Mesoscale-to-LES Modeling of the Sea-Breeze Front and Its Interaction with Turbulent Flows over a Coastal City

Abstract The sea-breeze front (SBF) intruding into coastal cities can lead to sudden changes in local weather and air quality. Its structures and disturbances over the urban surface with dense tall buildings have been a challenge for numerical modeling. In this study, we present the fine-scale structures of an actual SBF simulated by mesoscale-to–large-eddy simulation (LES) models at 3-m resolution in a 15-km domain of Sendai City, Japan. The head of SBF is shown to develop remarkable three-dimensional structures in the downtown area, where it interacts strongly with the microscale turbulent flows induced by buildings. Localized strong updrafts form at the lees of high-rise buildings within a few minutes just after the passing of the SBF, as ejections emerge in urban warm air mass surrounded by the cool air mass of the sea breeze. Downdrafts in the sea breeze are seen at the windward sides of high-rise buildings and help to increase wind speed in adjacent streamwise streets. After the passage of SBF, near-surface wind speeds exhibit high-frequency fluctuations with a period of 2–3 min, in good agreement with in situ observations in the downtown area. These results demonstrate the good capabilities of the building-resolving LES of both mesoscale weather conditions and microscale turbulent flows over a large city for the prediction of urban weather and environment in the streets. Significance Statement The sea-breeze front (SBF) has significant impacts on the local weather in coastal cities around the world. It is challenging to simulate the fine-scale structures of SBF and their interactions with turbulent flows over the cities. In this study, we present an improved building-resolving numerical simulation of an actual SBF that develops fine-scale structures in the downtown area with high-rise buildings. The SBF head is shown to be strongly disturbed by buildings and interacts with urban turbulent flows, which drive local strong updrafts and vertical transports of heat. The passing SBF also leads to high-frequency fluctuations in the simulated surface wind speed, in good agreement with observations. The results are helpful for improving our understanding and prediction of the local winds at street scales during the passing of the front.