Abstract
AbstractGravity 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.
Highlights
Gravity waves (GWs), or buoyancy waves, play an important role in the atmosphere
The preceding case study from 5 July 2015 illustrates the role of South Georgia in generating gravity waves and impacting the low-level flow
An innovative method was used to analyze GWs from 3D AIRS temperature retrievals and high-resolution Unified Model (UM) simulations. Both AIRS and the model simulations show evidence of orographic gravity waves being generated in a strong westerly flow, and large resolution-dependent differences in the maximum GW momentum flux over and downstream of South Georgia
Summary
The preceding case study from 5 July 2015 illustrates the role of South Georgia in generating gravity waves and impacting the low-level flow. An innovative method (the 3D S-transform) was used to analyze GWs from 3D AIRS temperature retrievals and high-resolution UM simulations Both AIRS and the model simulations show evidence of orographic gravity waves being generated in a strong westerly flow, and large resolution-dependent differences in the maximum GW momentum flux over and downstream of South Georgia. We can have a high degree of confidence in using high-resolution simulations as a “truth” to develop parameterization schemes This will be important in addressing the ongoing small-island problem in global weather forecast and climate models, which have a coarser resolution (typically several tens of kilometers), and in properly simulating low-level wakes in the lee of South Georgia and other islands like New Zealand and Hawaii, which can extend several hundreds of kilometers downstream. Such improvements will allow us to represent significant effects from resolving the small-island problem that will improve climate forecasting
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