Stratospheric mountain waves generated by the southern Andes

Current global operational numerical weather prediction models potentially have sufficient spatial and temporal resolution to resolve much of the inertia-gravity wave spectrum. The effects of gravity-wave dynamics are usually considered to be sub-gridscale and parametrizable, either as stationary, mountain waves or travelling, non-orographic waves. Simple techniques for isolating these waves in both the Met Office and ECMWF forecast models are presented and their associated three-dimensional Eliassen-Palm wave fluxes are computed for August 2006. In addition, a limited area configuration of the Met Office Unified model with 4 km horizontal resolution is run over a region that includes the southern half of the Andes mountain range. Spectral decomposition of the wave energy and momentum fluxes show a distinct peak in power at horizontal wavelengths of about 400 km irrespective of whether a 4 or 40 km grid is used. The computed Eliassen-Palm flux divergence suggests flow deceleration rates of up to 40 metres per second per day near a height of 40 km, a value which frequently exceeds the deceleration rates implied by non-orographic gravity-wave drag parametrization. Comparison of the time-mean temperature amplitude of the waves in the mid-stratosphere with estimates derived from limb-sounder satellite measurements from the HIRDLS project suggests that forecast models are capable of capturing the correct overall strength and distribution of gravity-wave activity.