Empirically Evaluating Divergence Rates of Atmospheric Trajectories

Abstract

Observed Northern Hemisphere divergence rates of phase-space trajectories initially emanating from neighboring points are estimated, using a simple, seasonal, weakly GCM-dependent linear method applied to Ertel potential vorticity on the 315-K isentrope. Using the same data and essentially the same method, atmospheric persistence, the rate of trajectory departure from an initial state, is also estimated. With the results applying only to the truncated state considered (comprising only a single variable observed at a single level), it is found that the time scale for divergence of two, initially reasonably similar, trajectories is about 4?6 days, between the time scales of cyclogenesis and blocking. It is also found that the time scale for divergence of a given trajectory from an earlier point along it is about 3?5 days.