Dry and moist convection forced by an urban heat island

This study numerically investigates dry and moist convection forced by an u rban heat island using a two-dimensional, nonhydrostatic, compressible mode l with explicit cloud microphysical processes (Advanced Regional Prediction System). The urban heat island is represented by specified heating. Extens ive numerical experiments with various heating amplitudes, representing the intensity of the urban heat island, uniform basic-state wind speeds, and b asic-state relative humidities, are performed to examine their roles in cha racterizing urban-induced convection. Two flow regimes can be identified in dry simulations. One regime is characterized only by stationary gravity wa ves near the heating region and is revealed when the urban heat island inte nsity is very weak. The other regime is characterized both by stationary gr avity waves near the heating region and by a downwind updraft cell that mov es in the downstream direction. The intensity of the downwind updraft cell increases as the heat island intensity increases or the basic-state wind sp eed decreases. Results of moist simulations demonstrate that the downwind u pdraft cell induced by the urban heat island can initiate moist convection and result in surface precipitation in the downstream region when the basic -state thermodynamic conditions are favorable. As the urban heat island int ensity increases, the time required for the first cloud water (or rainwater ) formation decreases and its horizontal location is closer to the heating center. It is shown that for the same basic-state wind speed and heat islan d intensity a stronger dynamic forcing-that is, a stronger downwind updraft -is needed to trigger moist convection in less favorable basic-state thermo dynamic conditions.