1999 Volume 77 Issue 5 Pages 1039-1059
When the winter monsoon prevails over the Japan Sea, many shallow convective snow clouds appear in the atmospheric mixed layer, and provide much snow over Japan. The behavior of the snow clouds and airflow affected by a complex terrain in the Tsugaru district of Japan, was studied based on single Dopplerradar observations. At ∼15km upstream of the Tsugaru mountains, which is a low-altitude (200-700m ASL) ridge, a weak-wind zone accompanied by upward developing snow clouds appeared parallel to the mountains. Above and in the lee of the Tsugaru mountains, the tops of the snow clouds were significantly lower towards the east accompanied with wind acceleration. In accordance with a theory of 2-D two-layer flow passing over a ridge, sonde-observed environmental parameters described a flow regime, where the flow is partially blocked and associated with a hydraulic lee jump. The observed flow pattern was consistent to this theoretical prediction, although the hydraulic lee jump could not be conclusively identified due to observational difficulties. Low-level wind convergence in the front of the weak-wind zone sustained the upward development of the snow clouds. The upward development of the snow clouds, however, did not lead to an obvious increase of snowfall intensity. A WSW to ENE elongated zone of increased snowfall appeared from the north of the Shirakami mountains and Mt. Iwaki (both high-altitude mountains with peaks exceeding 1000m ASL), to Mutsu Bay, passing through a corridor of the Tsugaru mountains. The orographic effects of the Shirakami mountains, Mt. Iwaki, and the corridor of the Tsugaru mountains were examined with respect to maintaining this elongated snowfall zone.