Simulation Study On The Relationship Between Urban Thermal Environment And Underlying Surface In Nanjing Based On WRF-Mosaic Schem

Urbanization has changed the radiative,thermal and dynamic characteristics of the underlying surface,which causes a series of urban thermal environment problems.It is of great importance to quantify the relationship between urban thermal environment and the underlying surface characteristics,and to decompose the attributions of air urban heat island（AUHI）.Studies on this topic can provide scientific basis for improving urban thermal environment and promoting sustainable urban development.Based on the replacement of land use data and the comparison between WRF-Dominant and WRF-Mosaic schemes,this paper firstly evaluated the impact of high-resolution land use data and Mosaic scheme on WRF simulation of urban thermal environment.Secondly,the spatiotemporal pattern of urban thermal environment was investigated in winter and summer season.Then,the relationship between thermal environment and the underlying surface characteristics was quantified.At last,an attribution analysis of the AUHI was performed in Nanjing.The conclusions are summarized as follows:（1）Using high-resolution underlying surface data can improve the simulation of temporal variations in relative humidity,wet bulb temperature and wind speed with WRF model.Compared to WRF simulation with default underlying surface data,the root mean square error（RMSE）is reduced by 1.08%,0.16℃and 0.69 m s^-1,respectively by simulation with high-resolution surface data.The Mosaic scheme can improve the simulation of air temperature,wet bulb temperature and wind speed in winter and summer,and the RMSE is reduced by 0.12℃,0.31℃and 0.15 m s^-1,respectively compared to the Dominant scheme.As for simulation of spatial pattern,the simulation RMSE of relative humidity,wet bulb temperature and wind speed is reduced by 0.87%,0.11℃and 0.54 m s^-1,respectively after replacing underlying surface data.Compared to the default Dominant scheme,the Mosaic scheme reduces RMSE by 0.32℃and 0.15℃for air temperature and wet bulb temperature,respectively.Therefore,using high-resolution underlying surface data and WRF-Mosaic approach can improve the simulation of urban thermal environment.（2）The air temperature in winter and summer is positively related to the fraction of impervious surface and water body,but negatively related to the fraction of vegetation.The water vapor pressure is positivelyrelated to the fraction of water body and vegetation,but negatively related to the fraction of impervious surface.All the above correlation coefficients show distinct seasonal variation with higher value in summer.The influence of air temperature and water vapor pressure on the spatial distribution of wet-bulb temperature can cancel each other,which results in non-correlated relationship between wet-bulb temperature and the underlying surface characteristics.So,there is no significant difference in wet-bulb temperature between urban area and rural.Within the city,high correlation coefficients（>0.37,p<0.01）are found between air temperature,wet-bulb temperature and landscape indexes,and the radius of maxmium influence is 3 km.Air temperature and wet-bulb temperature are lower for the more compact urban landscape.For the landscape with higher degree of segmentation and heterogeneity,the air temperature and wet-bulb temperature are also lower.（3）AUHI is stronger at night than during the daytime in Nanjing city,which is more obvious in summer.During the daytime in summer,weak evaporative cooling is the main contributor to AUHI,which makes the temperature of urban area is 0.69℃and 0.56℃higher than that of farmland and forest,respectively.At night in summer,air temperature in urban is1.98℃and 1.78℃higher than that of farmland and forest,respectively.The higher temperature at urban blending height is the main contributor to the AUHI.In addition,convection efficiency difference between urban and rural land is also a important contribution to AUHI,especially during summer nightime when compared to forest land.During the daytime in winter,the high convection efficiency in urban area leads to the formation of air urban cold island（-0.69～-0.71℃）.At night in winter,anthropogenic heat emission in urban is the main contributor to AHUI.