Effects of Unstable Thermal Stratification on Vertical fluxes of Heat and Momentum in Urban Areas

A Reynolds-averaged Navier-Stokes microscale model is applied for the simulation of the effect of unstable thermal stratification on the flow within an aligned configuration of building-like cubes as used in Santiago et al. (Urban Climate, 2014, Vol. 9, 115–133). The spatially-averaged results are analysed, showing an increase of the dispersive flux of heat and momentum, the turbulent length scales and the sectional drag coefficient. An extension of K-theory is presented in order to represent the dispersive fluxes in the urban canopy parametrizations, in which the sum of the turbulent and dispersive components is calculated. In addition, the observed increases in the length scale and drag coefficient are parametrized as functions of the ratio of buoyant and inertial forces. This approach improves the results of urban canopy parametrization simulations inside and above the urban canyon and represents the first attempt to account for the dispersive fluxes and the effect of solar radiation on the flow within the mesoscale.