The representation of non-orographic gravity waves in the IFS Part I: Assessment of the middle atmosphere climate with Rayleigh friction

TitleThe representation of non-orographic gravity waves in the IFS Part I: Assessment of the middle atmosphere climate with Rayleigh friction
Publication TypeMiscellaneous
Year of Publication2009
AuthorsOrr, A, Wedi, NP
Secondary TitleTechnical Memorandum
Number592
Abstract

The middle atmosphere climate of cycles 32R3 and 33R1 are assessed. The middle atmosphere climate is determined by the dominating processes of radiation, and wave drag arising from the deposition of momentum from the breaking of small-scale non-orographic gravity waves and large-scale planetary waves. In the current ECMWF Integrated Forecast System (IFS), the averaged effect of the non-orographic gravity wave drag is crudely approximated by Rayleigh friction on the zonal flow. Both cycles have a summer polar upper mesosphere warm bias, which suggests a lack of upwelling, and consequently an underestimation of the poleward circulation between the summer and winter hemispheres and downwelling over the winter pole, i.e. that the Rayleigh friction forcing of the mean flow is unrealistically weak and underestimates the required gravity wave drag. Given realistic descriptions of radiation and planetary waves, the weak downwelling should be associated with excessively cold winter polar stratospheric temperatures. Both cycles show a southern winter polar cold bias. However, 32R3 has a northern winter polar warm bias, while 33R1 has a northern winter cold pole (but not a cold bias), which is consistent with an improvement in its representation of planetary waves. The failure to simulate a northern winter polar cold bias indicates that the radiation scheme is associated with a northern winter warm bias. There is also a problem with unrealistically large orographic gravity wave drag at the model top, with a consequent strong compensation by the dynamics. This is alleviated by linearly depositing the orographic gravity wave momentum flux at 0.1 hPa between 0.1 hPa and the model top of 0.01 hPa. However, the northern winter temperature structure and general circulation are more realistic if the smoothing is removed. The results indicate a large imbalance at the model top due to orographic gravity wave drag, which warrants further attention.