A numerical study of the interactions of urban breeze circulation with mountain slope winds

The two-dimensional interactions of urban breeze circulation with mountain slope winds are investigated using the Weather Research and Forecasting (WRF) model coupled with the Seoul National University Urban Canopy Model (SNUUCM). A city is located near an isolated mountain, and there is no basic-state wind. Circulation over the urban area is asymmetric and characterized by the weakened mountain-side urban wind due to the opposing upslope wind and the strengthened plain-side urban wind in the daytime. The transition from upslope wind to downslope wind on the urban-side mountain slope occurs earlier than that on the mountain slope in a simulation that includes only an isolated mountain. A hydraulic jump occurs in the late afternoon, when the strong downslope wind merges with weaker mountain-side urban wind and stagnates until late evening. The sensitivities of the interactions of urban breeze circulation with mountain slope winds and urban heat island intensity to mountain height and urban fraction are also examined. As mountain height decreases and urban fraction increases, the transition from urban-side upslope wind to downslope wind occurs earlier and the urban-side downslope wind persists longer. This change in transition time from urban-side upslope wind to downslope wind affects the interactions between urban breeze circulation and mountain slope winds. Urban heat island intensity is more sensitive to urban fraction than to mountain height. Each urban fraction increase of 0.1 results in an average increase of 0.17 °C (1.27 °C) in the daytime (nighttime) urban heat island intensity. A simulation in which a city is located in a basin shows that the urban-side downslope wind develops earlier, persists longer, and is stronger than in the simulation that includes a city and an isolated mountain.