Multiple Polar Mesocyclones over the Japan Sea on 11 February 1997

Abstract

The characteristics and development mechanism of four meso-?-scale polar mesocyclones (M?PCs), which appeared as a vortex train over the Japan Sea from 0000 to 1700 UTC on 11 February 1997, are studied by means of observational analyses and numerical simulations. These four M?PCs were first observed at 0000 UTC along the southern edge of a meso-α-scale cloud system that looked like an occluded extratropical cyclone in a Japanese Geostationary Meteorological Satellite-5 (GMS-5) infrared satellite image. Detailed evolutionary processes of these M?PCs were described based on satellite and radar images. The horizontal scales of these M?PCs were on the order of 100 km, and the spatial intervals between their centers were about 200 km. Their lifetimes were around 10 h. The occlusion-like features of the meso-α-scale cloud system and the synoptic situation prior to the development of these M?PCs are examined by using regional objective analysis (RANAL) data of the Japan Meteorological Agency (JMA). In order to clarify the detailed structures and the development mechanism of these cyclones, numerical simulations using the JMA Regional Spectral Model (RSM) are performed. The simulation initiated from 0000 UTC 11 February 1997 reproduces the four M?PCs reasonably well: their life times and positions are in good agreement with the satellite observation. Detailed analyses of the simulated vortices indicate that they have shallow structures accompanied by relatively stronger upward motion and they have ?warm core? structure. The developments of these vortices are attributed to a combination of baroclinic instability and condensational heating, but barotropic instability could not be ruled out.