Orographic Effects on the Severe Snowstorm Associated with an Explosive Cyclone

Extratropical cyclones often develop rapidly in the cold season, called explosive cyclones, cause wide-ranging heavy snowfall and strong winds, which make enormous impacts on lifelines; the damage to electric power cables, the cancellation of flights, ships and trains, traffic jams, and so on. Severe snowstorms associated with explosive cyclones occasionally cause sudden whiteouts, resulting in fatal accidents. Therefore, elucidating processes causing such a sudden change in weather leads to the improvement of weather forecast and disaster prevention in the cold season.

An extratropical cyclone occurred over the Japan Sea on March 1 2013 and passed through the Hokkaido Island in Japan developing rapidly, and then peaked over the Sea of Okhotsk on March 3, which is classified into an explosive cyclone according to the rapid deepening rate of the surface central pressure of the cyclone. The severe snowstorm associated with the explosive cyclone caused a sudden whiteout in the eastern part of the Hokkaido Island and killed nine people on March 2.

To investigate the mechanism of the sudden whiteout by the severe snowstorm, numerical simulations are performed using the Weather Research and Forecasting (WRF) model. The control simulation with the realistic topography (denoted as CNTL) successfully reproduces the track and evolution of the cyclone, the distribution of strong winds associated with the severe snowstorm, and the sudden change in weather in the eastern part of Hokkaido. To examine orographic effects on the sudden change in weather, an additional simulation, in which the Hokkaido topography is modified into a flat, (denoted as HOK-FLAT) is performed. A comparison of the low-level geopotential and wind distributions between CNTL and HOK-FLAT clearly shows the barrier effects due to the Hokkaido topography. The positive geopotential anomaly generated on the upstream side of the topography in the CNTL simulation rapidly shifts strong winds eastward and also enhances the strong winds.