Mixed convection in an idealized coastal urban area with multi-scale surface heterogeneity.*

Marine heatwaves (MHWs) modulate the coastal climate through interacting with local circulations, such as meso-scale land-sea-breeze circulation and micro-scale urban-rural circulation. These circulations result from the meso-scale surface heterogeneity between land and sea surfaces, and micro-scale surface heterogeneity between the urban built-up areas and the surrounding rural areas, respectively. In this study, we consider the turbulent mixed convection within the planetary boundary layer (PBL) of the coastal region, and investigate how two different scales of surface heterogeneities impact the mix convection. The MHW is modeled as a turbulent inflow with elevated air temperature. Informed by dimensional analysis, four non-dimensional parameters are considered for a fixed setup of the urban-rural-sea system, in which two of them are related to the relative inflow speed and temperature , and the other two are related to the relative buoyancy flux of urban, rural and sea areas. Then, a suit of numerical experiments are carried out using large-eddy simulations (LESs) to systematically study the impact of the above four non-dimensional parameters. We find that the MHW, land-sea-breeze circulation and urban-rural circulation interact synergistically. The temperature difference between urban air temperature and inflow temperature is larger in the case with elevated inflow air temperature comparing to that of the base case with low incoming air temperature. It also increases with decreasing inflow velocity. The elevated urban air temperature will further strengthen the land-sea-breeze circulation. This study highlights the importance of multi-scale surface heterogeneity in modulating mixed convection.