Abstract
The results of synoptic analysis are described concerning the two typical examples of winter-time polar lows over the Japan Sea area. That occurred in the period of 24-27th FEB. 1981.
In this report, the following results are obtained.
(1). Non-frontal cyclones of small dimensions are formed in polar air masses over the Japan Sea in winter. They are located on the forward sides of the positive vorticity maxima at 500 mb and accompanied with a spiral or comma-shaped cloud pattern as they mature. They are also associated with the convectively neutral layers through a substantial depth of the troposphere and situated in regions marked by well-developed baroclinity throughout the troposphere in the poleward side of the jet stream. These features show that these cyclones are so-called polar lows.
(2). There are two-kinds of vorticity maxima at 500 mb. One is located in the central regions of cold vortex, and the other in the trough areas. The surface cyclones combined with the cold vortices are also of small dimensions below about 1000 km over the Japan Sea area and can be regarded as polar lows.
(3). The formation of deep layers of convective neutrality is investigated. It is shown by the stability change equation that the convectively neutrae layers of the middle troposphere between 700 and 500 mb on the forward areas of the positive vorticity maxima at 500 mb are mainly formed by the horizontal convergence effect, while in the rear areas of these maxima the layers are stabilized by the horizontal divergence effect. The layers below 700 mb are destabilized by low-level heating over the oceans in winter. These two layers combine and form the convectively neutrae layers through a considerable depth of the troposphere in front of the upper positive vorticity maxima.
(4). Instability mechanism for the formation of these cyclones are discussed by using of Petterssen's development equation and the distributions of Richardson numbers. It is suggested that the baroclinity effect, including the influence of the vorticity advection in the upper atmosphere on the surface cyclogenesis, and the non-adiabatic effect in the atmosphere of small Richardson numbers below about 10 are the main cause of the formation of the cyclones of small size below about 1000 km.
(5). The heavy snowfalls caused by the convective cloud bands accompanied with these polar lows in the coastal plain regions along the Japan Sea are also investigated. It is shown that the distributions of daily precipitation amounts are interpretable by the distributions of these convective cloud bands.
