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

Much effort has been made to detect urban induced weather and climate changes and find responsible mechanisms for them. In a recent work, we demonstrated that an urban heat island is able to induce a strong updraft cell downwind of the heat island, which can dynamically initiate moist convection under favorable environmental thermodynamic conditions. This helps to explain observed precipitation enhancement downwind of urban heat islands. Here, we extend our previous work to examine the effects of boundary-layer stability on dry and moist convection induced by an urban heat island. For this, dry and moist simulations using a nonhydrostatic numerical model with explicit cloud microphysical parameterization are performed and a theoretical two-layer gravity wave model is developed. The results of numerical model simulations show that the downwind updraft cell induced by an urban heat island becomes stronger as the boundary layer is less stable. For a two-dimensional, linear, steady-state, Boussinesq flow system in which the stability in the lower layer being different from that in the upper layer, analytical solutions for each layer are found. The analysis results of the gravity-wave solutions in this flow system indicate that gravity waves can amplify in the lower layer when the lower layer is less stable than the upper layer. This is consistent with the numerical model result. Our numerical and theoretical results clearly explain why urban induced thunderstorms are more frequent in the late afternoon than in the nighttime.