Urban heat island circulations of an idealized circular city as affected by background wind speed

The Weather Research and Forecasting (WRF) Model is used to study the interaction between urban heat island circulation (UHIC) and its background wind. Two distinct wind regimes are found depending on the ratio of the horizontal convective scale velocity to the background velocity, which is referred to as the buoyancy velocity ratio. When the buoyancy velocity ratio is above two, coherent asymmetric vortex rings are exhibited, similar to the UHIC structure under calm conditions. For buoyancy velocity ratios below two, upper return UHIC currents at the upstream disappear in the two-dimensional cases and the coherent asymmetric vortex rings are destroyed in the three-dimensional cases. In these low buoyancy velocity ratio cases, background wind inhibits UHIC development at the leading edge of the urban area and UHIC is transported downstream from the urban center. As a result, a trailing vortex forms instead of a vortex ring. In all simulated cases with a background wind speed, a counter-rotating vortex pair exists. This remarkable feature does not exist under calm conditions and is attributed to the corresponding background wind speed. Holding urban heat flux constant, as the background wind speed decreases, the reverse region expands.