Impacts of Local Topography on Snowfall Distribution in the Kanto Plain: A Case Study

On 5 February 2024, an extratropical cyclone passing the south coast of Japan brought 8 cm of snow to Tokyo, while in Tsukuba, located about 50 km from Tokyo, the snow cover was 0 cm. We investigated the mechanism generating the inhomogeneous distribution of snow cover using a numerical weather model with 1 km grid spacings. Our numerical simulation shows that a coastal front is located on the eastern coast of the Kanto Plain. On the other hand, a relatively warm area broadly spreads from the leeward side of mountainous areas including Mt. Tsukuba. A strong downward flow related to a gravity wave caused by the mountainous area brings adiabatic warming on the leeward side of the mountains. This adiabatic heating accelerates snowfall melting at the low troposphere, and the melting layer is higher on the leeward side of the mountains. Also, the adiabatic heating reduces relative humidity and decreases total precipitation amount. As a result, there is much less snowfall on the leeward side of the mountainous areas, including Tsukuba observational station, as compared with neighboring areas. A numerical simulation with 5 km grid spacings cannot simulate the local-scale snowfall distribution around the mountainous areas.