Microphysical Structures of Early-winter Snow Clouds during a Cold Air Outbreak of 23-25 December 2010

During Siberian cold air outbreaks in winter, the relatively warm sea supplies a large amount of heat and moisture into the atmosphere, and numerous convective clouds then form and develop over the Sea of Japan. These clouds often bring heavy snowfall over the coastal areas of Hokuriku, which is the northwestern region of Honshu that faces the Sea of Japan, and this heavy snowfall sometimes causes problems, such as snow accretion and snowbound traffic. To prevent these problems caused by snow, it is important to understand the mechanism of snowfall and to predict the temporal and spatial patterns of snowfall. In order to understand the mechanism of snowfall, it is essential to investigate the microphysical structures of clouds, such as the types, sizes, and concentrations of precipitation particles. In the Hokuriku coastal regions, intense lightning activity is often observed during early winter snowfall, and such an event is popularly referred to as “yuki-okoshi” in Japanese, which means the event is an indication of the beginning of winter. Investigations of the microphysical structures of snow clouds are also important to understand the mechanism of winter lightning.

In-situ observations are essential to gain a better understanding of the detailed microphysical structures of a cloud. Videosondes (Takahashi 1990) are powerful tools for observing solid precipitation particles such as graupel and snowflakes, since they can measure precipitation particles without contact and can obtain images of the particles as they fall through the air. In order to understand the process of heavy snowfall and the electric charge distribution in Hokuriku during a winter thunderstorm, an observation campaign using videosondes was conducted in Kashiwazaki (37.35°N, 138.58°E), which is located in the Hokuriku area of Japan, in the early winter of 2010 (Sugimoto et al. 2011). A total of 15 videosondes were launched into snow clouds that were associated with the cold air outbreak that occurred from December 23 through December 25. We succeeded in continuously launching the videosondes, and the vertical distributions of the precipitation particles in the snow clouds that developed through the entire stage of the cold air outbreak are revealed. To the best of our knowledge, this was the first time that numerous videosondes were launched into snow clouds continually and frequently during a cold air outbreak. In this study, the evolution of the microphysical structures associated with the cold air outbreak in early winter is investigated.

During an intense cold air outbreak event that occurred from December 23 to 25, 2010, 15 videosondes were successfully launched into snow clouds, and the precipitation particle distributions of the snow clouds were revealed for the entire cold air outbreak. To our knowledge, this was the first time that numerous videosondes were launched into snow clouds continuously within the period of a cold air outbreak. The 15 videosonde soundings were classified into three stages of the cold air outbreak (the Pre-CAO, CAO, and Post-CAO stages) on the basis of the evolution of the surface temperatures. We found that the precipitation particle distributions and the microphysical features in the clouds were different among the three stages. Graupel mainly contributed to the total precipitation during the Pre-CAO stage. As the graupel mass contribution ratio decreased, the contribution of snowflakes to the total precipitation increased as the cold air outbreak developed. The vertical distributions of the number densities of graupel varied among the different stages of the cold air outbreak. The altitude of the peak for the number density of graupel was located at lower levels during the late CAO stage, at which the temperature was warmer than -10°C. These results suggest that the evolution of the microphysical structures associated with the cold air outbreak, especially the graupel vertical distributions, is related to the lightning activity.