Kinematic and Thermodynamical Structures of Longitudinal-Mode Snow Bands over the Sea of Japan during Cold-Air Outbreaks Part I: Snow Bands in Large Vertical Shear Environment in the Band-Transverse Direction

摘要

The kinematic and thermodynamical structures of two longitudinal-mode (termed “L-mode”) snow bands over the Sea of Japan occurring on February 8, 1991 and January 21, 1993 are analyzed mainly based on dual-Doppler radar data. The L-mode snow bands with multicellular structure in 1991 and 1993 formed, respectively, at the early onset of and toward the end of cold-air outbreaks, where the magnitude of the band-transverse vertical shear was roughly 2 × 10-3 s-1 approximately in the lower-half of the mixed layer. This magnitude was larger than that associated with L-mode snow bands characterized by axi-symmetric circulation, which will be described in Part II. Thermodynamical structures and the spatial distributions of water substances in the two snow bands were inferred from variational-based retrieval.A pronounced feature of the airflow structures in both snow bands was upshear-tilting updrafts in the band-transverse vertical cross-section. At least two factors could account for their formation: the existence of a certain depth of the vertical layer of the environmental band-transverse system-relative wind components directing to the upshear, and the lower terminal fall velocities of snow and graupels. The retrieval results showed that both snow bands had a subsaturated cold pool beneath the band around the surface, whose collision with the unstable ambient air could have been responsible for overall upward motion in the bands. With regard to the energetics of the band circulation, energy production by buoyancy and the band-transverse shear was dominant.The repeated formation of new cells was observed in the two snow bands in the downshear side, which may have been triggered by the low-level collision of the outflow from active cells or a cold pool with the unstable ambient air. As the new cell developed enough, the older cell significantly decayed. Consequently, the successive formation of cells did not change the overall echo pattern of the L-mode snow bands without producing elongated echoes branching o. into the downshear direction at large angles to the orientation of the L-mode snow bands.