Warm Island Effect In The Lake Group Area Of The Badain Jaran Desert

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.