Stratospheric gravity waves revealed in NWP model forecasts

Abstract

Current operational numerical weather prediction models potentially have sufficient spatial and temporal resolution to resolve much of the inertia-gravity wave spectrum (e.g. horizontal wavelengths in the region of 100-1000 km typically, and even longer zonal wavelengths in the Tropics). The effects of gravity wave dynamics are usually considered to be subgrid-scale and parametrizable, either as stationary, mountain waves or travelling, non-orographic waves. Simple techniques for isolating these waves in both the

High gravity‐wave activity observed in Patagonia, Southern America: generation by a cyclone passage over the Andes mountain range

“…The ray-tracing technique shows that the wave event is forced by surface winds over mountains reaching heights of 1700 m located over the South American continent. Orographic waves that propagate long distances downstream have also been found in satellite observations (Wu and Jiang, 2002;Wu et al, 2006;Alexander et al, 2008) and in modelling studies (Shutts and Vosper, 2011). In this work, we emphasize the long lateral propagation of the waves (by lateral we mean perpendicular to the downstream direction), which are found to propagate laterally over 1500 km with amplitudes of about 5 m s −1 at those distances.…”

Middle atmosphere predictability in a numerical weather prediction model: revisiting the inverse error cascade

“…small-scale Courant number) may be more appropriate (Bartello and Thomas, 1996). Shutts and Vosper (2011) have shown that gravitywave activity in the operational version of the MetUM is in better agreement with observations from the HighResolution Dynamics Limb Sounder (HIRDLS) instrument when the timestep is decreased from 15 min to 2 min. Figure 5 shows the effect of the timestep on the mean relative error, r. In all cases, the qualitative evolution is unchanged.…”

“…Shutts and Vosper, 2011) and coarse vertical resolution. In particular, an increase in the predictive skill of forecasts could be obtained from both modelling and assimilation.…”

“…Mountain waves, for example, typically have scales of ∼ 10 km (Doyle et al, 2002). However, it has been shown that present-day NWP forecasts can reproduce a significant fraction of the gravity-wave spectrum (Shutts and Vosper, 2011). Thus the problem may be more complicated.…”

“…Moreover, gravity waves are typically underresolved in NWP models on account of limited spatial and temporal resolution (e.g. Shutts and Vosper, 2011). Thus, in light of the Boussinesq results of Ngan et al (2009), the predictability of NWP models could be influenced by the numerics, especially in the middle atmosphere.…”