A Simple Method of Discriminating between Occurrences of Freezing Rain and Ice Pellets in the Kanto Plain, Japan

Abstract

Based on a freezing rain and ice pellet event observed on 3 January 2003 in the Kanto Plain, Japan, the atmospheric conditions necessary for the occurrence of such phenomena in this area were investigated, and a simple method of discriminating precipitation types, including freezing rain and ice pellets, was examined.
Freezing rain and ice pellets fell from 1800 to 2400 JST on 3 January in the western portion of the Kanto Plain. During the precipitation, northwesterly cold-air advection near the surface dominated in this region, with southerly warm-air advection associated with the approach of a synoptic low present above the cold air layer. This atmospheric structure produced the conditions required for freezing rain and ice pellets to form, namely a melting layer with an air temperature above 0°C to melt snow particles into raindrops, and a refreezing layer with an air temperature below 0°C to supercool the raindrops into freezing rain or freeze them into ice pellets.
To establish a method of discriminating between freezing rain and ice pellets, the conditions required for the complete melting of snow particles and the complete freezing of raindrops during the event were examined theoretically. We found that the mean air temperatures, the mean relative humidities, and the depths of the melting and refreezing layers can be used to distinguish the environmental conditions required for freezing rain and ice pellets to form. In addition, determination of whether the wet-bulb temperature at the surface is above or below 0°C was used to discriminate rain and freezing rain. The distribution of precipitation types at 2100 JST on 3 January 2003 in the Kanto Plain was estimated using the method described above, and the results agreed closely with the distribution actually observed. The diagnostic method presented in this study enables the discrimination of precipitation types, including freezing rain and ice pellets.