SYNOPTIC SIGNIFICANCE OF DYNAMIC INSTABILITY

Abstract

The subsequent motion of a perturbed air-parcel is outlined, together with the criteria that determine the four fundamental types of dynamically stable and dynamically unstable motions. The specification of such motions for synoptic studies is presented, and the physical significance of the motions themselves discussed. The importance of lateral diffusion in the atmosphere is deduced, especially when dynamic instability is present. A region of dynamically unstable motions is identified as one showing lateral ?perturbation divergence.? The sectors of compensating convergence can also be established, especially for the class of instability characterized by a unique ?axis of outflow? for the perturbed parcels. In theory, marked instability should be associated with a negative development tendency for the corresponding surface pressure system, and marked stability with a positive development tendency. Statistical studies of a large number of dynamic-instability computations adequately verified these concepts and revealed the fact that at 500 mb only one quarter of all cases showed actual dynamic stability. When the computations are grouped with respect to distance from the 500-mb polar front, maxima of stability are found in the quasibarotropic cold and warm air-masses and in the frontal transition zone; a significant maximum of instability is found in the baroclinic warm-air zone. This instability is especially well marked for strongly baroclinic cases. The physical significance and the consequences for forecasting of these synoptic results are discussed.