The representation of non-orographic gravity waves in the IFS Part II: A physically based spectral parametrization

TitleThe representation of non-orographic gravity waves in the IFS Part II: A physically based spectral parametrization
Publication TypeMiscellaneous
Year of Publication2009
AuthorOrr, A
Secondary TitleTechnical Memorandum
Number593
Abstract

The middle atmosphere climate of cycle 33R1 of the IFS is simulated using the Scinocca (2003) parametrization of the momentum deposition from non-orographic gravity waves (hereafter SO3). A comparison is made with a basic cycle 33R1 simulation using Rayleigh friction, which is the standard method of parametrizing the effects of non-orographic gravity waves in this cycle of the IFS (hereafter RF). The SO3 scheme assumes hydrostatic and non-rotational wave dynamics, and describes the vertical evolution of a broad, constant, isotropic spectrum of gravity waves, emanating from the troposphere. The SO3 simulation shows a number of improvements to the zonal-mean circulation relative to the RF simulation; noticeably an improved overturning circulation resulting in more realistic winter westerly winds, a reduction in the southern winter polar stratosphere cold bias, and an improvement in the capture of summer easterlies in the stratosphere and mesosphere. Estimates of gravity wave drag and gravity wave momentum flux agree well with the limited observations available. Investigation into the response of stationary planetary waves indicates a good agreement between SO3 simulated and observed amplitudes and an improved representation of planetary wave driving of the zonal-mean flow. However, the simulation is marked by a failure to capture the westerly components of the quasi-biennial and semi-annual oscillations (QBO and SAO). An additional comparison was made with a cycle 33R1 simulation with the Doppler spread parametrization of gravity waves replacing Rayleigh friction. This scheme controls momentum deposition by Doppler shifting due to the background winds and from wave-induced fluctuations of the gravity wave winds themselves. This scheme produced unrealistically strong values of gravity wave drag, resulting in an overly strong meridional circulation and excessive downwelling, and an overly warm and elevated winter stratopause.