Effects of boundary-layer stability on urban heat island-induced circulation

The effects of atmospheric boundary-layer stability on urban heat island-induced circulation are numerically and theoretically investigated using a nonlinear numerical model (ARPS) and a two-layer linear analytical model. Numerical model simulations show that as the boundary layer becomes less stable, a downwind updraft cell induced by the urban heat island strengthens. It is also shown that as the boundary layer becomes less stable, both the height of the maximum updraft velocity and the vertical extent of the downwind updraft cell increase. Hence, in the daytime with a nearly neutral or less stable boundary layer the urban heat island-induced circulation can become strong, even though the urban heat island is weak. It is suggested that these findings can be a mechanism for urban-induced thunderstorms observed in the late afternoon or evening with a nearly neutral or less stable boundary layer. The boundary-layer stability affects the spatial distribution of scalar concentration through its influencing urban heat island-induced circulation. Analytical results from a two-layer model with different boundary-layer stabilities in the lower and upper layers are in general qualitatively consistent with the numerical simulation results, although the low-level maximum vertical velocity does not change monotonically with lower-layer stability.