The South Georgia Wave Experiment: A Means for Improved Analysis of Gravity Waves and Low-Level Wind Impacts Generated from Mountainous Islands

Abstract

Gravity waves (GWs) play an important role in many atmospheric processes. However, the observation-based understanding of GWs is limited, and representing them in numerical models is difficult. Recent studies show that small islands can be intense sources of GWs, with climatologically significant effects on the atmospheric circulation. South Georgia, in the South Atlantic, is a notable source of such “small island” waves. GWs are usually too small scale to be resolved by current models, so their effects are represented approximately using resolved model fields (parameterization). However, the small-island waves are not well represented by such parameterizations, and the explicit representation of GWs in very-high-resolution models is still in its infancy. Steep islands such as South Georgia are also known to generate low-level wakes, affecting the flow hundreds of kilometers downwind. These wakes are also poorly represented in models. We present results from the South Georgia Wave Experiment (SG-WEX) for 5 July 2015. Analysis of GWs from satellite observations is augmented by radiosonde observations made from South Georgia. Simulations were also made using high-resolution configurations of the Met Office Unified Model (UM). Comparison with observations indicates that the UM performs well for this case, with realistic representation of GW patterns and low-level wakes. Examination of a longer simulation period suggests that the wakes generally are well represented by the model. The realism of these simulations suggests they can be used to develop parameterizations for use at coarser model resolutions.

The South Georgia Wave Experiment (SG‐WEX): radiosonde observations of gravity waves in the lower stratosphere. Part I: Energy density, momentum flux and wave propagation direction

“…The model lid was at 78 km but there was a damping layer applied to the top 20 km. This UM version is designed to capture gravity waves generated due to orography and no gravity wave parametrizations are included (Jackson et al, ).…”

“…This strongly indicates that the large increases we see in the energy density are due to orographic wave activity from South Georgia. The low-resolution radiosonde data and UM data are close in agreement during the period where upward waves dominate; the differences can be put down to non-orographic waves also being present in the radiosonde data and in the UM results; differences can occur due to small model errors in the wave field accumulating with height (Jackson et al, 2017).…”

Gravity waves in the winter stratosphere over the Southern Ocean: high-resolution satellite observations and 3-D spectral analysis

“…Alexander et al, 2008;Yan et al, 2010;Hendricks et al, 2014;Hindley et al, 2015) and could suggest a connection between wave activity over the southern Andes and, at least, some of the central and western portions of the observed belt of gravity wave activity around 60 • S. A possible mechanism for this could be downwind propagation of gravity waves from the mountains either directly (Sato et al, 2012) or through the generation of secondary gravity waves with non-zero phase speeds as a result of wave breaking or intermittency due to variability in the stratospheric wind over the mountains (Woods and Smith, 2010;Bossert et al, 2017;. Increased gravity wave activity over the Southern Ocean at all longitudes around 60 • S has also been attributed to waves generated by spontaneous adjustment mechanisms resulting from baroclinic instability around the vortex edge (Hendricks et al, 2014;Hindley et al, 2015;Plougonven and Zhang, 2014) or convection within fronts (Jewtoukoff et al, 2015;Plougonven et al, 2015;Holt et al, 2017). During August, non-orographic processes could begin to be more dominant over any connection between gravity wave activity over the southern Andes and over the Southern Ocean, leading to the more zonally uniform distribution of gravity wave activity within this latitude band that we observe in Fig.…”

Current Challenges in Orographic Flow Dynamics: Turbulent Exchange Due to Low-Level Gravity-Wave Processes

“…However there still remain many challenges in observing and quantifying the importance of gravity waves, in accurately modelling their effects in high-resolution models, and in parametrizing their effects in lower-resolution models. This is an active area of research with a number of significant field campaigns in recent years [3,4].…”