Effects of unstable thermal stratification on the flow characteristics in an idealized rural-to-urban transition region: A large-eddy simulation study

Abstract

Large-eddy simulations are performed to study the impacts of unstable thermal stratification on the mean wind and turbulence in an idealized rural-to-urban surface transition region. In the unstable stratification case, a convective boundary layer is developed over the urban region and much longer adjustment distances for the mean wind and turbulence within the canopy are identified compared with that in the neutral case. Moreover, the flow recirculation and turbulence within the canopy are substantially enhanced by the unstable stratification. Specifically, at the far downstream positions of the surface transition, the mean wind is observed to flow at oblique angles through the street channels under the effects of buoyancy, which is qualitatively different from that in the neutral stratification case. Despite the magnitudes of the turbulent and dispersive stresses are significantly larger in the unstable case, the horizontally space-averaged mean streamwise velocity component within the canopy shows similar magnitudes as those in the neutral case. At the downstream region where the flow is already adjusted to the urban surface, the turbulent viscosity and Prandtl mixing length calculated from the horizontally space-averaged quantities are found to be significantly larger in the unstable case than those in the neutral case, while the turbulent Prandtl number is approximately uniform with height with a value of 0.5.