|Alternative Title||Effects of Vegetation Changes in Northwest China on Summer Climate in China|
|Place of Conferral||兰州|
|Keyword||WRF模式 西北地区 LUCC 生态恢复 t检验 非局地气候效应|
我国西北地区地处欧亚大陆内部，地貌类型和下垫面特征复杂多样，生态环境脆弱，受人类活动影响较大。西北地区大部分都处于干旱和半干旱区，大气干燥，荒漠广布，长期遭受干旱，水土流失，沙漠化，生态环境恶化等一系列气候问题的影响。人类在生产生活的过程中会对陆地表面覆盖状况进行直接影响，随着生产力水平的提高和人口的增长，人类影响地表覆盖状况的能力也逐渐增强。近年来由于我国一系列生态恢复工程的开展，我国的森林资源和草地面积逐渐恢复，西北地区的生态退化的现状得到了有效遏制，局部开始出现缓解。西北地区土地利用/土地覆盖的改变（Land Use and Land Cover Change，LUCC）对于中国的气候将产生如何影响，不仅影响到经济的发展，也和生活在西北地区的1亿人的日常生活息息相关，对我国西北地区区域气候的模拟研究有着重要的现实意义和指导意义。
为了研究西北地区大面积土地覆盖由裸土改变为草地的气候效应，本文采用了WRF（Weather Research and Forecasting Model）模式对1979-2008年30年夏季的气候进行了数值模拟。设计控制试验和敏感性试验两组试验，敏感性试验将西北地区所选区域内的裸土下垫面修改为草地，控制试验保持土地覆盖类型不变。将敏感性试验和控制试验的模拟结果和观测结果进行对比，研究植被改变对中国不同区域温度、辐射、降水和环流特征等产生的影响，并针对降水偏多年份和偏少年份进行了个例分析，研究不同降水年份不同区域的响应特征。得到的主要结论如下：
The northwestern part of China is located within the interior of Eurasia. The features of landforms and features of the underlying surface are complex and diverse. The ecological environment is fragile and affected by human activities. Most of the northwestern China are in arid and semi-arid regions, where the atmosphere is dry, the desert is widespread. It suffers from a series of climate problems such as drought, soil erosion, desertification, and deterioration of the ecological environment. In the process of production and living, humans directly affects the land surface coverage. With the improvement of productivity and population growth, the ability of humans to affect the surface coverage has gradually increased. In recent years, due to the development of a series of ecological restoration projects in China. China's forest resources and grassland areas have gradually recovered, and the status of ecological degradation in the northwest region has been effectively curbed, and local conditions have begun to ease. The land use/land cover change (LUCC) in the Northwest China has an impact on China's climate, which not only affects economic development, but also has close relationship with the daily life of 100 million people living in the northwestern China. The simulation of regional climate in northwest China has important practical significance and guiding significance.
In order to study the climate effect of the land cover transition from bare soil to grassland on the climate changes in the northwest region, the WRF model (Weather Research and Forecasting Model) is used to simulate the climate in the summer of 1979-2008. Two sets of control tests and sensitivity tests are designed. In the sensitivity tests the underlying bare surface in the selected area in the northwestern China is changed into grassland, and in the control experiment the land cover is kept constant. Comparision was conducted among among the sensitivity test results and the control experiment results and the observation results to study the effect of vegetation change on temperature, radiation, precipitation, and circulation characteristics in different regions of China, and case study is conducted for the years of anomalous increased precipitation and those with anomalous decreased rainfall, in which the response characteristics of different regions in different precipitation years is Analyzed. The main conclusions are as follows:
(1) When the bare soil is changed to grassland, the shortwave radiation and the upward shortwave radiation decrease in the vegetation modification area, The decrease in surface albedo and the increase in surface emissivity have an opposite effect on net radiation, but former have a greater impact on net radiation than the latter, resulting in the increase in net radiation. Changes in vegetation will lead to changes in transpiration, surface emissivity, and albedo. The effects of changes in albedo on temperature during this experiment play a dorminant role in affecting the temperature, resulting in the increase in the average summer temperature over the vegetation modified area. The sensible heat flux and latent heat flux in the modified area of vegetation is also increased, and the increase in net radiation was less than the increase in sum of the latent heat . When the underlying surface of the bare soil was changed to grassland, the net radiation allocated to latent heat is increased according to the Bowen ratio.(2) The summer precipitation in the provinces of Xinjiang, Inner Mongolia, Northeast China, East China and Southwest China increases when vegetation is changed in the northwest China, and the precipitation decreases slightly in the central and southern Gansu, Ningxia, Shaanxi, Shanxi and Hebei. In Region 1 (northwest China) and Region 2 (northeast China), the precipitation in the sensitivity test is overall greater than the control test; the precipitation sensitivity test in Region 3 (Central and South China) is less than that in the control experiment in other years.(3) The precipitation of Sensitivity test in 1981 and 1988 was extremely low and extremely high In the Region 3 (Central and South China), respectively. Through the diagnostic analysis of the circulation field, it can be found that in 1981, In 1981, there were low-level anticyclone anomalies over the southern China-China region, abnormal high-level cyclones, and heightened geopotential height, which were not a favorable condition for the occurrence of precipitation. In 1988, the circulation over the southern China-China region was contrary to that in 1981, favoring the formation of precipitation. The low-level cloud cover was anomalously low and high in 1981 and 1988, respectively, which is also consistent with the precipitation anomalies.(4) In the vegetation-modified area, the absolute values of the precipitation increase in sensitivity tests for the summer and summer rainy years are the same, but the precipitation increase rate in the low rainfall years is slightly greater than that in the rainy years. In Region 2 (northeast China) and Region 3 (Central and South China), the absolute value and percentage of precipitation increase caused by vegetation change in the northwest China during the rain deficiency year are higher than those in the rainfall abunduncy year. In general, the vegetation changes in the northwest China have more pronounced effects on precipitation in China in the rainfall deficiency year than in rainfall abunduncy years.(5) LUCC will also cause non-local climate effects outside the region. Non-local impacts of changes in vegetation within the vegetation-modified regions cause an increase in precipitation in the Northeast China. This may be due to the fact that the vegetation changes in the northwest China significantly affect the tropospheric energy and water vapor transport in the downwind region. Through the horizontal vapor advection, the local water vapor and energy are transported to the downstream regions (ie, the northeastern China), resulting in downstream precipitation increase. With the increase, the increase in vegetation cover in the northwest China can inhibit the decline in precipitation in Northeast China.
|苗文辉. 西北地区植被变化对中国夏季气候的影响[D]. 兰州. 兰州大学,2018.|
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