Future Projection of Extreme Heavy Snowfall Events With a 5-km Large Ensemble Regional Climate Simulation

We have recently experienced several heavy snowfall events, but still do not sufficiently understand how global warming will impact changes in local extreme snowfall events. The analysis relevant to the extreme events requires ensemble experiments with high-resolution regional climate modeling. In this study, we use a large number of ensemble warming projections downscaled dynamically to 5-km grids to examine the differences in distribution and mechanism in extreme snowfall events over Japan between present and future climates. Japan has two typical snowfall patterns, the winter monsoon and the south coast cyclone patterns. The domain-averaged amount of extreme snowfall in the south coast cyclone pattern regions appear to decrease with the rise in temperatures. Meanwhile, the winter monsoon pattern regions showed little change as the increases in extreme snowfall in the mountainous areas are canceled out by the decrease in the coastal areas. Based on the existing mechanism of normal snowfall, the increase in mountain snow is caused by a temporary intensification of cold air outbreaks. The amount of the cold air blowing from Japan Sea clearly increases only during the days preceding the extreme snowfall event, and as the cold dry air obtains more latent heat over the ocean, the precipitable water vapor increases. The winter monsoon then transports this air over land where the intensification of precipitation results from orographic updraft. This sequence of processes beginning with cold air outbreaks may have a stronger control over extreme snowfall phenomena on the warmer field.