衡。最近在一系列气候条件下进行的几项实验表明，测量到的夜间地表感热通量太小，无法产生平衡。这可能是由测量误差或非代表性测量引起的。对流和辐射通量散度分量也不确定。提出了一个简单的深谷日风温结构演变的概念模型。在早晨的过渡期间，上坡气流形成过热的山谷侧壁，而山谷中心的补补性下沉产生变暖，最终使山谷下风逆转。在清晨过渡期间，垂直运动在山谷大气中传递能量的关键作用已通过实地和模拟研究得到证实。晚上的过渡时期很少受到观测的关注，关键的物理过程还不为人所知。斜坡风系统的研究主要集中在夜间风流。在外部流动较弱的情况下，孤立山两侧的流动可以很好地理解，但由于山谷内温度结构的持续演变和上覆沿山谷流动的强烈影响，山谷侧壁的斜坡流动变得复杂。最近的实验表明，地形内的热驱动流动可能受到上覆环流的微妙影响。这种影响几乎总是在某种程度上存在，但尚未得到系统的研究。简要总结了近年来关于夜间山谷出口产生的强风和支流流的研究，并对Maloja风和防风系统作了一些评述。本章最后总结了需要进一步研究的课题。
Slope and valley wind systems are local thermally driven circulations that form frequently in complex terrain areas. Recent research has focused on the temperature structure along the slope and valley axes that leads to the wind systems. Two new tools being used in these analyses include the topographic amplification factor, which quantifies the role of the topography in producing bulk temperature gradients along a valley’s axis, and atmospheric heat budgets, which identify key physical processes leading to changes in temperature structure. Both tools are in an early stage of development, are being applied primarily to steady-state nighttime periods, and are leading to new concepts and understanding.Recent climatological evidence in Austria’s Inn Valley and in several Colorado valleys supports the concept that valley winds are driven by horizontal pressure gradients that are built up hydrostatically by the changing temperature structure along a valley’s length. Topographic amplification factors appear to be useful in assessing the strength of valley wind systems. Several components of valley atmospheric heat budgets have proven difficult to measure, and large imbalances are being experienced. Several recent experiments, in a range of climatological regimes, suggest that measured nighttime surface sensible heat fluxes are too small to result in balances. This may be caused by measurement errors or by nonrepresentative measurements. The advective and radiative flux divergence components are also uncertain.A simple conceptual model of diurnal wind and temperature structure evolution in deep valleys is presented. During the morning transition period, upslope flows form over heated valley sidewalls and compensatory subsidence over the valley center produces warming that eventually reverses the down-valley winds. The key role of vertical motions in transferring energy through the valley atmosphere during the morning transition period has been demonstrated by field and modeling studies.The evening transition period has received little observational attention, and the key physical processes are not yet well known. Investigation of slope wind systems has focused mostly on the nighttime flows. Flows on the sides of isolated mountains are reasonably well understood when external flows are weak, but slope flows on valley sidewalls are complicated by the continued evolution of temperature structure within the valley and the strong influence of the overlying along-valley flows.Recent experiments have shown that thermally driven flows within the topography may be influenced in subtle ways by the overlying circulations. This influence is nearly always present to some extent, but has not yet been systematically investigated. Recent research on strong winds that issue from a valley’s exit at night and on tributary flows is briefly summarized, and some comments are made on Maloja winds and antiwind systems. The chapter ends with a summary of topics needing further research.