A numerical study of an urban breeze and its interaction with convective cells

An urban breeze and the interaction between an urban breeze front and convective cells formed in both the urban and rural areas are investigated numerically using the Weather Research and Forecasting (WRF) model coupled with a new single-layer urban canopy model. Two-dimensional idealized urban-breeze simulations, with an isolated urban area surrounded by rural areas, are performed for a summer solstice condition under no background wind. An urban breeze, characterized by a convergent/divergent flow in the lower/upper part of the boundary layer, is well developed in the daytime with a maximum vertical velocity in the late afternoon but is greatly weakened in the late nighttime. The initiation and development of the urban breeze are associated with horizontal temperature gradient related to the difference in surface sensible heat flux between the urban and rural areas, whereas those of convective cells result from thermal instability in the daytime boundary layer. As the urban-breeze front penetrates the urban area, a strong updraft at the front interacts (or merges) with convective cells advected from the rural areas and also with those formed in the urban area. This results in strengthening or weakening of the updraft at the front. Extensive sensitivity experiments are performed to investigate the impacts of urban-related parameters such as urban size, aspect ratio, and surface albedo as well as environmental conditions on the intensity of the urban breeze and convective cells.