Abstract
Increasing evidence indicates that surface cyclogenesis is predominantly a response to the approach of a preexisting trough at upper levels. A question then arises about the origin of the upper-level predecessor. As an initial approach to this question, mobile troughs in the major band of westerlies were crudely tracked in daily Northern Hemispheric 500-mb analysis during nine recent cold seasons. These troughs were identified only in the 552-dam height contour. Between 8 and 15 of them were present on a given day. Study of a particular cold season showed a median duration of 12 days and a mode of 5 days. Average zonal phase speed was 13 m s−1.
Locations or origin and termination of individual troughs were distributed over all longitudes, but births greatly exceeded deaths over and east of the Rocky Mountains in North America and the highlands of central Asia. Trough terminations dominated over the eastern portions of the oceans. Within the quasi-steady planetary waves, origins and terminations of the smaller mobile troughs occurred preferentially in northwesterly and southwesterly flow, respectively.
More detailed studies of the structure during episodes of origin over North America showed prominent vertical and lateral shear in the time-averaged 500-mb flow, rapid growth of the perturbations through the depth of the troposphere, with a vertical tilt upshear only in the lower half, pronounced maximum amplitude near the tropopause, and a variety of circumstances in which troughs became organized in the belt of major westerlies.
