On shear lines in the upper troposphere over Europe

One of the features often associated with the life cycle of large-amplitude disturbances in the upper troposphere is the formation of shear lines. A region noted for this phenomenon is western Europe.

Approximate statistics concerning the occurrence of shear lines over Europe in 1974–1976 are presented. The main emphasis is, however, given to a case study of potential vorticity budget in the region of a shear line which occurred over Europe on November 1–4, 1975. The shear line area is a region of high (potential) vorticity. For the case studied, the total time derivative of potential vorticity indicated, in the area of the shear line, a sink of this quantity with an e-folding time of about 1 day. If the potential vorticity is assumed to diffuse mainly vertically, an exchange coefficient of the order of 10 m² s^-1 is needed to explain the sink. This value agrees with that obtained by Shapiro (1980) from the observed vertical fluxes of ozone and some chemical constituents.