The Impact Of City Clusters On Boundary Layer Structure And Regional Climate In Yangtze River Delta

Urbanization changes the land surface properties,which lead to the modification of dynamic processes in the atmospheric boundary layer and the surface energy budget.At the same time,the large amount of heat generated from urban structures,as they consume and reradiate solar radiations,and from the anthropogenic heat sources are the main cause of urban heat island(UHI)and UHI circulation,which will have a larger impact on regional climate system.In order to analyze the influence urbanization has on Yangtze River Delta city clusters;this paper utilizes a WRF/UCM model data which uses a cloud resolving(3km)regional climate modelling technique in two seasons—summer and winter,without the cumulus convection parameterization scheme.The three paralleled cases are named:(1)AH experiment,which uses the 2001 MODIS data as land surface type and adds an extra anthropogenic heat profile directly in the surface sensible heat flux of all urban grids;(2)UB experiment,which is the same with AH experiment except for the absence of anthropogenic heat flux;(3)NU experiment,which is the same as UB experiment,but for the replacement of urban land use to cropland land use.AH experiment denotes the influence of anthropogenic heat release and urban canopy effects,while UB experiment stands for the urban canopy effects only.And NU experiment is the case without urban impact.In order to ensure the reasonability of WRF/urban model in reproducing weather and climate patterns for the further analysis,this paper compares the output from the model with MICAPS gauge data and high spatial and temporal satellite data CMORPH.As a result,WRF/urban model is capable of reproducing the distribution feature of surface temperature and the diurnal variation of temperature in both seasons.In addition,the modelling of precipitation matches well with those depicted by MICAPS gauge data,which shows the three concentrated higher values in East China.The model also represents the precipitation diurnal cycle of Yangtze River Delta quite well,especially along the Yangtze River Basin,which shows a peak value appearing at 1700 LST with a single-peak structure.The downside is the estimation in Jianghuai Valley has some sort of deviation from observational data.Through cross-contrasts among three experiments and carrying out the WRF-LES model,the following conclusions are obtained.1.Anthropogenic heat has a potential role in causing an urbanization-induced regional warming in Yangtze River Delta which owns a seasonal variation and diurnal cycle.Accompanying with the rapid urbanization in Yangtze River Delta,large amounts of anthropogenic heat are released into the atmosphere,which is becoming a more and more severe issue for the local climate.To understand what role anthropogenic heat release plays in the urbanization-induced regional warming,WRF/urban model experiments are carried out with different configurations of urban land use and anthropogenic heat.The results show that anthropogenic heat can cause notable warming in almost the whole YRD rather than confining to the local cities where it releases,which is more significant in winter than in summer if the same amounts of anthropogenic heat apply,especially at winter night.And furthermore,it can decrease(somewhat increase)the diurnal temperature range in winter(summer).On the contrary,urban canopy effect due to the change from rural to urban land use can produce a stronger warming in summer than in winter,and always reduce diurnal temperature range in both winter and summer.Contribution of anthropogenic heat to the sensible heat flux in urban regions is responsible for those anthropogenic heat-caused warming and diurnal temperature range features,because it is larger in winter and at winter night but much smaller in summer and at summer night.Without ambient wind effect,warming centers in cities and spreads around homogeneously.However,under the prevailing wind,it has less magnitude,shifts to the downwind city edges and forms a warming belt along the wind direction,which may favor linking the adjacent anthropogenic heat caused warming together.On the other hand,the warming depends on the city sizes that are proportional to the total amount of anthropogenic heat emitted.2.The heterogeneity features of city clusters turn to homogeneous warming in boundary layer with a mesoscale circulation under the influence of ambient wind in the late afternoon in summer.UHI intensity is always stronger under breeze situation rather than with an ambient wind of southeasterly(SE).All cases are discussed under clear-sky situation in summer.When the ambient wind is SE,UHI intensity is generally weaker.With the help of UHI circulation and surface friction,a convergence zone is formed above urban areas except from the direction where the wind comes from.However,in a breeze situation,UHI intensity can be up to 3?,the near-surface temperature is remarkably increased which lead to the increase of mixing layer height at night.The vertical influence of urbanization can reach the middle and upper layer of PBL at 1700LST,which is clearly more obvious under the SE situation.Urban impact on horizontal wind speed experiences different diurnal features when it comes to SE and breeze situation.The deceleration is not quite discernible in breeze,while with the SE wind,wind speed profile has a jet nose in the middle PBL.The existence of urban slows down the surface wind speed by lm s-1 and cause the jet intensity lower and position higher.The jet nose has a good correspondence with inversion height.Friction velocity remains high in SE situation,which means the turbulence mixing is always increasing with a stronger background wind.And the cross section over Suzhou-Wuxi-Changzhou(SXC)indicate that in the late afternoon the city cluster's heterogeneity features on the ground turn into a homogeneous warming at the top of PBL,along with the forming of a mesoscale circulation across three cities.3.Thermal inhomogeneities induced by urban and rural land cover present homogeneous behaviors under the influence of ambient wind when the distance of downtown urban is small enough.Results of convective boundary layer feature under different thermal inhomogeneous forcing are studied using WRF-LES model.The outcomes show the key points in changing mesoscale circulation and PBL structure lie in the thermal inhomogeneity feature at surface and the ambient wind feature.As to the single city case,the greater wind speed is,the lower mixing layer height is.Downwind rural areas' MLH(mixing layer height)is always higher than the upwind rural area.In the case where wind profile behaves like a lower-level jet,MLH of the whole region is increasing including the downwind rural areas.As to the three-city case,it is found that when the distance of two urban cores is smaller than the sum of their own scales and when the ambient wind speed reaches 3m s-1,the urban-rural heterogeneity surface show a homogeneous warming feature and becomes stratified in the upper PBL.The MLH above rural areas rises up to the same value as urban areas,and an organized mesoscale circulation forms across three cities.However the MLH of the whole region decreases as a result of the weakening of upward motion.The mesoscale sensible heat flux remains positive in PBL and turns negative above PBL,whose maximum value decreases with wind speed and increases with urban numbers.Mesoscale momentum flux does not change with wind speed linearly;it is influenced more by heterogeneity thermal features of underlying surface.When it comes to the three-city case,mesoscale momentum flux has a negative maximum value in PBL,which indicates that momentum is dominated by downward transport.But when the wind speed reaches up to lm s-1,it is dominated by upward momentum transport.Cities in the upwind area forms a city cluster,downwind side urban is affected and shows a higher MLH.