A Classification of Snow Clouds by Doppler Radar Observations at Nagaoka, Japan

A classification of snow clouds, called the "snowfall mode," is proposed based on Doppler radar observations at 10-minute intervals at Nagaoka in 1999/2000 winter season. Using 795 hours of data at an altitude of 1.6 km, six snowfall modes were defined: longitudinal line (L-mode), transversal line (T-mode), spreading precipitation (S-mode), meso-beta scale vortex (V-mode), mountain-slope precipitation (M-mode), and local-frontal (discontinuity) band (D-mode). In migrating snow clouds, a subclass, referred to as snowfall with coastal intensification (xI-mode, where x is L, T, S and V) was defined. A sample snapshot and the mean Ze are shown for each snowfall mode. The frequency of occurrence of the snowfall modes indicated that both of the longitudinal cloud streets and the meso-alpha scale disturbances occupied about 1/3 of the analysis period. About 18% of the precipitation in the analysis period was considered to be under orographic effects. The prevailing wind direction differed between the snowfall modes although a west-northwesterly wind dominated.