The Characteristics And Influencing Factors Of Oasis Effect

In the context of global climate change and intensification of human activities,huge changes in land use and land cover have significant impacts on ecology and climate change at local,regional and global scales.The arid regions are fragile parts of terrestrial ecosystems,more sensitive to global climate change and human activities than any other places.Oasis is the unique landscape in arid region,and the essence of arid land.Although oasis occupies a small proportion in the area of arid land,it carries almost all the social and economic activities in the area and concentrates most of the natural productivity.Hexi oasis is the foundation for the survival and development of the people in Hexi region.The rational development and utilization of the oasis area is the key to realize the sustainable development and rural revitalization strategy of this region.Oasis effect is a unique climate feature formed by the flow of material and energy in oasis-desert system,which charactered by cooling island effect.The study of spatial-temporal characteristics at multi-scale and formation mechanism of oasis cooling effect is the key to the research on climate change and oasis sustainable development in arid area.Compared with the studies on urban heating effect,the distribution,formation mechanism and ecological benefits of oasis cooling effect are still the scientific problems to be solved.Based on the basic theories of ecology and geography,this study takes the oasis and its desert background of Hexi Corridor as the research area,and uses multi-source remote sensing observation and regional climate model numerical simulation to reveal the three-dimensional spatial and temporal characteristics of the cooling effect in arid oasis.Based on the inner biophysical model,the effects of surface parameters of different land cover and land use,such as reflectance,Bowen ratio and surface flux,on the oasis cooling island effect and the biophysical mechanism of its cooling phenomena were quantitatively analyzed.Firstly,multi-source remote sensing products were used as data sources to analyze the oasis cooling island characteristics of different patches in Hexi Corridor at daily,monthly and seasonal scales.Secondly,the dynamic downscaling method based on WRF model was used to simulate the continuous weather in summer and winter of the Oasis-Desert system in Hexi,and the two-and three-dimension characteristics of the temperature field,humidity field,wind field and energy field of the oasis effect were analyzed.Thirdly,based on high-precision remote sensing products,the relationship between oasis effect and key land surface parameters,such as land surface albedo and land surface evapotranspiration,was analyzed by linear regression,and the energy differences caused by land surface parameters were further discussed from the perspective of energy balance.Finally,the IBPM model is used to quantitatively analyze the energy difference caused by the difference of key surface parameters into the temperature forcing caused by the surface parameters including albedo,Bowen ratio and ground flux.This study can provide theoretical basis for climate change research and sustainable development of oasis in arid area.The main conclusions are as follows:1.The double features of oasis effect are found,which are the strong"cooling island effect"in summer and the weak"heat island effect"in winter.The cooling effect of oasis is dominant,but the warming effect in winter cannot be ignored.During the growing season,cooling effects predominate both day and night,the intensity of oasis effect in the daytime was-5.2?,-12.8?and-4.56?in spring,summer and autumn,while the intensity of cooling effect in the night was-1.43?,-3.34?and-0.6?,respectively.In winter,the warming effect is dominant.Except for water body and grassland,the warming effect appears in oasis in day and night.The intensity of overall heat island effect in oasis was 0.43?,and is was 0.54?at night.At the same time,the cooling was much stronger during the day than at night,while the warming was relatively small both during the day and at night.The oasis area of Hexi Corridor has expanded from 10781 km~2 in 1986 to 16449 km~2 in 2020,increased 52.6%in 35 years.With the expansion of oasis scale,the intersity of the cooling effect in the daytime increased significantly in summer with the R~2 between oasis area and cooling effect intensity of 0.85.On the contrary,the intensity of the cooling effect at night and the heat island effect in winter had no significant trend.2.Introducing the new land surface datasets and optimal parameterization schemes,the temperature and humidity field,wind field and energy field of oasis-desert system in summer and winter were simulated with high accuracy by WRF model.The maximum height of cooling island effect and wet island effect of oasis effect was about1500m,above which the difference of water vapor and temperature between desert and oasis disappeared.3.By analyzing the relationship between the oasis effect intensity and the albedo and the evapotranspiration,it is concluded that the evapotranspiration in oasis area is the main factor of oasis cooling effect in the growing season,while the low albedo in oasis area is the main factor of winter heat island effect.The difference of evapotranspiration(?ET)and albedo(?albedo)between oasis and desert were negative in all season.The seasonal dynamic trend of?ET is unimodal pattern and reached its peak in the summer for 55 mm,this number is three times of that in the spring or autumn.This trend is consistent with OEI trend,indicating that ET is the main contributor to the cooling effect.In the vegetation growing season,albedo is low and has little change,while in winter,albedo is higher,which is the feedback of snow cover and vegetation litter,and can partly explain the warming phenomenon in winter.4.IBPM theory can successfully simulate the forcing forces that cause oasis effect:albedo,surface heat flux,and Bowen ratio.The positive force of albedo is balanced by the negative force of Bowen ratio,and soil heat flux contributes a small negative force in the oasis area.Combined with these three temperatures forcing values,IBPM model can well predict the strength of oasis effect and quantitatively decompose the magnitude of each forcing force.This study can provide a theoretical basis for climate change research in arid area,ecological environment protection and sustainable development of oasis in arid area.