Structure of Shear Lines near the Tropopause in Summer

A detailed study is made of a shear line in the tropopause region. A summer case was purposely selected, in which the lower half of the atmosphere was relatively barotropic, in order to minimize the role that could possibly be played by a subsiding cold air mass in producing a vertical stretching and consequent concentration of vorticity in the higher atmosphere.

During the period studied, the circulation over North America was characterized by the existence of two distinct jet streams, the southernmost of which, though initially having the form of a sinusoidal trough, appears to “buckle”, forming a very sharp trough separating two jet streams of opposite direction. A possible mechanism is suggested for the production of strong northerly flow leading up to shear-line formation, which depends upon a mutual interaction between the mass and velocity fields of the jet-stream systems located at different latitudes.

An attempt to determine whether potential absolute vorticity is conserved during formation of the shear line is not wholly conclusive, but indicates a reasonable possibility that vorticity may be generated in the free atmosphere.