Analysis Of Extreme High Temperature In Southern China And Evaluation Of WRF Simulation Performance

In this study,the total days,mean duration and intensity of extreme hot events over southern China during the 2008-2017 warm seasons are analyzed with daily maximum and minimum temperatures(Tmax and Tmin),by comparing the newly proposed independent hot day(IHD),independent warm night(IWN),and compound extreme(CMPD,i.e.,the continuous occurrences of hot days and hot nights)to the traditionally defined hot day and warm night.Relationships between the hot extremes and urbanization are explored with 1-km resolution population density(P)data.Results show that(1)there are obvious differences in the spatial distributions between IHD,IWN,and CMPD over southern China.IHD occurs most frequently in the west of Sichuan Basin,the east of Yunnan,the south of Gansu and the Pearl River Delta,while IWN occurs more in the south and north part of the analysis region than in the middle(27°N-33°N).CMPD occurs more frequently in areas with high population like the Yangtze River Delta,and the duration is longer with stronger intensity.In the Yangtze River Delta,CMPD and the traditionally defined hot day and warm night exhibit high index values,while IHD and IWN exhibit the opposite.(2)The three hot-extreme indices of CMPD are positively correlated with the population density,which is qualitatively similar to the traditional hot days and warm nights,reflecting the enhancement of hot extremes by the urban heat island effects.In contrast,negative correlations between the IHD and IWN indices and the population density are found,as their indices are more apparent over rural areas.In addition,IHD and IWN show less evident annual variations,while CMPD exhibits more meaningful annual variations resembling the traditional hot days and warm nights.(3)The analysis results obtained are used to evaluate convection-permitting climate modeling with the WRF model.The spatial and temporal characteristics of Tmax and Tmin can be well simulated.While Tmax is mainly underestimated in rural stations and Tmin is overestimated in large-urban stations.(4)The simulation underestimates IHD at most stations,with larger biases at the stations with lower population density(i.e.,ln(P)<7).IWN and CMPD are also underestimated at the lower-population-density stations,but overestimated at the megacities with ln(P)>9.The observed relationships between population density and hot extreme are reproduced for CMPD,but not for the independent hot extremes.(5)Sensitivity experiments suggest that use of a multi-layer urban canopy model significantly improves the temperature simulation of urban and large-urban stations and improves the simulation of IWN and traditionally defined warm night in respect of the correlation coefficient and standard deviation ratio between the simulation results and the observation ones,especially the cumulative intensity.While improved treatment of sub-grid cloud fraction and the aerosol effects could not.