A Numerical Study of Sea Breeze and Spatiotemporal Variation in the Coastal Atmospheric Boundary Layer at Hainan Island, China

Numerical simulations of sea breezes and the coastal atmospheric boundary layer (ABL) at Hainan Island, China during summer and winter are discussed. The different behaviour of sea breezes and the ABL on the leeward and windward sides of the island are examined, and it is found that offshore flows are more likely to create a strong sea-breeze signature, whereas the process of sea-breeze development under onshore flows is difficult to capture. At the location where the sea-breeze signal is remarkable, the height of the coastal ABL displays an abnormal decrease, corresponding to a transitional point from a continental ABL to a thermal internal boundary layer (TIBL) formed under sea-breeze conditions. This is corroborated by the sudden increase in the water vapour mixing ratio and/or wind speed, indicating the arrival of the sea breeze. Regarding the spatial distribution, the TIBL height decreases abruptly just ahead of the sea-breeze front, and above the cold air mass. When the sea-breeze front occurs with a raised head, a cold air mass is separated from the sea-breeze flow and penetrates inland. This separation is attributed to the interaction between the sea breeze and valley breeze, while the dry airflow entraining to the sea-breeze flow may also partially contribute to this air mass separation.