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

A large number of cities around the world are located in or near complex terrain. In these regions, urban breeze circulation and mountain/valley winds are observed. Understanding interactions between urban breeze circulation and mountain/valley winds is important in a view of mesoscale atmospheric dynamics as well as urban air pollution. In this study, we theoretically examine the interactions of urban breeze circulation with mountain slope winds in the context of the response of the atmosphere to specified thermal forcing. Starting from linearized governing equations in two dimensions, analytical solutions for perturbation kinematic pressure, buoyancy, horizontal velocity, and vertical velocity are obtained. Then, the analytical solutions are used to examine the interactions. Urban breeze circulation and mountain slope winds are, respectively, evolved with time due to respective time-varying thermal forcing that has steady and diurnal components, and they linearly interact with each other. Asymmetric flows are developed over the urban and mountain areas. In the daytime, converging flows induced by mountain heating (upslope winds) and those induced by urban heating (urban breeze) effectively interfere with each other, resulting in weakened flows over the urban-side mountain slope and the mountain-side urban area. A transition from the upslope wind to downslope wind on the urban-side mountain slope occurs earlier and thus the downslope wind persists longer when compared with the case that has mountain thermal forcing only. In the nighttime, flows become strong in the region between the urban center and the mountain center due to the additive interaction of the downslope wind with the urban breeze, but the flow intensity weakens with time because of weak nighttime urban heating. In the nighttime, the downslope wind on the rural-side mountain slope and converging flows on the plain-side urban area are weakened. The interactions of urban breeze circulation with mountain slope winds are shown to be sensitive to the intensities of urban and mountain thermal forcings.