Characteristics of Momentum, Heat and Moisture Fluxes in the Urban Roughness Sublayer

Abstract: Eddy covariance (EC) observations above the densely built-up center of Nanjing were carried out during the measurement campaign for urban surface energy balance. Separate EC systems operated at two heights in the urban roughness sublayer from 1 December, 2011 to 8 August, 2012. The measurement data show two typical characteristics of momentum and heat fluxes. When the airflow comes from the area in which almost all the buildings are lower than the measurement heights (i.e. Type A), the momentum and heat fluxes at the upper level are systematically larger than those at the lower level in the daytime. When the airflow comes from the area in which there are some nearby buildings taller than the measurement heights (i.e. Type B), the momentum and heat fluxes at the upper level are systematically smaller than those at the lower level in the morning, and the fluxes at the two levels are almost same in the afternoon. The moisture flux at the upper level is always larger than that at the lower level in the daytime, and the difference between the two levels is more obvious for Type A. Analysis suggests that the different characteristics of fluxes are caused by different turbulent behaviors.