The characteristics and structure of mesoscale snow clouds generated in the western Hokuriku District of Japan is examined, mainly through Doppler radar data. These clouds were produced in the convergence zone between the land-originated local wind (land breeze) and the northwesterly monsoon flow. This convergence zone was located in a convective mixed layer over the relatively warm sea, producing mesoscale snow bands found along the coast.
The snow bands periodically appeared in synoptic situations during which the northwesterly monsoon was not intense and moderately cold air existed in the lower troposphere. They occurred at an interval of 3-4 days and brought about moderate snowfall to the coastal regions. Their horizontal scale was approximately 100km×20km with a lifetime of 1.0-1.5 hours classified them as meso-β scale phenomena. The land breeze circulation was produced by a thermal contrast between the warm sea and the cold mountainous land. The cold gravity current associated with the circulation flowed downward toward the sea. The current was modified by the relatively warm sea surface and converged with the northwesterly monsoon flow 10-30km offshore, producing the snow bands.
The snow bands exhibited a marked evolution from the growing stage to the decaying stage, as they moved from the sea to the coast. The successive production of the snow bands resulted from the coupling of three air flows, the northwesterly monsoon, the land breeze and the low-level outflow originating from the anvil-like cloud at the rear of the snow bands. The low-level convergence between the monsoon flow and the land breeze resulted in the development of the snow bands in the growing stage. The pumping of land breeze air into the updraft of the snow bands caused the land breeze front to retreat toward the coast, accompanied by the snow bands. The low-level outflow from the decaying snow bands acted as a trigger or a seed for the formation of a new snow band. The dissipation of the snow band and the extinction of the updraft pumping allowed the front to advance offshore again.