植被变化对西北地区陆气耦合强度的影响

西北地区地处欧亚大陆腹地,生态系统对于气候变化和人为影响十分敏感,同时该区也是湿润的东亚季风区与干燥的中亚干旱区的过渡区域,陆气相互作用比较强烈.本文对西北地区植被变化对当地的陆气耦合强度及其与之相关的地表水文过程的影响进行了分析研究,并且找出适于增加植被以缓解西北地区荒漠化趋势的最具成效的地区.本文利用美国国家大气科学研究中心(NCAR,National Center for Atmospheric Research)研制的通用大气模式CAM3(Community Atmosphere Model Version 3)对西北地区植被变化的影响进了数值模拟.本文共设计了三个试验,使用正常地表植被覆盖的参考试验,地表下垫面变为裸土的去植被试验和植被增加的生态环境好转试验.首先,本文对西北地区植被变化对于当地降水量、地表水分盈余量、径流量、地表土壤含水量等地表水文变量的影响进行了分析研究.然后对西北地区植被变化对当地的陆气耦合强度的影响进了分析研究,陆气耦合强度是衡量局地陆气相互作用强弱程度的一个新标准,基于计算年降水量与蒸散量的协方差与降水量方差之比而得到.它利用观测数据或模式输出数据,计算起来简便容易,物理意义明确清晰,陆气相互作用越强烈的地区,其陆气耦合强度也越高.最后,本文计算了一个蒸散-水汽通量散度指数来衡量植被变化对局地蒸散与大气水汽通量散度的影响,其在一定程度上反应了植被变化对局地陆气相互作用和大尺度大气环流输送作用的影响,也可以视为一个评估人为生态环境工程效果的指标.西北地区陆气耦合强度由东南向西北递增.去植被之后,西北地区降水与蒸发普遍减少,其中在东南部区域,地表径流增加约10~40mm,渗流量与地表土壤含水量分别减少约40~80mm和5~20mm3·mm-3,陆气耦合强度上升,这有可能导致水土流失,不利于当地植被的恢复.生态环境好转之后,内陆地区降水与蒸发明显增加,但地表盈余水分有所减少,主要原因是蒸散量相较于降水量增加的更多.其中在沙漠戈壁区边缘的新疆南部与内蒙西部,渗流量与地表土壤含水量分别上升约5~20mm和5~20mm3·mm-3,陆气耦合强度降低,蒸散-水汽通量散度指数较高,这可能主要是由于植被变化对局地陆气相互作用的改变而造成的.植被对于西北地区地表水文过程有着明显的影响,植被的存在能加速西北地区地表水文循环过程,减小陆面蒸散的变化,降低陆气耦合强度.在有限的人力与财力条件下,集中力量在在沙漠戈壁区边缘的新疆南部与内蒙西部适当种植灌木与青草并防止过度放牧,能有效降低当地陆气耦合强度,缓解西北地区荒漠化加剧的趋势.本文下一步还需考虑如模式地表植被数据与真实情况的差异性,海洋因素变化对于植被变化的反馈,以及进行集合实验来增加研究结果的可靠性.

Research on the impact of vegetation change on land-atmosphere coupling strength in northwest China

Northwest China is located in the inner center of the Eurasia continent, its ecosystem is sensitive to climate variability and anthropogenic alterations. It is the transition zone of the humid East Asia monsoon region and the arid central Asia area, the land-atmosphere interactions are strong there. In this research, the effects of vegetation change on land-atmosphere coupling strength and related land surface hydrological processes in northwest China were examined, and we also tried to confirm the most efficient areas of vegetation increasing for desertification mitigation in northwest China.Using a state-of-the-art Community Atmosphere Model (CAM3) developed by National Center for Atmospheric Research,the effects of vegetation change in northwest China were studied by three numerical experiments: with realistic vegetation characteristics varying monthly (CTL run), without vegetation (NOVEG run), and with increased vegetation (VEG run). Firstly, the influences of vegetation change in northwest China on local hydrological variables (precipitation, surface water surplus, surface runoff, surface soil moisture and so on) were examined. Then, the effects of vegetation change on local land-atmosphere coupling strength were analyzed, land-atmosphere coupling strength is new standard to evaluate the degree of local land-atmosphere interaction, it is based on the ratio of the covariance between annualy precipitation and evaporation anomalies (from their climatological means) over the variance of precipitation anomalies. The coupling strength is easy to compute from observations and standard model output and has a relatively clear physical meaning, and the stronger the local land-atmosphere interaction is, the higher the land-atmosphere coupling strength will be. At last, an Evapotranspiration-Moisture flux divergence Index was calculated to evaluate the influences of vegetation change on evapotranspiration and moisture transport in the atmosphere. This index could reflect the effects of vegetation change on local land-atmosphere interactions and large scale atmosphere circulations to a certain degree, and it also could be used as an indicator of evaluating the effects of artificial ecological restoration programs.The land-atmosphere coupling strength increased from southeast to northwest in this area. Desertification led to reduced precipitation and evapotranspiration in most area of northwest China. In the southeastern area, infiltration and soil moisture were decreased for about 40~80 mm and 5~20 mm³·mm^-3 respectively, yet surface runoff increased for about 10~40 mm. Land-atmosphere coupling strength also increased. These might cause happening of soil erosion, this might go against for vegetation recovery in this area. An increased precipitation and evapotranspiration were found in the inland area of northwest China by vegetation increasing, yet surface water surplus was reduced slightly, mainly because evapotranspiration was increased more compare with precipitation. In the southern part of Xinjiang and western part of Inner Mongolia which are the edge of desert, increased infiltration and soil moisture increased for about 5~20 mm and 5~20 mm³·mm^-3 respectively, the coupling strength reduced. The Evapotranspiration-Moisture flux was also high in this region, and it seemed mainly because the local land-atmosphere interaction was altered by vegetation change.Vegetation has a significant influence on land surface hydrological processes in northwest China, the existence of vegetation could accelerate the hydrological cycle, reduce the variability of evapotranspiration and land-atmosphere coupling. Given limited financial and human resources, concentrated shrub-grass planting and preventing overgrazing in this particular region where the coupling strength could be substantially reduced by vegetation, might be an effective strategy to mitigate serious desertification in Northwest China. In the further research, we should consider the differences between the model land surface vegetation cover and the real situation, the feedback of ocean variability to vegetation change. ensemble simulations are necessary to make this research robust.