Propagation paths and source distributions of resolved gravity waves in ECMWF-IFS analysis fields around the southern polar night jet

Abstract

Abstract. In the southern winter polar stratosphere, the distribution of gravity wave momentum flux in many state-of-the-art climate simulations is inconsistent with long-time satellite and superpressure balloon observations around 60∘ S. Recent studies hint that a lateral shift between prominent gravity wave sources in the tropospheric mid-latitudes and the location where gravity wave activity is present in the stratosphere causes at least part of the discrepancy. This lateral shift cannot be represented by the column-based gravity wave drag parameterisations used in most general circulation models. However, recent high-resolution analysis and re-analysis products of the European Centre for Medium-Range Weather Forecasts Integrated Forecast System (ECMWF-IFS) show good agreement with the observations and allow for a detailed investigation of resolved gravity waves, their sources, and propagation paths. In this paper, we identify resolved gravity waves in the ECMWF-IFS analyses for a case of high gravity wave activity in the lower stratosphere using small-volume sinusoidal fits to characterise these gravity waves. The 3D wave vector together with perturbation amplitudes, wave frequency, and a fully described background atmosphere are then used to initialise the Gravity Wave Regional or Global Ray Tracer (GROGRAT) gravity wave ray tracer and follow the gravity waves backwards from the stratosphere. Finally, we check for the indication of source processes on the path of each ray and, thus, quantitatively attribute gravity waves to sources that are represented within the model. We find that stratospheric gravity waves are indeed subject to far (>1000 km) lateral displacement from their sources, which already take place at low altitudes (<20 km). Various source processes can be linked to waves within stratospheric gravity wave (GW) patterns, such as the orography equatorward of 50∘ S and non-orographic sources above the Southern Ocean. These findings may explain why superpressure balloons observe enhanced gravity wave momentum fluxes in the lower stratosphere over the Southern Ocean despite an apparent lack of sources at this latitude. Our results also support the need to improve gravity wave parameterisations to account for meridional propagation.

3D Numerical Simulation of Secondary Wave Generation From Mountain Wave Breaking Over Europe

“…The extension of the mountain wave in Figure 7 is reminiscent of discussions described in other works (Alexander & Teitelbaum, 2011; Jiang et al., 2013, 2019; Preusse et al., 2002; Sato et al., 2009; Sato et al., 2012; Strube et al., 2021). Discussions of these MW extensions are often related to meridional shear in the ambient mean wind and refraction into the stratospheric jet.…”

“…A study of mountain wave propagation over New Zealand by Strube et al. (2021), however, suggested that waves propagating far distances have the favoring direction already from the beginning of their propagation. ECMWF data on the 12 January 2016 shows the presence of the polar stratospheric jet over Scandinavia (and meridional shears), and the AIRS data suggests extension of the wave to the north‐east.…”

Impacts of Limited Model Resolution on the Representation of Mountain Wave and Secondary Gravity Wave Dynamics in Local and Global Models. 1: Mountain Waves in the Stratosphere and Mesosphere

“…Rather, they typically employ parameterizations relying on linear theory and discrete or idealized GW spectral forms (Kim et al., 2003). Our results provide quantitative insights into, and confirmation of, significant deficiencies, especially large‐scale circulation biases, in current parameterizations of orographic GW drag (GWD; Sandu et al., 2019; Strube et al., 2021; van Niekerk et al., 2020; Vosper et al., 2019) and of non‐orographic GWD (Geller et al., 2013; Müller et al., 2018; Stephan et al., 2019).…”

Towards a more “scale‐aware” orographic gravity wave drag parametrization: Description and initial testing

“…There have been a number of studies indicating that the gravity wave dynamics within the Southern Hemisphere stratosphere could be more complex than in the Northern Hemisphere and that not representing this complexity in models is the cause of the bias at 60°S. Observations (Wright et al ., 2017), high‐resolution models (Sato et al ., 2012), and ray‐tracing theory (Perrett et al ., 2021; Strube et al ., 2021) all suggest that the lateral propagation of gravity waves, a process not accounted for in parametrizations, could be the cause of the missing drag at 60°S. Poor representation of other gravity wave processes, such as nonorographic gravity wave generation (Garcia et al ., 2017), could also contribute to this bias.…”