Characteristics Of Turbulent Momentum And Heat Flux Exchange In The Near-surface Layer Over Urban Area

The urban surface layer is divided into an inertial sublayer and an underlying roughness sublayer,with the lowest part of the RSL from the ground up to the mean building height referred to as the urban canopy layer.In the urban RSL,time-averaged turbulence statistics and flux densities are horizontally and vertically inhomogeneous,and in particular,the Reynolds stress and sensible heat flux vary with height,in contrast to the non-urban surface layer over flat terrain.It has long been realized that within the RSL Monin-Obukhov similarity theory is invalid.This characteristic affects predictions of the evolution of the boundary layer and surface variables if included in numerical models.So it's important to understand the characteristics of turbulent momentum and heat flux exchange in the roughness sublayer.The present experimental study addresses turbulent momentum and heat flux exchange processes in the urban roughness sublayer.The results are as follows:First,the normalized standard deviations of velocity components(?u,v,w/u*)and temperature(?Tv/Tv*)are calculated.The results show that the statistics of ?u,v,w/u*and ?Tv/Tv*are in good agreement with the local similarity theory.For the normalized standard deviations of velocity components,the values of ?u,v,w/u*increase with increasing stability and instability,and are larger than the reference values over homogenous underlying surface.For the normalized temperature standard deviations,the values are found to increase strongly towards neutral conditions at each height.Thus the exponent in the power-law relationship is close to-1 rather than-1/3 under near neutral conditions.While under stable/unstable conditions,the-1/3 power-law relationship is obeyed well.However,the influence of stability on ?Tv/Tv*increases significantly with height under stable conditions.Then,the observation data are analyzed to derive the flux-gradient relationships,and its integration form(the flux-profile relationships).The results show that under unstable conditions,the classic momentum flux-gradient relationships preforms well in urban near surface layer,but deviate from the observation data under stable conditions.And the coefficient in the fitted relations increases with height and gradually approaches the value in the classical similarity relations under stable conditions.This trend suggests an estimation of roughness sublayer depth by linear extrapolation of the derived relations.Although the momentum flux-gradient relationships perform well but its integration form do not.The reason is suggested to be the dependence of roughness lengfh on the stability.The calculated roughness lengths are found to increase with observation heights under neutral conditions,while decrease with instability/stability increasing.A parameterization of roughness length is made,and the classical momentum flux-profile relationship is improved with parameterized roughness length.The results also suggest that the flux-gradient relationship of temperature is failed in the urban near surface layer.This is possibly because of the turbulent counter-gradient heat flux transport.Finally,in order to find out the vertical profile of turbulent flux,observations are grouped into two sectors(A,B)according to the prevalent wind directions in Nanjing.For sector A,where the nearby buildings are all below the lower measurement level,the sensible heat and momentum fluxes are generally greater at the upper level.For sector B,where several high-rise buildings are located upwind,the sensible heat and momentum fluxes at the upper level are close to those at the lower level.The analysis shows that the difference of turbulent eddy characteristics between the two wind sectors leads to a different behavior of turbulent exchange between the two levels.We focus on sector A where there is no influence of high-rise buildings.In this sector,the turbulence transfer efficiency are found to increase with height.The 'blocking effects','wake effects' and some other effects are believed to be the reasons.This result shows that in the urban roughness sublayer,the observed turbulent flux increase with observation height,which indicates that the observation of turbulent flux is underestimated in the urban roughness sublayer.Therefore,the observation of turbulent flux should be conducted in the inertial sublayer.This study is concluded in the following:1.The turbulence behavior can be described by the local similarity theory in urban roughness sublayer.2.In the urban roughness sublayer,the turbulence statistical characteristics,the flux-gradient relationship and the flux-profile relationship vary with observation height.The higher the height is,the closer the fitted relations are to the classical relations which are valid over homogeneous flat surface,suggesting that above the roughness sublayer there is an inertial sublayer,in which the classical relations are valid.3.The roughness length determined in the urban roughness sublayer varies with the stability and observation height.4.The observation of turbulent flux is underestimated in the urban roughness sublayer.