강릉공항 활강 강풍 사례의 수치 모의 및 역학적 메커니즘 분석

Abstract

This study investigates the physical mechanisms of a downslope windstorm that occurred in the Gangneung region on April 11, 2023, using numerical simulation and the shallow water theory. In shallow water theory, the mechanism of a downslope windstorm involves the flow transition from subcritical to supercritical due to changes in the fluid thickness and velocity, which is identified by the Froude number (Fr = U/ ). While previous studies calculated the Fr using a fixed characteristic depth, this study applied the spatially varying flow thickness for the Fr calculation, assuming that the lower tropospheric inversion layer with the maximum Brunt-Väisälä frequency could be considered as the free surface in shallow water theory. Numerical simulation conducted using the Weather Research and Forecasting (WRF) model revealed that the new Fr calculated with the varying flow thickness aligns better with shallow water theory and effectively explains the physical mechanisms of the downslope windstorm. On the upwind side, the westerly flow is well identified as subcritical until it reaches the crest, several kilometers west of Daegwallyeong, where its Fr transitions to supercritical by decreased flow thickness and increased wind speed. The flow further accelerated while descending the lee side under supercritical conditions. The subcritical-to-supercritical transition made the westerly flow keep accelerating, causing the downslope windstorm at Gangneung Airport. The new spatially varying Fr revealed clear distinctions between the downslope windstorm and turbulent regions closely matching observations at Gangneung Airport.