【作者】 陈磊；

【导师】 田文寿；

【作者基本信息】 兰州大学 ， 大气物理学与大气环境， 2008， 硕士

【摘要】 利用中尺度模式WRF,首先对以兰州为中心包括周围数十公里的地区进行了一系列的敏感性数值试验。在数值实验中分别对不同区域减少了植被覆盖或改变下垫面性质,以对半干旱地区植被和下垫面发生改变后大气产生的局地和非局地的响应进行分析研究;然后论文进一步以干旱区中的一个孤立绿洲-兴隆山区为研究对象,选取兴隆山地区有降水而兰州城区无降水的几个个例进行了实况模拟,对比分析了影响兰州地区和兴隆山地区空中云水分布和收支差异主要因素。诊断分析结果表明:植被变化对区域内温度的影响比较复杂,并有明显的日变化。相比于区域城市化形成的强而稳定的增温中心,植被减少只会在区域形成较弱而间断的增温中心并敏感地依赖于地面净能量在感热、潜热和土壤热通量之间的分配。区域内植被变化和下垫面特性的改变导致的局地温度变化在背景风场的作用下向区域外传播,其传播的细节与风场的特征和地形密切相关。在适当的环流背景下,迎风坡下垫面改变导致的温度变化可在背景风场的输送下,绕过很高的山脊在背风坡形成一个持久的变温中心。植被减少导致的变温中心会在原有的环流形式上叠加一个强迫的二级环流,这一环流深度约为1.1 km。区域植被的减少,一方面减少了地面向上的水汽输送,导致了区域内气柱水汽含量的减少;另一方面增温引起的强迫二级环流会使区域外水汽向内输送,部分地补偿了地面向上输送水汽的减少,但是二者总体的效果是区域内的气柱水汽总量减少。在实验区域之外,上风向趋于增湿而下风向趋于减湿。在兴隆山地区,以植被为主的下垫面向大气的水汽输送较强,有明显的季节变化,以夏季的个例为最大。除春季外,山区主要受东南方向的较湿润的气流影响,同时在所有研究个例中该地区与周围区域都有较强的水汽交换,可以获得区域外的水汽输入。山区上空水汽充沛,云水和雨水含量较高,降水可能性较大;在兰州城区,以城市为主的下垫面对大气有稳定的水汽输送,没有明显季节变化,在夏秋季个例中远低于兴隆山区。城区一直处于来自北方、东北方的较干气流的影响,同时与周边地区的水汽交换微弱,水汽的平流输入小。城区上空云水和雨水含量都较小,降水概率比兴隆山地区小。更多还原

【Abstract】 Using the mesoscale model WRF, a series of numerical simulations are first performed over the area around Lanzhou City of China. In these numberical experiments the model vegetation cover or the type of the landsurface in different patches of the study area has been arbitrarily changed to analysis the local and non-local responses of the atmosphere to changes in vegetation cover and surface types over a semi-arid region. A group of numerical experiments are further performed to simulate some water vapor budget and transport properties over two distinc areas, i. e., Xinlong mountain and Lanzhou city. The cases are selected when precipitation occurred over Xinlong mountain but have no precipitation over Lanzhou city. The results show that:Changes in vegetation cover have a complex effect on the surface temperature with a significant diurnal cycle. Contrast to a strong and persistent warming caused by urbanization of the area, decreases in vegetation cover only give rise to a weak and intermitted warming depending on the partition of the surface net available energy between sensible heat flux, latent heat flux and soil heat flux.The local temperature anomalies caused by the vegetation and surface type change can propagate out of the source region by background winds with details of the propagation depending on wind fields and underlying topography. Under suitable wind conditions, those temperature signals can pass around a mountain to form a persistent warming region lee of the mountain. A decrease in vegetation cover tends to force a 1.1 km deep secondary circulation. Decreases in vegetation cover also lead to decreases in column total water vapour over the area for one hand, and force secondary circulations due to temperature contrast on the other. The forced secondary circulations tend to transport inward the water vapour outside of the area to compensate the decrease of water vapour input from the surface. The integrated effect of a decrease in vegetation cover is to decrease column total water vapour over the area. Outside of the area, the colume total water vapour tends to increase upwind of the area and decrease downwind of the area. Over Xinglong mountain, the vegetated land-surface supply plenty of water vapor to the atmosphere with obvious seasonal variation. The maximum water vapor input into the atmosphere from the surface occurs in the summer cases. The Xinglong mountain area is mainly affected by the moist airflow from southeast except for the spring cases and has strong exchange of water vapor with the area around it. The area has plenty of cloud water and rain water above, and hence, high possibility of rain; Over Lanzhou city, the water vapor supply from surface to atmosphere is stable and has no evident seasonal variation, The total water vapor over Lanzhou city is much lower than that over Xinglong mountain in summer and autumn cases. Lanzhou city is mostly controlled by the dry airflow from north or northeast and the water vapor transport from outside of the city is quite small. Consequently, the cloud water and rain water as well as the possibility of rain over the city are lower than those over Xinlong mountain.更多还原