半干旱区植被减少与城市化对大气的局地和非局地影响的数值模拟

Numerical Simulation of Local and Non-Local Effects of Vegetation Change and Urbanization in a Semi-Arid Region in China

利用中尺度模式对一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了模拟分析。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。相比与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。在中等风速条件下,近地面约1K的区域变温所造成的二级环流深度可达到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。

Using a mesoscale model, the local and non-local responses of the atmosphere to changes in vegetation cover and surface types in a semi-arid region are analysed. The results show that the 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.