Cascading effects of the Changbai Mountains on an extreme weather disaster in northern Japan in January 2021

Abstract

A sequence of a snowstorm, a quasi-stationary convergence zone (Japan Sea polar airmass convergence zone, JPCZ) and a mesoscale coastal front (MCF) caused an extreme weather disaster along the Sea of Japan (SOJ) coast in northern Japan from January 7 to 11, 2021. To examine how the presence of the Changbai Mountains (CMs) affects these multi-scale successive phenomena, we conducted numerical simulations with and without modified topography of the CMs and their surrounding regions using a regional atmospheric model with a horizontal resolution of 4 km. Backward-trajectory analyses and an additional experiment that removed the sublimation process of snow were also performed to clarify the formation and maintenance processes of the MCF. The CMs lead to the structural change of the snowstorm developing over the SOJ, enhancement of the JPCZ, and formation of the MCF, affecting heavy snowfall in the SOJ coastal areas. The MCF, which is a mesoscale phenomenon smaller than the JPCZ, is formed and maintained by the monsoonal flows that detour from the north and south of the CMs. Several of the parcels that detour from north of the CMs intrude into the MCF through the boundary layer, which is thermodynamically modified by the heat and moisture supply from the sea, whereas the inland flows that penetrate the MCF from its south are composed of two major routes: one is the flow detouring from south of the CMs that crosses the high mountains on the mainland of Japan, and the other is the parcels detouring from north of the CMs that penetrate from the inland after they land on the northern coast. The difference in equivalent potential temperature between the inland flows and the monsoonal flow from the sea is responsible for the MCF formation. A cold pool near the surface also contributes to the prolonged MCF. Understanding of the CMs’ cascading effects on multi-scale phenomena from synoptic scale to mesoscale is necessary for the prediction of extreme weather disasters over Japan in winter.