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The Vegetation Feedbacks of Drought Events in North China Based on the Dynamic Vegetation Module in RegCM-CLM4.5

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【Author in Chinese】 夏爽

【Supervisor】 李耀辉

【Author's Information】 兰州大学, 大气科学·气象学, 2018, 硕士

【Abstract in Chinese】 陆气耦合数值模拟是探究干旱半干旱地区的干旱过程与区域植被变化之间响应及互反馈的重要途径,近期耦合在区域气候模式RegCM下的CLM陆面模型为我们量化这种影响提供了可能。围绕RegCM-CLM模式及其动态植被模块对过去30年(1983年01月至2012年12月)的东亚地区设计并进行了两组试验:叶面积指数(LAI)与植被覆盖类型(PFT)固定的RCM-CLM试验;植被覆盖与类型随大气耦合动态变化的RCM-CLM-CN-DV试验。并以中国西北地区东部与华北地区(33°N-42°N,100°E-120°E)的干旱过程为例,探讨了两组实验中干旱个例发展与植被改变的相互作用。两组模拟结果与资料对比,均能准确地重现过去30年中国区域降水与气温的分布特征,整体气温模拟略偏低,但西北地区尤其是祁连山两侧地区误差较小;植被的分布型也能较为准确地模拟再现,因RCM-CLM-CN-DV试验对降水较敏感,在湿润地区的LAI模拟值明显偏大,但变化梯度较大,使得其在干旱与半干旱区与观测以及RCM-CLM试验基本一致。研究区域内30年间的几次典型干旱事件:1986-1987、1994-1995、1997-1998、2004-2005,均可在两次试验结果中得到显著体现。其中CN-DV试验无论干旱强度还是持续周期均高于原RCM-CLM试验。经比较两组试验标准化降水指数(SPI)变化,发现CN-DV试验中的干旱过程解除得较迟缓。又对比两组试验的植被变化、辐射平衡、风场、蒸散与水汽通量变化,结果表明:在我国北方一次干旱过程发生后,受旱地区地表植被强度将因降水减少而出现萎缩,且此萎缩效应会滞后于干旱过程约1-2个月;北方半干旱地区一次干旱过程造成的植被萎缩与地表条件的改变,可相当程度增加当地的地表返照率,从而影响大气能量收支;这种大气能量收支影响将使对流层中低层出现垂直速度向下正距平,作为局地次级环流影响干旱解除阶段的天气系统;还可减少局地蒸散,阻碍水汽通量传输,从而造成不利的水汽条件。在干旱结束阶段,此类植被枯萎导致的环流信号通过边界层传递至高层,逐次影响接连而来的历次降水过程,最终使得干旱解除周期显著延长。

【Abstract】 The regional climate model RegCM4.5 which coupled with land model CLM4.5 was introduced and used to derive a 30-year-long simulation over east Asia.Two groups of experiments were carried out based on the different configurations of dynamic vegetation module in CLM4.5: 1)Group RCM-CLM in which the Leaf Area Index(LAI)and the Plant Functional Type(PFT)of the land surface system are prescribed;2)Group RCM-CLM-CNDV in which the vegetation type and intensity change dynamically with atmospheric forcing.Both outputs from these two groups can well reproduce the spatial distribution characteristics of precipitation and air temperature in the simulated 30 years.As group RCM-CLM-CNDV is more sensitive to precipitation,the simulated LAI values in humid regions are slightly higher than observations.While with a larger spatial gradient,the LAI values in arid and semi-arid regions thus are appeared consistent with observations.Part of Northwest-China and North-China(33°N-42°N,100°E-120°E)were respectively isolated to discuss the relationship between drought events and the changes in vegetation.Several recorded drought events in the study area over the 30 years,including 1986-1987,1994-1995,1997-1998,and 2004-2005,can be significantly reflected in outputs from the two groups.Both the intensity and duration of these drought events in group RCM-CLM-CNDV are higher than in group RCM-CLM.A slower recovery of drought was perceived in group RCM-CLM-CNDV after comparing the changes of the Standard Precipitation Index(SPI).Also,changes in vegetation intensity,radiation balance,wind field,evapotranspiration,and water vapor flux in the outputs of the two groups of experiments were compared.Results suggested that the vegetation atrophy and the changes of surface conditions caused by a drought process in the semi-arid region in Northern China can increase the local surface albedo to a considerable extent,thus affecting the energy budget of the atmosphere.The vertical velocity anomaly in lower troposphere appears positive downward and can act as local sub-circulation to influence the weather system during the drought recovery stage.Reduced local evapotranspiration and hindered water vapor flux transmission can cause an unfavorable moisture condition.When the drought comes to its end phase,such resulting circulation signals can be transmitted to the upper layer and interfere with continuous precipitation processes,eventually prolonging the drought events.

  • 【Contributor】 兰州大学
  • 【Year of Internet Publish】201902
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