Observed evolution of vertical planetary wave structure in relation to the propagation characteristics of the basic state

The extent to which linear wave theory explains the structure and time evolution of observed planetary waves at a given time or season in the stratosphere is not well known. The sensitivity of linear wave models to details of the basic state and model damping, both of which are not determined from observations in great accuracy, makes it hard to determine why the observations deviate from modeled waves in any given case.

Looking at the vertical structure of waves, both in models and in observations allows us to examine this more rigorously. Temperature and geopotential height are the most directly observed wave fields, and the latitude and height variations of their phase are related to the wave propagation in these directions. By understanding the effects of the wave propagation geometry and damping on the vertical structure of the amplitude and phase of both geopotential height and temperature of the waves in a model, we can relate specific large scale (and easily observed) features of vertical structure to specific features of the basic state. We can then test linear theory by examining to what extent observed wave structures are consistent with the propagation characteristics of the basic state. In particular, good diagnostics of the propagation characteristics of the basic state are the meridional and vertical wavenumbers calculated from the steady state linear wave solution to the observed zonal wind and temperature.

We apply this to two cases of wave evolution with very different time scales, daily and seasonal. First, we show how daily time scale variations of the vertical structure of a wave during its life cycle result from time variations in the basic state. Second, we show how the seasonal evolution of vertical wave structure from mid winter to late winter in the southern hemisphere can be explained in terms of the changes in the vertical wavenumber, calculated from mid-winter and late-winter basic states.