Orographic and coastal impacts on the unexpected development of the snowstorm in Seoul Metropolitan Area

Seoul Metropolitan Area (SMA) is one of the most densely populated megacities in the world, which is significantly vulnerable to high-impact weather events. Under this situation, an accurate weather forecast is required but is challenging due to the complex geographic and topographic features on upstream in combination with limited observational data arising from the geopolitical constraints. On 6 January 2024, a heavier-than-expected snowfall event occurred in SMA, with a peak intensity of 3 cm per hour and a total of 6 cm within just 3 hours, causing significant public discomfort, traffic congestion, and flight delays. A numerical simulation of the Weather Research and Forecasting model with a kilometer of horizontal grid spacing reproduced well the local distributions of the snowfall events, which revealed that a meso-γ-scale convective cell at Haeju Bay in North Korea was a main contributor to this unexpected snowfall event in SMA. Upwind orographic and coastal contributions to the system development were investigated with various topographic and coastal sensitivity simulations. The upwind topographic features induced a stationary local convergence-vorticity area (LCVA) at Haeju Bay, which was a key factor in the formation of a convective cell under northwesterly prevailing wind condition. The LCVA was significantly weakened or disappeared in sensitivity experiments in which the terrain was flattened or prescribed as ocean, resulting in reduced snowfall over Seoul compared to observations. These findings highlight that even local topographic features can modulate snowfall intensity, providing insights into the prediction of high-impact weather events affecting the SMA.