Simulations of an observed stratospheric warming with quasigeostrophic refractive index as a model diagnostic

Abstract

SUMMARYA three-dimensional primitive equation model of the stratosphere and mesosphere is described. The model was forced at its lower boundary by observed lOOmb height fields for the wavenumber 2 stratospheric warming period of February 1979, and correctly simulated the reversal of the high latitude circulation. This behaviour contrasts with earlier model simulations of wavenumber 2 warmings in which forcing of a climatological zonally symmetric initial circulation by a stationary wave perturbation led to an initial reversal of the circulation in low latitudes. In a series of idealized experiments we show that wave-wave interaction played no essential role in this simulation and that the ingredients leading to its success were firstly the non-climatological initial wind structure and secondly an imposed longitudinal phase speed for the upward propagating planetary wave at the lower boundary. These studies also demonstrate that in the prewarming period the direction of propagation of planetary wave activity, as represented by integral curves of the so-called Eliassen-Palm flux, may be qualitatively described by the WKBJ limit of the quasi-geostrophic potential vorticity equation. In this limit, the Eliassen-Palm flux is simply related to the zonal mean refractive index, leading to two complementary diagnostics for studying wave, mean-flow interaction.

How Well do we Understand the Dynamics of Stratospheric Warmings?

“…cit.). Essentially similar results have been independently obtained by Butchart et al (1982), using a different numerical model.…”

“…Results like these add to the growing body of evidence suggesting that, no matter what the troposphere is doing, conditions in the stratosphere have to be prepared in some special way before a major warming can take place (Butchart et al, 1982;Dunkerton et al 1981;Kanzawa, 1980;Labitzke, 1981;Palmer 1981b;Quiroz et al, 1975, §2b), especially a warming dominated by wave 2. Something is needed which can overcome the defocusing effect and guide planetary waves upwards into the polar cap.…”

“…The observations are not only giving a much better idea of how, for instance, Eulerian-mean zonal wind profiles change from day to day, but even some idea (Butchart et al, 1982;Chapman and Miles, 1981;Kanzawa, 1980;Kanzawa and Hirota, 1981;O'Neill and Youngblut, 1982;Palmer, 1981a, b;Simmons, 1982b) of the harderto-estimate quantities which theory tells us must be central to the dynamics.…”

“…For a recent review the reader may consult section 2 of the paper by Edmon et al (1980). Examples of the use of the EP flux to describe stratospheric planetary waves have been given by Butchart et al (1982), Dunkerton et al (1981), Kanzawa and Hirota (1981), Youngblut, 1982, Palmer (1981a, b), and Sato (1980). I have been using the term "EliassenPalm cross-section" to refer to latitude-height cross-sections showing both F and its divergence.…”

Some aspects of the sensitivity of stratospheric climate simulation to model lid height

“…In the simulations discussed in this paper, we use a derivative of the United Kingdom Meteorological Office (UKMO) stratosphere and mesosphere model (SMM) [Butchart et al, 1982] Although these results were based on a comparison of simulations which used one gravity wave parameterization, the effects of changing the model lid height were very similar to the changes seen when simulations are performed with and without a gravity wave parameteriztion [e.g., Garcia and Boville, 1994], which leads one to conclude that these effects would be seen with any physical gravity wave parameterization (as opposed to unphysical parameterizations like RF) and are dependent on the amount of parameterized gravity wave flux (as would be expected from downward control) rather than the details of the implementation.…”