On the mix-down times of dynamically active potential vorticity filaments

A simple model is used to study the evolution of potential vorticity filame nts, viewed in cross-section, subject to steady shear and deformation flows representative of the large-scale atmospheric circulation. It is found tha t the balanced, ageostrophic circulation induced by the anomalous potential vorticity can cause the evolution of a dynamically active filament to diff er substantially from that of a dynamically passive filament in a similar b ackground flow. It is suggested that estimates of the mix-down time of mate rial contained in atmospheric filaments need to be corrected to allow for t his effect.