In early winter season, thunderclouds are frequently generated in Hokuriku (locates on the south of the sea of Japan.) These thunderclouds are remarked with a high occurrence of positive lightning and huge amounts of electric charge which rushes toward the ground, hence accidents associated with such a lightning frequently happen in Hokuriku during this season. For warning of the lightning, it is important to detect a generation of graupel, because a riming process caused the electrification. The C-band dual polarization radar of Hokuriku Electric Power Company is available for the study on the winter thunderstorm.
In this paper, we analyze the radar data during late December (cold air outbreaks begin in this period) in 1999, 2000 and 2001, and make a comparison between the radar data and ground-based observation on precipitation particles to investigate a detectability of the graupel in the winter thunderclouds by the radar. Moreover, we clarify a geographical distribution of precipitation particles (rain, snow, graupel, etc.) by the polarization method around the coastline to derive the physical process of the thundercloud generation.
As a result of the classification of precipitation particles using the lookup table based on the horizontal reflectivity factor (Zh) and the differential reflectivity (Zdr), the graupel was detected by the radar, when the lightning occurred near the ground observation site. Interestingly, it was observed that the thundercloud including the graupel caused the both positive and negative lightnings, while the thundercloud in which the graupel had already fallen to the ground caused a positive lightning with large current only once. This result supports the hypothesis that it cause the positive lightning with large current that the ice particle with the positive electric charge stays in the thundercloud for a long time after the graupel with the negative electric charge falls to the ground.
As for the geographical distribution of the precipitation particles, the graupel tended to exist in the range of 5 km from the coastline. Furthermore, wet snow and rain drops distributed near the sea surface while dense snow distributed on the ground. These distributions suggest that the warm and moist air near the sea surface was ascended by the horizontal convergence near the coastline to generate a large quantity of cloud water. Therefore, the graupel was efficiently generated by riming process below 2 km in height. Then, the generated graupel immediately fell to the ground near the coastline because the graupel was generated in the low altitude. Consequently, the ice particle with the positive electric charge remained in the thundercloud.
This study derived the generation mechanism of the thunderclouds around the coastline on the south of the sea of Japan during early winter and indicated the detectability of the graupel by the dual polarization radar.