丘陵地形上の降雪と気流に関する観測的研究

Abstract

The characteristics of snowfall and airflow over a low mountain barrier (the Dewa Hills; the height of the ridge is about 0.6 km) were investigated using a Doppler radar and rawinsondes. A sequence of snow clouds appeared accompanying a cold-front passage on 3 February 1990. Radar echoes were observed to pass over the Dewa Hills for about five hours, and they had diameters of tens of kilometers. The mixed-layer top was lowered and the wind was accelerated over the low mountain barrier, and the flow was considered to be subcritical flow. The lowered mixed-layer top boundary suppressed the cloud- and echo-top height. The echoes contained the cells whose sizes were 5 km to 10 km. The cells were accelerated by 3 m/s when they were over the mountain barrier. The precipitation enhancement occurred over and as far as 10 km upwind of the mountain barrier. It is considered that the dominant process of the precipitation enhancement was a seeder-feeder process between the snow clouds and the clouds formed by the orographic lifting. On the lee side, colder air in the lowest layer suppressed descending flow and prevented the precipitation reduction by sublimation. It is considered that aggregation and melting mainly increased reflectivity on the lee side. In this case, the behavior of the snow clouds was strongly affected by the flow over the low mountain barrier, while the mean flow was not significantly modified from subcritical flow by the existence of the snow clouds