Unusual inland intrusion of nocturnal sea breeze in the North China plain during summer

A month-long numerical simulation investigates summertime sea breezes in the North China Plain (NCP), occurring predominantly from late afternoon to night-time and penetrating over 100 km inland. The key factor driving this phenomenon is identified as a persistent lee-side cyclone in the western NCP, formed by dynamic interactions between upper-air north-westerly winds and mountain barriers upstream. Throughout a summer month, the lee-side cyclone varies in strength diurnally, significantly influencing sea breeze development. A case study of high-resolution simulation provides detailed insights into the wind field and the evolving lee-side cyclone structure. In the evening peak, three sectors of air flows—westerly, southerly, and south-easterly—encompass the cyclone core, demarcated by fronts with sharp gradients in temperature, moisture, and wind speed and direction. As the south-easterly sea breeze intensifies, it swiftly advances inland along the mountainside, forming an arch-like intrusion path. With the weakening and south-westward movement of the lee-side cyclone, the sea breeze transforms into an inertial current, guided further south-westward. The vertical structure of the sea breeze is revealed, including the depth of its moisture-layer and its uplifting effect on the warmer inland air mass at the cyclone core. The interaction between the lee-side cyclone and sea breeze facilitates the transport of substantial water vapor from the Bohai Sea coastline to the interior of the NCP. These findings provide new insights into the summertime sea breeze mechanism in the NCP, with implications for local weather patterns, water vapor budget, and air pollutant transport.