Synoptic-Scale Environmental Features of the Long-Lasting Ice Pellet Event in Northern Japan on 10 April 2005

An extremely long ice pellet event with extensive episodic ice pellet falls occurred over northern Japan on 10 April 2005. The synoptic-scale environmental features of this event were examined based on Japan Meteorological Agency (JMA) mesoscale model (MSM) datasets. A synoptic-scale dry environment was formed under the melting layer ahead of the surface warm front by an intense secondary circulation. Back-trajectory analysis suggested that dry air was located to the south of the precipitating area and was conveyed to the precipitating area by strong geostrophic winds. The surface air at Sapporo, Japan, originated from the ocean surface to the southeast and was cooled to almost the same temperature as that of the sea surface (+3°C); that is, the minimum temperature at 600 m progressively decreased to −3°C at the dry adiabatic lapse rate, during the ice pellet event. Sapporo and its surrounding areas, which were at the southern edge of the precipitating area until the passage of the low pressure center and continued to experience midlevel dry and low-level cold inflows maintained by persistent synoptic-scale structures, were identified as the favorable locations for long-lasting ice pellets.