The effect of reflecting surfaces on the vertical structure and variability of stratospheric planetary waves

In this work we study the effects of downward reflection of stratospheric planetary waves from upper stratospheric turning surfaces on their vertical structure and evolution. Past studies have mostly considered the effect of lower stratospheric reflecting surfaces on blocking wave propagation into the stratosphere (e.g. Charney and Drazin, 1961). Only little attention has been given to upper stratospheric reflecting surfaces which do not block wave activity completely, but affect wave structure. In this work we establish that reflecting surfaces do form in the upper stratosphere, and that downward reflection from these surfaces can have a large effect on the vertical structure of the waves, and its time evolution.

To diagnose the wave propagation characteristics of the basic state, and in particular, to determine the existence and geometry of turning surfaces, we develop a wavenumber diagnostic that is based on the steady state wave solution to a given basic state. Our diagnostic is a more accurate indicator of vertical wave propagation regions than the index of refraction, because it takes into account the meridional structure of the wave even when the basic state is nonseparable in latitude and height.

We then apply our diagnostic to observations from the southern hemisphere winter of 1996 and show that reflecting surfaces exist in the upper stratosphere, and have quite a large effect on wave structure. We find that the differences in vertical wave structure between middle and late winter can be explained as a linear response to the seasonal evolution of the basic state which involves a formation of a reflecting surface in late winter. We also show that on daily time scales wave mean flow interactions cause significant changes in the basic state propagation characteristics for periods of a few days. These changes, along with the time variations in the forcing of the waves are responsible for the observed daily time scale variations in wave structure. The fact that the observed evolution of the waves and the basic state are consistent with linear or quasi-linear wave theory (depending on the time scale we look at) increases our confidence in the applicability of the theory, as well as in the observations.