Impact of eddy characteristics on the turbulent heat and momentum fluxes in the urban roughness sublayer

Eddy covariance (EC) observations above the densely built-up center of Nanjing were carried out from December 2011 to August 2012. Separate EC systems operated at two heights on a 36 m tower standing on a building roof. The morphology of buildings around the observational site provides opportunities to compare the turbulent eddy characteristics and turbulent fluxes at the two heights in different wind sectors. In sector A, the buildings within a distance of 500 m from the tower are all below the lower measurement height. The sensible heat flux and the momentum flux at the upper level are larger than at the lower level. In sector B, several high-rise buildings stand nearby at the upstream locations. The sensible heat flux at the upper level is smaller but the momentum flux is still larger than at the lower one. Analyses show that the turbulent eddy characteristics in the two sectors are different, which leads to different behaviors of turbulent exchange of heat and momentum at the two levels, especially at the higher one. To address the vertical variation of turbulent heat flux in the urban roughness sublayer, a conceptual model is proposed to explain why the sensible heat flux increases with height in this layer.