Daytime urban breeze circulation and its interaction with convective cells

Abstract

AbstractThe structure and evolution of daytime urban breeze circulation (UBC) and its interaction with convective cells in two dimensions are examined through high‐resolution numerical model simulation. The UBC is the thermally forced, solenoidal circulation that results from the difference in surface energy balance between the urban and rural areas. As the temperature excess in the urban area increases, the UBC becomes larger and stronger with time and the two urban‐breeze fronts (the leading edges of the UBC) that initially form at the urban‐rural boundaries in the morning move toward the urban centre. Meanwhile, due to strong surface heating in the daytime, a number of convective cells form in both the rural and urban areas and the different characteristics between rural and urban convective cells are identified. The aspect ratio of the urban cells is smaller than that of the rural cells, which is partly attributed to the deeper urban boundary layer. The cell updrafts originating from the urban area are stronger, warmer and drier than those from the rural area. As the UBC develops, the convective cells that form in the rural area are advected toward the urban area by the UBC. Under the influence of the UBC, the cell updrafts originating from the rural area weaken and the water vapour mixing ratio in the updrafts decreases. As the cell updrafts originating from both the rural and urban areas merge with the urban‐breeze front, the front intensity increases and the water vapour mixing ratio at the front is modulated. Copyright © 2012 Royal Meteorological Society