Observational And Simulation Study Of The Urban Surface Energy Balance In Central Shanghai

The process of urban surface-atmosphere energy,mass exchanges is one of the key focuses in the field of urban climate.Observed surface energy balance fluxes are essential to improve model forecasting ability.However,Due to the extreme heterogeneity of the underlying urban surfaces,direct urban surface energy balance measurements are challenging and scarce especially for sub-tropical cities and urban areas with tall buildings.This research addresses the following prospects:By utilizing a full year of eddy covariance flux data measured from a dense neighbourhood with a wide range of building heights(8 to 150 m,mean by direction)in Shanghai,the flux source area,seasonal,diurnal and spatial characteristics of surface energy balance,and key parameters such as bulk atmospheric transmissivity,albedo and Bowen ratio have been analyzed.Combining measurements of cloud cover,PM2.5 concentration,visibility,relative humidity and precipitation,the impact of sky conditions and air quality on the solar radiation has been studied.Particular attention is directed to the impact of urban surface heterogeneity on the fluxes and the challenges of making EC measurements in this environment.Moreover,the performance of the surface urban energy and water balance scheme(SUEWS)is evaluated over the central business site of Shanghai driven by the above mentioned 1-year surface energy flux observations.Special focus is directed to the impact of the seasonally varying diurnal profiles of the anthropogenic heat flux(QF)derived from city scale annual energy consumption data,hourly electricity power load data,and traffic count data.The impact of urban irrigation on the simulation of latent heat flux(evaporation)is also addressed using the empirical irrigation scheme in SUEWS.The main conclusions are as follows:(1)The downward shortwave radiation is weakened by about 25.8%under cloudy conditions compared with that under sunny conditions.The haze conditions decrease hourly median bulk atmospheric transmissivity by 11.3%,which can reach nearly half of the cloud attenuation effect.The effect of cloud cover on downward longwave radiation is opposite to downward shortwave radiation that the downward longwave radiation is strengthened,thereby reducing radiative cooling at night.Cloud cover fraction and type both have a significant impact on the radiation fluxes with the reduction of the hourly median downward shortwave radiation by about 25.8%.This is especially evident during the Meiyu period in June when opacus clouds are frequent with the reduction of downward shortwave radiation by 350 W m-2.The effect of cloud cover on downward longwave radiation is opposite to downward shortwave radiation.The downward longwave radiation is increased under cloudy conditions compared to that under sunny conditions,thereby reducing radiative cooling at night Median daily maximum values of the net all-wave ration(Q*)under clear conditions range from about 400(winter)to 640 W m-2(spring).The atmospheric transmissivity is larger in winter and spring than in summer and autumn under sunny conditions.This is mainly attributed to larger air moisture in summer and autumn,which enhances the absorption of the incident shortwave radiation.The atmospheric transmissivity under cloudy and rainy conditions is much lower than that under sunny conditions.The effect of air quality is evident:haze conditions decrease hourly median atmospheric transmissivity by 12.9%in winter and 11.2%in summer,which can reach nearly half of the cloud attenuation effect.(2)The effect of the three-dimensional geometry of buildings on surface albedo is significant.Building shadows and the urban canopy have obvious weakening effect on the surface albedo.The diurnal curve of the surface albedo in each season shows a typical U shape.The surface albedo varies with the solar altitude and azimuth due to the inhomogeneity of the underlying surface.Based on variation of the surface albedo with the solar geometry,together with the distribution of tall buildings within the radiative source area reflected by the fisheye camera,it is found that shadows play an important role in decreasing the surface albedo in the late afternoon.A bulk surface albedo of 0.14 was calculated to represent the overall characteristics of this site across radiative conditions.This value is similar to other dense urban sites,but lower than typical suburban values,indicating the multiple interception effect of the 3-D urban canyon(3)Analysis of a series of turbulence indices by wind direction revealed highly correlation with building heights.Based on this and considering different packing density of roughness elements,the constant flux layer heights for different flow regimes are determined.A method of identifying turbulent flux data as local and micro-scale is proposed.The temporal patterns are similar between local and micro-scale fluxes.The biggest difference is that the micro-scale sensible heat flux is larger than that of the local scale,which is mainly due to the stronger vertical turbulence and the heat release of a large number of high-rise dry walls.Analysis of drag coefficient and other turbulence indices by wind direction,revealed high correlation with building height,standard deviation of wind direction and blending height.Based on these findings,together with the consideration of packing density of roughness elements,a method of identifying turbulent flux data as local and micro-scale is proposed.At the local-scale,median daily peaks of sensible heat flux occur in the early afternoon.The latent heat flux is smaller than the sensible heat flux.The median hourly sensible and latent heat fluxes remained positive,reflecting the thermal storage capacity of urban street canyons at night.The time variation of micro-scale flux is similar to that of local scale,but the sensible heat flux is larger,which is mainly due to the stronger vertical turbulence and the heat release of a large number of high-rise dry walls.(4)Since the study site is dominated by impervious surface,the enhancement of evaporation following rainfall only lasts for about 12 hours.Compared with other urban sites,the normalized sensible heat flux is among the largest observed to date,while the normalized latent heat flux is typical for dense urban sites.The diurnal variation of CO2 flux has two peaks in relation with morning and evening rush hour traffic emissions.The absorption effect of vegetation is weak due to its small coverage.The monthly daytime median Bowen ratio is between 2 and 4.7,indicating the dominant role of the sensible heat flux.At this site,with predominately impervious surface(85%buildings/pavement,14%vegetation),the enhancement of evaporation following rainfall usually lasts for about 12 h.Consistently larger Bowen ratios at the micro-scale than the local-scale are attributed to roughness effects and the impact of extensive areas of dry walls.Compared with other urban sites,the normalized sensible heat flux is at the upper end,while the normalized latent heat flux is typical for dense urban sites.This dense urban site is a net source of CO2 for the whole year,dominated by traffic emissions with two rush hour peaks in the morning and evening(larger).Vegetation does offset some CO2 emissions,but the uptake effect is relatively small given the limited vegetation around the site.Analysis by wind direction highlights the role of land cover on partitioning of turbulent heat fluxes and CO2 fluxes.(5)The SUEWS parameters are localized based on observed flux data and GIS survey.Evaluation results show that SUEWS can generally reproduce the urban surface energy fluxes well in Shanghai.By collecting city-specific hourly electricity consumption data,hourly traffic data,high spatial resolution population data,the energy consumption-temperature response function is improved,which improves the estimated seasonal variability of the anthropogenic heat flux.The diurnal profile of the anthropogenic heat flux suitable for megacities is carefully derived with consideration of the industry/electricity ratio and the dynamic population density.Sensitivity experiments revealed the importance of appropriately including anthropogenic heat flux and irrigation process in urban land surface models.The SUEWS land surface is evaluated based on the whole year tower-based measured data.Particular attention is directed to the estimation of the anthropogenic heat flux(QF)and irrigation,and the associated influence mechanism on surface energy balance.Diurnal profiles of QF are carefully derived based on city-specific hourly electricity consumption data,hourly traffic data and dynamic population density.A new building heat emission/air temperature response function using two balance points made the seasonal variation of the building anthropogenic heat flux more distinct.The diurnal patterns of the building anthropogenic heat flux for weekday,weekend and holidays are similar with a peak around 11:00 LST.On holidays there is a larger evening peak around 19:00.Weekday diurnal profile of traffic anthropogenic heat flux has two peaks associated with rush hours.The morning peak is more distinct than the evening peak.Weekends have no distinct peaks.QF is estimated to be largest in summer(mean daily peak 236 W m-2).When QF is omitted,the SUEWS sensible heat flux(QH)reproduces the observed diurnal pattern generally well,but the magnitude is underestimated compared to observations for all seasons.When QF is included,the QH estimates are improved in spring,summer and autumn,but poorer in winter indicating winter QF is overestimated.Inclusion of QF has little influence on the simulated latent heat flux(QE),but improves the storage heat flux estimates except in winter.Irrigation,both amount and frequency,has a large impact on QE.When irrigation is not considered,the simulated QE is underestimated for all seasons.The mean summer daytime QE is largely overestimated compared to observations under continuous irrigation conditions.Model results are improved when irrigation occurs with a 3-day frequency,especially in summer.Results are consistent with observed monthly out-door water use.These findings highlight the importance of appropriately including the QF and irrigation in urban land surface models-terms not generally considered in many previous studies.