A Quantitative Study Of A Directional Heat Island In Hefei Based On Multi-Source Data

Since the initiation of China’s reform and opening-up policies,the country has undergone rapid urbanization,characterized by the expanding scale of urban construction land and the consequent intensification of the urban heat island(UHI)effect.This phenomenon has exacerbated the disparity in thermal environments between urban and rural areas,further amplifying the vulnerability of urban thermal environments.Surface Urban Heat Island(SUHI)is an important indicator for studying the UHI effect and evaluating urban thermal environments.Quantitative studies of SUHI are essential for revealing the mechanisms underlying the formation of the UHI effect,developing urban planning and management strategies,and thus reducing the impact of the UHI effect.However,current quantitative studies of SUHI ignore the thermal radiation directionality(TRD)and lack an assessment of the urban heat island angle effect on the quantitative studies of SUHI,which directly interferes with the precision of such studies.Therefore,the objective of this study is to quantitatively investigate the impact of TRD on the assessment of SUHI,aiming to enhance the precision of quantitative studies on SUHI.In order to achieve this purpose,this article utilized Moderate Resolution Imaging Spectroradiometer(MODIS)land surface temperature(LST)data and station air temperature data in Hefei city from 2010 to 2020.The nearly homogeneous nature of the water surface was utilized to eliminate the interference of atmospheric attenuation angle effect and daily temperature variations on the quantitative analysis of the urban heat island angular effect.And the impact of the TRD on the quantitative studies of SUHI was evaluated by comparing TRD under different land-use intensities in Hefei.The main conclusions of this article are as follows:1)There are two significant TRD hotspots for daytime urban surfaces,where the sensor zenith angle is approximately the same as the forenoon solar zenith angle,and where the sensor zenith angle is near its nadir in the afternoon.The maximum intensity of daytime TRD can reach up to 4.7K,occurring in areas with the highest urban land-use intensity.During nighttime,the maximum intensity of TRD can reach up to 2.6K,situated in areas with medium urban land-use intensity.2)Based on previous research and the findings of this study,it has been discovered that urban morphological characteristics,water and land layout,as well as climate environment are important factors contributing to the differences in the TRD intensity between cities.Additionally,urban scenes with high-rise buildings and a medium-density distribution exhibit more significant TRD characteristics.3)The difference in TRD intensity between urban and rural areas can reach up to 2.7K during the same overpass time.The TRD can introduce uncertainty of approximately 2.0K in the results of quantitative studies of SUHI in Hefei based on MODIS LST,which accounts for about 31%-44% of the total SUHI in Hefei.This study further elucidates the impact of TRD on the urban-rural thermal environment differences,providing methodological references for accurate quantitative estimation of the UHI effect using remote sensing.It is expected to promote the quantitative development of remote sensing monitoring of urban thermal environments.This study separates the impacts of atmospheric attenuation and daily temperature variations on the angle effect of the urban heat island by utilizing the approximate Lambertian characteristics of water surfaces.It quantifies the intensity of TRD in urban areas and refines the influence of TRD on the differences in urban-rural thermal environments.These findings provide a methodological reference for the quantitative and accurate remote sensing estimation of the urban heat island effect,thus promoting the quantitative development of remote sensing monitoring for urban thermal environments.