【作者】 陈磊；

【导师】 田文寿；

【作者基本信息】 兰州大学 ， 大气物理与大气环境， 2012， 博士

【摘要】 干旱半干旱地区受全球变暖和人类活动因素的共同影响,存在严重的植被退化、土地退化以及荒漠化的形势,特别是在中国西北的干旱半干旱区,植被的减少或退化对当地和生态和气候环境形成了潜在的威胁。深入了解干旱半干旱区天气、气候对植被变化的响应机制和特征,可为防治土地的进一步退化和荒漠化提供重要的理论依据和技术支撑。本文首先利用一个中尺度数值模式(WRF),通过对实际天气个例的数值模拟、敏感数值实验并结合长期观测数据,研究了一个在中国西北的半干旱区长期存在和维持的森林山区(兴隆山区,103.84E,35.86N)的降水特征以及与周边地区的降水特征的差异,并探讨了造成这种差异的主要原因。模拟分析表明,有利于的水汽输入的大尺度环流形势、地形对空中水汽的截留以及该地特殊的地质因素是兴隆山区的孤立森林岛在半干旱地区中长期的存在和维持的原因,其维持机制与沙漠绿洲有明显的差异。兴隆山区与周边地区的降水差异主要出现在夏秋季。兴隆山区在夏秋季受到东南湿润气流的影响,可以获得较多的水汽输入和较稳定的水汽来源,而山地地形则有利于截留东南气流携带的水汽并形成降水；兴隆山区与其周边地区局地的蒸散差异对二者之间的降水差异的贡献不大。另外,兴隆山区的土壤堆积覆盖的石质山构造和森林下垫面也有利于降水的截留和贮存以及植被的生长。本文进一步利用一个区域气候模式(Regcm3),选择中亚的中蒙干旱半干旱区作为研究区域,研究了该区域不同程度的植被减少引起的区域气候响应,探讨了其响应机制和不同下垫面上的响应差异。模拟分析表明,植被减少总体上会导致地表气温的上升和年降水量的下降,并且年降水的下降主要是由对流性降水的下降导致的。不同程度的植被减少导致的降水下降是接近线性的。研究结果还表明草地区域气候相对于森林而言是对植被减少响应最为敏感的区域,植被减少将会导致降水和对流性降水的显著下降。植被减少引起的区域之间地表热力对比差异的变化和地形的共同作用会在草地和半荒漠之间的过渡区形成一个异常反气旋环流。这一异常反气旋一方面与地形绕流的方向有着密切的联系,同时受到地面温度梯度改变引起的二级环流方向的影响。异常反气旋环流的存在会加强过渡区气候对植被减少的响应。在该异常环流和植被减少导致对流增强的共同作用下,中亚半干旱过渡区草原区域的气候对植被减少的响应最强。在全球变暖的大背景下,植被减少会减缓区域降水下降趋势,加强区域的增温趋势。植被减少造成的温度和降水趋势的变化相对于区域温度和降水的背景趋势变化而言不是很显著,不过在局部地区可以显著的改变温度和降水背景变化趋势。本文还进一步比较了相同程度植被增加和植被减少导致的气候响应的差异以及植被变化导致气候响应的季节变化。分析结果表明,两者产生了基本上相反的气候响应,但都是通过对流性降水影响总降水。由于对波文比的影响不同,植被增加导致的蒸散和降水的增幅小于植被减少导致的蒸散和降水的降幅,表明区域整体降水对植被减少更为敏感。区域气候对植被增加最敏感的响应区域是落叶森林而森林到草地的过渡区并不敏感。另外,小幅度的植被增加与植被减少都会导致试验区域水汽输入增加。同时,植被变化导致的气候响应在6月和夏季最强,其他季节较弱,气候响应的年变化主要由夏季的变化决定。研究结果还表明,植被增加导致的地面温度梯度变化在过渡区形成一个向北倾斜的异常气旋,对降水有进一步促进作用。更多还原

【Abstract】 Under the combined effect of global wanning and human activities, arid and semi-arid areas are faced with severe vegetation degradation, land degradation and desertification. The ecololy and environment in the arid and semi-arid regions of northern China, in particular, are under potential threaten to the vegetatioan degradation and landuses. A better understanding of the mechanism and the characteristic of weather and climate response to vegetation change is important for preventing semi-arid areas from further degradation and desertification.Using a meso-scale model (WRF) and long term observations, this paper first analysed and simulated the precipitation differences between an isolated mountain forest (Xinglong,103.84E,35.86N) and its surrounding area located at the arid/semi-arid region of northwest of China,in an attempt to understand the mechanisms responsible for the persistence of this isolated forest. The results indicate that the precipitation differences between Xinglong Mountains and its surrounding area are the largest in summer and autumn. The Xinglong mountain area is dominated by the southeastern wet flow in summer and autumn which provides a stable water vapor source for the region. In addition, the meso-scale high terrain blocks the water vapor from the southeastern flow and enhances precipitation. The contribution of local evapotranspiration to the precipitation is less than that of advection effects of water vapor. On the other hand, the rock tectonics of Xinglong Mountain covered by soil and forest also help to hold and store water for the growth of plants. Therefore, being different from the oasis, the persistence of the forest island (Xinglong mountains) in semi-arid regions is the consequence of the large-scale circulation, which transports more water vapor, the orography and the special geology which tend to help and store more water vapor for this area.Using a regional climate model, the climate responses to different degrees of vegetation degradation are further investigated in China-Mogolia arid and semi-arid areas (CMASA). The mechanisms of climate responses and the differences in climate responses upon different land-cover types are studied. Overall, vegetation decreases can cause an increase in the surface air temperature and a reduction in annual total precipitation. The total precipitation decreases nearly linearly with decreasing vegetation coverage. It is found that the reduction in total precipitation caused by vegetation changes is mainly due to the decrease in convective precipitation. Our analysis reveals the most vulnerable land-cover type in CMASA to the vegetation degradation is the dry grassland where vegetation decreases can cause significant reduction in convective precipitation. The local circulation responses to reduced vegetation cover are characterized by an anomalous anticyclone covering transient zone between the well vegetated surface and semi-desert area and this anomalous anticyclone is found to be related to the orography flow and variations in secondary circulation induced by the surface temperature gradient changes. This anomalous circulation tends to enhance the climate responses to vegetation changes over the transient zone. It is also found that the vegetation changes can weaken the decreasing trend of local precipitation and enhance the increasing trend of surface temperature. The effect of vegetation change on the local precipitation and surface temperature trends are relatively small compared to their corresponding background trends in CMASA. But vegetation change can largely affect the background trends in some areas.Finally, the comparison is made between the climate response of revegetation and vegetation degradation. The seasonal patterns of climate response to the vegetation change are also investigated. The results indicate that the climate responses to revegetation are overall opposite to those responses to vegetatation degradation, but both of revegetation and vegetation degradation affect the precipitation through changing convective precipitation. Our analysis reveals that the increasing amounts of evapotranspiration and precipitation induced by revegetation are smaller than the corresponding decreasing amounts induced by vegetation degradation owing to their different influences on Bowen ratio. And this makes regional climate of CMASA more sensitive to vegetation degradation than to revegetation. Our analysis indicates that the climate response to revegetation is most sensitive over deciduous forest while the responses over the transition zones between forest and grassland are not significant. A small increase and decrease in vegetation coverage can both lead to an increase in the moisture import. The climate responses induced by vegetation change are strongest in June and summer and are relatively weak in other seasons. It is also found that local circulation responses to revegetation are characterized by an anomalous cyclone tilting to the north which can enhance precipitation.更多还原