Improved middle atmosphere climate and forecasts in the ECMWF model through a non-orographic gravity wave drag parametrization

In model cycle 35r3 (Cy35r3) of the ECMWF Integrated Forecast System (IFS) the momentum deposition from small-scale non-orographic gravity waves is parametrized by the Scinocca (2003) scheme which uses hydrostatic non-rotational wave dynamics to describe the vertical evolution of a broad, constant, isotropic spectrum of gravity waves emanating from the troposphere. The Cy35r3 middle atmosphere climate shows: i) an improved representation of the zonal-mean circulation and temperature structure, ii) realistic parametrized gravity wave drag, iii) reasonable stationary planetary wave structure and stationary wave driving in July, and an underestimate of the generation of stationary wave activity in the troposphere and stationary wave driving in January, iv) improved representation of the tropical variability of the stratospheric circulation, although the westerly phase of the semi-annual oscillation is missing, and v) realistic horizontal distribution of momentum flux in the stratosphere. By contrast, the middle atmosphere climate is much too close to radiative equilibrium when the Scinocca scheme is replaced by Rayleigh friction, which was the standard method of parametrizing the effects of non-orographic gravity waves in the IFS prior to Cy35r3. Finally, there is a reduction in Cy35r3 short-range high resolution forecast error in the upper stratosphere.

Contents

1. Introduction
2. The S03 gravity wave scheme
3. Climate experiments, parameter settings, and validating data
4. The extratropical temperature structure and circulation
5. Planetary waves
6. Tropical oscillations
7. Parametrized and resolved momentum flux
8. High resolution middle atmosphere forecast error and analysis
9. Summary and discussion
10. References