利用RAMS模式对山谷城市冬季局地风场的数值模拟

利用美国科罗拉多州立大学和MRC/ASTER发展的中尺度数值模式RAMS， 采用三重嵌套的方法， 模拟研究了兰州山谷地区局地环流特征。结果表明： （1）兰州市近地面流场以偏东风为主， 在城市东西部之间的狭窄地带， 风速相对较大些， 在东西部山谷城市中心区域有大片的静风区； 冬季兰州市山谷夜间是辐合流场， 白天是辐散流场。受城市热岛环流的影响， 白天热岛环流抑制谷风环流， 夜间增大山风环流， 夜间的山风风速大于白天的谷风风速。（2）白天， 兰州市区山顶和山谷之间上空气柱以下沉气流为主， 这主要是由于地形作用使得白天盛行谷风环流和山峰加热作用的共同影响。夜间， 兰州市区山顶和山谷之间上空以上升气流为主， 这主要是由于地形作用使得市区和山谷在夜间盛行山风环流， 但是冬天夜间兰州市区和山谷上空有较厚的逆温层存在， 抑制了气流的上升运动。（3）在午后13：00左右， 兰州市区山谷从近地面到400 m高度， 辐散场在逐渐减弱， 在510 m左右的高度转变为辐合场； 皋兰山顶上空从近地面到400 m高度， 辐合场在逐渐减弱， 在510 m左右的高度转变为辐散场。在凌晨01：00左右， 兰州市区山谷从近地面到400 m高度， 辐合场在逐渐增强， 到400 m高度达到最强， 从400 m到510 m高度又逐渐减弱； 皋兰山顶上空从近地面到220 m左右的高度， 辐散场在逐渐减弱， 在400 m左右的高度辐散场转变为辐合场， 从400 m到510 m左右的高度， 皋兰山顶的辐合场逐渐增强。

Numerical Simulation of Local Circulation in Valley City in Winter Using RAMS Model

The characteristics of local circulation in valley city were simulated using the mesoscale numerical model RAMS developed by Colorado State University and the MRC/ASTER and triple-nested simulation method. The results are as follows:(1) Wind direction near the ground of the Lanzhou city is main from east to west,and the wind speed is relatively larger more in the narrow area between the eastern and western city. In the eastern and western center of the valley city have large areas of the static wind zone;near the surface wind in the valley city of Lanzhou is convergence in the winter night,and during the day is a divergence flow. By the effect of the urban heat island circulation,the daytime heat island circulation inhibiting the valley breeze circulation,increased night-time mountain breeze circulation,and the velocity of the mountain breeze at night is larger than the velocity of the valley breeze in the daytime. (2) During the day,there is downdraft above the Lanzhou city and the valley between the mountains. It′s mainly due to the terrain effect and the heating effect of mountain peak. At night,there is updraft above the Lanzhou city and the valley between the mountains. It′s mainly due to the terrain effect which makes mountain breeze circulation in the night,while there is strong inverse temperature in exist above the Lanzhou city and the valley between mountains which restrain the updraft of the air in the winter night. (3) Around 13:00,the divergence gradually weakened from the near surface to 400 m heights above the Lanzhou city,then it turned into convergence at the heights of 510 m;the convergence gradually weakened from the near surface to 400 m heights above the Gaolan mountain,then it turned into divergence at the heights of 510 m. Around 01:00,the convergence gradually weakened from the near surface to 400 m heights above the Lanzhou city and was strongest at the height of 400 m,and then it gradually weakened from 400 m to 510 m heights;the divergence gradually weakened from the near surface to 220 m height,then it turned into convergence at the heights of 400 m. The convergence gradually enhanced from 400 to 510 m heights above Gaolan mountain.