Warm Island Effect in the Lake Group Area of the Badain Jaran Desert
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【Author in Chinese】 梁晓燕；
【Author's Information】 兰州大学， 地理学·地球系统科学， 2020， 博士
【Abstract in Chinese】 巴丹吉林沙漠是世界上沙丘高度最大的沙漠,同时是我国第二大沙漠。在其腹地和东南部分布有大量的湖泊,形成了独特的沙山-湖泊相间分布的地貌格局。湖泊群补给来源及其水循环演变、高大沙山的形成机制和地貌演化模式等科学问题,长期以来受到了国内外学者的广泛关注和研究。通过遥感反演冬季地表温度与基于野外观测数据均发现,巴丹吉林沙漠腹地东南部湖泊区存在温度明显高于沙漠其他区域及沙漠外围地区的暖岛现象。但由于暖岛效应的研究尚处在起步阶段,其稳定性与时空分布特征还需要进一步研究,暖岛效应的具体内涵及形成机制更需要定量化揭示。与此同时,也需要结合植被物候观测数据来对其进行验证。基于此,本研究利用兰州大学野外科学观测试验站与沙漠外围国家基本气象站的数据资料,对比分析了沙漠腹地湖泊区与沙漠周边地区和沙漠腹地积水湖盆与干涸湖盆间的温度差异。同时,利用沙漠腹地与外围地区的物候观测数据,分析了植被物候对暖岛效应的响应。最后从局地环流、辐射平衡、湖泊热源贡献等方面对暖岛效应的形成机制进行了探讨。本研究进一步明确了巴丹吉林沙漠湖泊区暖岛效应的概念内涵;对其形成机制的探讨也为湖泊补给来源研究提供了新的科学依据。凡此,均对巴丹吉林沙漠湖泊群水循环机制与地下水补给来源、陆面过程、生态系统服务及人类活动的环境效应研究具有重要意义。主要研究结果和结论如下:(1)巴丹吉林沙漠腹地湖泊区暖岛效应是指沙漠腹地湖泊区比周边地区热量丰富的现象。具体表现为巴丹吉林沙漠腹地湖泊区年平均气温比周边地区高1.5℃,春、夏、秋、冬季的平均气温比周边地区分别高1.9℃、1.5℃、1.5℃、1.4℃。沙漠腹地湖泊区入春时间要比周边地区早2～3 d,入夏时早约2 d,夏季长度比周边地区长12 d;入秋时间比周边地区晚约9 d,入冬时间晚约4 d,冬季长度比周边地区少6 d。日平均温度T≥0℃积温、T≥10℃积温和T<0℃负积温分别比周边地区高412.4℃、429.5℃和96.0℃。T≥0℃、T≥10℃积温持续时间比周边地区均长9 d,T<0℃负积温持续时间则比周边地区少2 d。(2)巴丹吉林沙漠腹地湖泊区暖岛效应还包括沙漠腹地积水湖盆比干涸湖盆热量资源丰富的现象。积水湖盆年、春、秋和冬季平均气温及T≥0℃积温和T≥10℃积温均高于干涸湖盆,气温日、年变化均小于干涸湖盆。上述现象在冬季表现更加明显,随着湖泊大小的差异而不同。据此可说明湖泊对局地气温起了一定的调节作用,且主要表现在冬季。此点与冷岛效应在时间尺度上是不同的,冷岛效应一般出现在夏季,且在白天表现更明显。(3)巴丹吉林沙漠腹地湖泊区车日格勒物候观测站比沙漠外围区两个井站沙枣树的返青期提前23 d,休眠期推迟11 d,生长季延长了34 d。二者的物候差异除与两个井站的海拔高度较高有关外,主要是巴丹吉林沙漠湖泊区的暖岛效应所致。数字相机作为“近地面”遥感的新方法,通过连续自动拍摄高时空分辨率的数字图像,可有效弥补了无人观测区传统物候研究的不足。(4)腾格里沙漠东北部由于存在大量湖泊,所以沙漠腹地湖泊区年、季夜晚地表温度也明显高于周边地区。而同纬度东部乌兰布和沙漠和西部塔克拉玛干沙漠由于没有大量积水湖泊存在,所以年、季白天和夜晚平均地表温度空间分布规律各不相同。因此,湖泊对于暖岛效应的形成起到了不容忽视的作用。(5)巴丹吉林沙漠湖泊区暖岛效应的形成首先是由于下垫面性质不同及沙漠腹地湖盆内风速较小,热量不容易散失。沙漠腹地湖泊区相对于周边地区是一个热源。另一方面,沙漠腹地湖泊区,因为地下水在补给湖泊过程中释放热量,并通过湖泊沙山之间的平流作用将湖泊存储的热量释放到局地环境中进而影响温度的空间分布特征。
【Abstract】 The Badain Jaran Desert has the highest megadune in the world.It is located on the northwestern Alxa Plateau in north-central China.Covering an area of 5.2×10~4 km~2,the Badain Jaran Desert is the second-largest desert in China.It is characterized by the coexistence of more than 110 perennial lakes and thousands of megadunes.Previous studies in this region have primarily been focused on the geochemistry of the groundwater and lakes,past fluctuations in lake level,the eolian geomorphologyand the key problems regarding the source(s)of lake water.The traditional view holds that an oasis or lake in the desert or Gobi is a cold source,namely,the cold island effect.As a unique region with more than 110 lakes,it is founded that the temperature in the lake group region of the Badain Jaran Desert was significantly higher than that in other regions of the deserts by using the field observation data and remote sensing and geographic information system technology(i.e.,a“warm island effect”).However,as the research on the warm island effect is still in its starting stage,its stability and reliability need to be further improved.At the same time,the specific connotation and the formation mechanism of the warm island effect are still unclear.More importantly,plant phenological observations should be employed to reveal the phenological responses of plants to the warm island effect in the lake group area of the Badain Jaran Desert.Based on this,we analyzed the temperature difference between the lake group region of the Badain Jaran Desert and the surrounding areas as well as between the water lake basin and the dry lake basin in the desert hinterland.Climatic data were obtained from Lanzhou University field scientific experimental stations and China National Climate Center standard meteorological stations.Then we studied the phenological responses of plants to the warm island effect in the lake group area of the Badain Jaran Desert by using the digital images.Finally,the formation mechanism of the warm island effect was discussed from the aspects of local circulation,radiation balance and lake heat source contribution.Our study clarified the concept and connotation of the warm island effect in the lake group region of the Badain Jaran Desert and also provided a new scientific basis for the study of lake recharge sources.It will be of important significance for the study on the lake circulation mechanism,the sources of groundwater recharge,land surface processes,ecological system and human activities.The main findings and preliminary conclusions are as follows:(1)The warm island effect in the lake group region of Badain Jaran Desert refers to the phenomenon that the temperature in the lake group region is higher than that of surrounding areas.More specifically,the annual mean temperature is 1.5℃higher in the lake region of Badain Jaran Desert than that in the surrounding areas.The mean temperature in spring,summer,autumn and winter is 1.9℃,1.5℃,1.5℃and 1.4℃higher than the surrounding areas,respectively.The starting date of spring and summer is 2～3 days and 2 days earlier and the starting date of autumn and winter is delayed 9days and 4 days in the lake region of Badain Jaran Desert than that in the surrounding areas.Thereby the length of summer is 12 days longer and the length of winter is 6 days shorter in the lake region than that in the surrounding areas.The T≥0℃,T≥10℃accumulated temperature and negative accumulated temperature is 412.4℃,429.5℃and 96.0℃higher in the lake region than the surrounding areas.The duration days of T≥0℃and T≥10℃are all 9 days longer and the duration days of negative accumulated temperature are 2 days shorter than in surrounding areas.(2)The warm island effect in the lake group region of Badain Jaran Desert also refers to the phenomenon that the temperature in the lake group region in the water lake basin is higher than that of dry lake basin.Compared the local climate between different stations in the Badain Jaran Desert,we found that the water lake basin has higher temperature in annual,spring,autumn and winter than the dry lake basin.At the same time,T≥0℃and T≥10℃accumulated temperature in the water lake basin are higher than in the dry lake basin.The diurnal and annual variations of temperature have less change in the water lake basin.The above phenomenon is more obvious in winter and varies with the size of the lake.Therefore,the lake plays an important role in regulating the local temperature,which is mainly reflected in winter.This is different from the cold island effect on the temporal scale.To be specific,the cold island effect generally appears in summer and is more obvious in the daytime.(3)Combine the advantages of the traditional phenology observation and satellite remote sensing technological,the use of digital cameras provides an opportunity to conduct long-term monitoring of plant phenology and to extract phenological transition dates.In the lake group region of Badain Jaran Dessert,the onset of greening occurred23 days earlier,the onset of dormancy began 11 days later and the growing season was34 days longer,compared to those of the surrounding area.This difference is partly related to the higher altitude in the LGJ;however,it is dominantly related to the warm island effect in the lake group region.(4)Due to the large number of stagnant lakes distributed in the southeast part of the Badain Jaran Desert and the northeast of the Tengger Desert,the annual and seasonal average night surface temperature is significantly higher than that in the surrounding areas.But this phenomenon is not obvious in the daytime.In addition,the above pattern does not occur in the Taklimakan Desert and Ulan Buh Desert.Thus,the lake plays an important role in the formation of the warm island effect.(5)The warm island effect in the lake group region of Badain Jaran Desert mainly contains two aspects.Firstly,the wind speed is smaller in the lake basin than the surrounding areas and the underlying surface is also different between lake group area and surrounding areas.The heat resource in the hinterland of the Badain Jaran Desert is richer than the surrounding areas.Secondly,much heat is being carried from the groundwater during the process of deep groundwater recharged lakes.Then the local air circulation between lakes and megadunes will affect the spatial distribution pattern of heat resource by way of air movements.