斜坡地形對邊界層風場日變化振盪的效應

The Effects of a Sloping Terrain on the Diurnal Oscillation of the Boundary Layer Wind

林沛練(Pay-Liam Lin)

《大氣科學》 12卷1期 (1985/03) Pp. 63-80

摘要

本文以理論採討以及數值模擬的方式來研究熱力強迫作用在斜坡地形上風場日變化振盪中所扮演的角色，我們知道對平坦底面上之旋轉成層流體而言，其主要的作用力為科氏力、氣壓梯度力以及摩擦力（或黏滯力），因此在這種我們所熟知的艾克曼邊界層（Ekman Boundary Layer）中成層之作用只扮演了一個次要的角色，但是當邊界層之底面非為平坦情況而是一種斜坡面時，由理論推演以及模擬之結果均指出只要底邊界之斜坡面上有一點熱力之強迫作用存在，艾先曼層的特性就會為平行於斜坡面的重立分量所修飾，這時浮力與其他的主要作用力具相同之數量級大小，對加熱之斜坡地形而言，由於動量場與熱力量之耦合將會產生一沿斜坡面之浮揚力而激發或加強跨越等壓線之氣流。其次由本文之模擬結果也指出不均勻的斜坡加熱分佈，將會因為有浮揚艾克曼抽拉作用的存在而加強斜坡靠近底面處之垂直運動，反之，對均勻的斜坡加熱而言，最強之垂直運動會發生在接近斜坡頂部的地方。

並列摘要

Diurnally periodic motions forced by diurnally fluctuating buoyancy forces above the sloping terrain are studied by numerical experiments and by simple theoretical scale analysis. The purpose of this study is to take interest in the understanding the physics of a Buoyant-Ekman layer under the influence of diurnal beating and cooling of the underlying sloping surface and the role of thermal forcing in the diumal oscillation of the planetary boundary layer wind above sloping terrain.The scale analysis and the numerical experiments show that the terrain slopes as gentle as 10^(-2) or 10^(-3), the buoyancy force parallel to the terrain becomes as important as the coriolis and pressure gradient forces. The characteristics of the Ekman layer will be modified as long as temperature perturbations occur at the bottom boundary.Because the gravitational force vector has a component parallel to a sloping boundary, the diurnal temperature oscillation in the boundary layer provides a source of potential energy which derives a diurnal oscillation in the boundary layer wind, thus the Ekman boundary layer and the thermal boundary layer are coupled. lf the surface heating is homogeneous, the pattern of the upslope wind circulation is located near the top of the sloping terrain. Otherwise, in the case of an uneven distribution of surface heating, the pattern of circulation is confined to the lower part of the slope.