AN INVESTIGATION OF A SELECTED COLD VORTEX OVER NORTH AMERICA

Abstract

An example of a cold vortex to the south of the maximum westerlies, with its entire life history within the network of aerological observations of the United States, is investigated. The conditions for the formation of such a cold vortex are found to be a rapid intrusion of cold air into the upper west-wind belt from the north, a strengthening and cyclonic rotation in time of the wind upstream, and a strong wind around the cold dome in the south. Such a situation leads to a creation of solenoids in the horizontal plane, a strong, unbalanced solenoid field in the vertical plane perpendicular to the boundary of the cold air, and most probably to a rapid descending motion of the cold air in the north; while in the south, conditions are more- or less balanced. This results in a residual cold dome. The further development and strengthening of such a cold vortex depends on the maintenance and the strengthening of the wind around and above the cold dome, which in turn depends on the energy supply upstream. If the surrounding wind is strengthened, the cold vortex will increase its revolving motion and may build down to the surface, forming a system extending from the surface to the lower stratosphere with an approximately vertical axis. This type of surface cyclogenesis is evidently different from the usual type associated with “waves” on a surface front. The displacement of the cold vortex is mainly dependent on the strength of the steering current. The influence of the motion within the cold vortex on the displacement is supposed to be of importance only when the steering current is weak. The approach of a strong west-wind belt to the north of the vortex will tend to destroy the cyclonic wind field around and above the cold dome, and result in rapid decay of the cold vortex. The observations showed definite evidence of indirect circulation in the sense that the cold air in the vortex center was lifted while the surrounding warmer air descended. A consideration of the tendency and vorticity equations shows that frictional forces in the upper atmosphere play an important role in the strengthening of this kind of vortex.