Numerical Study of the Daytime Urban Effect and Its Interaction with the Sea Breeze

Abstract

Under light gradient-wind conditions, the heat-island effect associated with large urban areas is characterized by a temperature rise after sunrise over the land (ΔθL)and a temperature difference between the urban and rural areas (Δθ) during the day, whereas by only Δθ during the night. A numerical experiment using a two-dimensional hydrostatic boundary-layer model was performed to examine the basic characteristics of the daytime heat-island circulation. The results show that a daytime circulation can develop that is much stronger than the nocturnal circulation in spite of a generally smaller value of Δθ. The circulation decays under the influence of a moderate gradient wind. On the other hand, it persists in the sea-breeze system, and has a notable effect on the sea-breeze pattern. Some characteristics of the sea breeze observed in the Tokyo metropolitan area, such as formation of a suburban stagnant region, frontal intensification, and the delay of its advance inland, were well simulated with the simplified model. Trajectory analyses for air parcels passing over the coastal city indicate that the heat island could prevent the dispersion of urban pollutants and delay their inland transport.