Coastal cities in South Korea, especially in the east coast, often experiences heavy precipitation events affected by terrain, i.e., the Taebaek Mountains (TM), and winds blowing from the East Seacalled the Kor’easterlies. Cities are located on a narrow coast along the TM. Here, two extreme rainfall events are analyzed in the coastal cities using the mesoscale model simulations: case A induced by a typhoon circulation, and case B related to a mesoscale convective system (MCS).

In case A, a coastal city, Gangneung, underwent a record-breaking 24-hr accumulated rainfall of 870.5 mm under the influence of Typhoon Rusa (2002) in late August. It was mainly caused by the followings: 1) strong low-level convergence of moist air by the Kor’easterlies into the coastal area; 2) consequent orographic lifting over the TM; and 3) release of potential instability to trigger deep convection.

In case B, a back-building MCS produced a daily rainfall of 226.2 mm on 6 August 2018 via the following mechanisms: 1) low-level convergence by the Kor’easterlies, carrying moist air into the coast; 2) strong vertical wind shear, making the updraft tilted and sustained; 3) coastal fronts and back-building convection bands, formed through interactions among the Kor’easterlies, cold pool outflows, and orography; 4) mid-level advection of cold air and positive relative vorticity, enhancing vertical convection and potential instability; and 5) vigorous updraft releasing potential instability.