A Polar Low over the Japan Sea on 21 January 1997. Part Ⅱ: A Numerical Study

摘要

A remarkable isolated polar low observed over the Japan Sea on 21 January 1997 is studied by numerical simulations using a 20-km-mesh regional primitive equation model [Regional Spectral Model (RSM) of the Japan Meteorological Agency] and a 5-km-mesh nonhydrostatic mesoscale model [Meteorological Research Institute Nonhydrostatic Model (MRI-NHM)] and by sensitivity experiments using the MRI-NHM. The 24-h integration of RSM starting from 0000 UTC 21 January reproduces quite well the polar low's movement and development, including the initial wrapping of the west part of an east-west-oriented vorticity zone. The vorticity budget analysis indicates that the stretching term plays a dominant role in the polar low development. The simulation by MRI-NHM reproduces surprisingly well the wrapping of the western part of the east-west-oriented vortcity zone associated with a cloud band, the spiral-shaped cloud pattern, and the "eye" structure near the vortex center. The wind speed and resulting sea surface heat fluxes of the simulated polar low exhibited a significant deviation from axisymmetry. The eye, almost free of cloud, was relatively dry and was associated with a strong downdraft and a warm core. A thermodynamic budget analysis indicates that the warm core is mainly caused by the adiabatic warming due to the downdraft. Two types of sensitivity experiments using a 6-km-mesh MRI-NHM are performed: In the first type, the physical processes such as condensational heating and/or surface heat fluxes are switched on-off throughout the whole integration time of 24 h, as in the past sensitivity studies. In the second type, on the other hand, both condensational heating and surface heat fluxes are incorporated up to a certain time, but then one or all of them are switched on-off for the following 1 h. This minimizes the deformation of the environment of the vortex and is suitable for studying the direct effect of the physical processes on the vortex development. The results of the sensitivity experiments show that the rapid development of the present polar low is principally caused by the condensational heating. The surface heat fluxes, on the other hand, are important for maintaining the environment that supports the vortex development; that is, the absence of the surface heat fluxes stabilizes the stratification in the boundary layer so that the vortex development is severely suppressed.