A Meso-Scale Low Family Formed over the Northeastern Japan Sea in the Northwestern Part of a Parent Polar Low

Abstract

Development of a meso-scale low family over the northeastern Japan Sea on 3-4 January 1987 is studied.
The meso-scale low family formed in the northwestern part of a parent polar low. Local low-level frontogenesis occurred as a result of the horizontal differential thermal advection between polar air streams from the continent to the southwestern side of the parent polar low and warm air streams from the Pacific to the northwestern side of the polar low. A low-level shear line of WNW-ESE orientation formed in the frontogenesis zone.
While upper-air observations showed a thermal gradient of ∼1.5K/100km and a lateral wind shear of ∼15(m/sec)/100km across the shear line, dense surface observations indicated a strong lateral shear of 5-10(m/sec)/10km within the shear zone of 20-30km width.
Two meso-scale lows developed along the shear line with an interval of ∼200km. Each meso-scale low showed the warm core structure and brought strong gusts of ∼25m/sec with intense precipitation.
While the generation of these lows in the strong shear zone suggested the barotropic process, the meso-scale cyclogenesis in association with the local frontogensis indicated the baroclinic generation process. It was examined whether the observed meteorological parameters would satisfy the criterion of barotropic shear or Eady's type baroclinic instability in the shallow layer obtained from linear instability analyses. It was shown that the maximum growth rates of these instabilities appeared around a wavelength of 200km, although the maximum growth rate of the baroclinic instability calculated from sparse aerological data was considerably smaller than that of the barotropic instability calculated from dense surface data.