A Statistical Study of the Snowfall Distribution on the Japan Sea Side of Hokkaido and Its Relation to Synoptic-Scale and Meso-Scale Environments

摘要

We carried out statistical analyses to investigate the characteristics of snowfall distributions over the Ishikari Plain of the island of Hokkaido, Japan. Using AMeDAS's hourly precipitation data between 1982 and 1991, we determined that the snowfall distribution could be classified into two regions and using the rotated empirical orthogonal function : one is a mountain-centered distribution, and the other is a plain-centered distribution. The characteristics of the temporal variations of the two spatial patterns showed that the plain-type snowfalls apt to occur when it is colder, i. e., in the morning or in the latter part of winter. Next, we investigated the relationship between the plain-type snowfall and synoptic-scale and mesoscale environments. We were able to establish that the plain-type snowfalls are accompanied by offshore cold winds that blow in the planetary boundary layer against the winter northwesterly monsoon. When the snowfall continues for more than one day, the offshore wind originates in the large scale katabatic winds that spread around the whole river basin behind the coast. This finding suggests that the source of the offshore wind is the pooled cold air on the plain produced by the confluence of the katabatic flow from the surrounding mountains. We proposed that continuous drainage from the pool enables the coastal snowfall to continue. We also investigated the large-scale atmospheric conditions, using the data for the objective analysis. When a plain-type snowfall occurs, the westerly wind speed and the temperature at the upper troposphere are respectively weaker and lower than these factors during snowfall with no offshore wind. These synoptic-scale environments encourage the katabatic flow, and eventually prolong the coastal snowfall. Moreover, these synoptic-scale environments are similar to those that operate during snowfalls in other regions facing the Japan Sea.