Abstract
The “parcel” method of treating dynamic instability is extended to the case of an arbitrary flow pattern. The criteria for instability become relatively complex and depend on the three-dimensional gradients of temperature and geostrophic wind, as well as on the latitude. Fundamentally different types of instability and stability may arise; these are illustrated by typical examples. Methods for the computation of the various terms in the fundamental stability-criterion parameters are presented, and these methods, as well as the generalized criteria themselves, are extended to the case of saturated air.
It is suggested that marked quasi-horizontal, or dynamic, instability will superimpose a strong field of “perturbation divergence” on the ambient field of mass divergence and hence should be a significant factor in the weakening of anticyclones and the formation and deepening of cyclones.
