Structure of Wind Field and Variations of Vorticity in a Summer Situation

An analysis of the observed wind field at 300 mb is shown for a period when iterative shear-line formations occurred. Each was preceded by the injection of a localized wind maximum into lower latitudes, southwesterly winds subsequently increasing east of the trough line, but not by advection of high winds around the southern end of the trough.

Series of cross sections at different times illustrate the wind variations as the localized jets move through them. These indicate that the local wind maxima at 300 mb reflect similar variations through a deep layer and thus are not due to vertical undulations in the height of the jet stream.

During a time of relatively steady state, changes of vorticity are examined in the region east of the trough, where the jet is weak in lower latitudes and increases in speed downstream. Locally large individual increases of absolute vorticity (of the order of the value of the Coriolis parameter in 12–24 hr) in the upper troposphere are accounted for by the conversion of vertical shear into vorticity about a vertical axis, through differential vertical motions in an indirect solenoidal sense. A simultaneous increase in vertical shear is accounted for by horizontal circulations in a direct solenoidal sense. Considering the trough as a whole, there was weak subsidence in the upper troposphere, with maximum descending motions near the jet stream.