Initiation of a Record-Breaking Rainfall Event in Beijing, China, Associated with a Penetrating Inland Sea-Breeze Front

In summer, many sea-breeze fronts (SBFs) are observed propagating from the sea to inland areas. However, there has been an absence of in-depth studies on whether and how these SBFs alone initiate convection initiation (CI) inland. We selected an inland CI event that occurred near Beijing on 17 May 2019 to analyze how the SBF triggers CI during its inland progression. The 3-km continuously cycled analyses with 12-min updates, produced by assimilating observations from radar and dense surface networks, revealed that as the northwestward-moving SBF reached Beijing, it interacted with the warm and dry southerly flow, mountains, and city landscape. These interactions created local conditions of strong convergence and high humidity, conducive to CI. The mountains and cities blocked and changed the direction of winds behind the SBF from southeasterly to easterly, enhancing local convergence and moisture along with the westerly downslope flow from the mountains. Meanwhile, the reduction in wind speed allowed the wet, cold air mass behind the SBF to catch up with the enhanced convergence zone, enabling the air parcel to rise from the surface to the level of free convection (LFC), thereby triggering convection. The new storm then merged with the eastward-propagating convective systems from the western mountains to form the record-breaking heavy rainfall. Sensitivity studies were conducted to quantify the effects induced by mountains, cities, and both. It was found that mountains played a vital role in enhancing convergence by changing the wind direction of the SBF, while cities primarily contributed to slowing down the SBF, thereby aligning wind convergence with water vapor and enabling the moist air to be lifted to the LFC.