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

The interactions of urban breeze circulation with mountain slope winds in two dimensions 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 developed asymmetrically, which is identified by the weakened mountain-side urban breeze due to the opposing upslope wind in the daytime and the intensified and horizontally extended plain-side urban breeze. A transition from upslope wind to downslope wind on the urban-side slope starts earlier, which accompanies the strong drainage wind. Around 1800 LST, a hydraulic jump occurs when the strong drainage wind merges with weaker mountain-side urban breeze and stagnates until the late evening. Sensitivities of the urban heat island intensity and the interactions of urban breeze circulation with mountain slope winds to the mountain height (100-1000 m) and urban fraction (10-90%) are examined. The urban heat island intensity is more sensitive to the urban fraction than to the mountain height. An increase in urban fraction by every 10% results in a mean increase of 0.17°C (1.27°C) in the daytime (nighttime) urban heat island intensity. In a case with the largest urban fraction and highest mountain, the urban-side drainage wind starts much earlier (around 1125 LST) and persists longer (85% of the total wind). Plain-side urban breeze is intensified with increases in urban fraction and mountain height. A case in which a city is located in a basin is examined. Drainage/plain-to-basin winds develop earlier and persist longer as compared to the case with a city and an isolated mountain. Moreover, hydraulic jumps persist longer in the basin geometry.