Stratospheric gravity waves over the mountainous island of South Georgia: testing a high-resolution dynamical model with 3-D satellite observations and radiosondes

Abstract

Abstract. Atmospheric gravity waves (GWs) play an important role in atmospheric dynamics but accurately representing them in general circulation models (GCMs) is challenging. This is especially true for orographic GWs generated by wind flow over small mountainous islands in the Southern Ocean. Currently, these islands lie in the “grey zone” of global model resolution, where they are neither fully resolved nor fully parameterised. It is expected that as GCMs approach the spatial resolution of current high-resolution local-area models, small-island GW sources may be resolved without the need for parameterisations. But how realistic are the resolved GWs in these high-resolution simulations compared to observations? Here, we test a high-resolution (1.5 km horizontal grid, 118 vertical levels) local-area configuration of the Met Office Unified Model over the mountainous island of South Georgia (54∘ S, 36∘ W), running without GW parameterisations. The island's orography is well resolved in the model, and real-time boundary conditions are used for two time periods during July 2013 and June–July 2015. We compare simulated GWs in the model to coincident 3-D satellite observations from the Atmospheric Infrared Sounder (AIRS) on board Aqua. By carefully sampling the model using the AIRS resolution and measurement footprints (denoted as model sampled as AIRS hereafter), we present the first like-for-like comparison of simulated and observed 3-D GW amplitudes, wavelengths and directional GW momentum flux (GWMF) over the island using a 3-D S-transform method. We find that the timing, magnitude and direction of simulated GWMF over South Georgia are in good general agreement with observations, once the AIRS sampling and resolution are applied to the model. Area-averaged zonal GWMF during these 2 months is westward at around 5.3 and 5.6 mPa in AIRS and model sampled as AIRS datasets respectively, but values directly over the island can exceed 50 mPa. However, up to 35 % of the total GWMF in AIRS is actually found upwind of the island compared to only 17 % in the model sampled as AIRS, suggesting that non-orographic GWs observed by AIRS may be underestimated in our model configuration. Meridional GWMF results show a small northward bias (∼20 %) in the model sampled as AIRS that may correspond to a southward wind bias compared to coincident radiosonde measurements. Finally, we present one example of large-amplitude (T′≈15–20 K at 45 km altitude) GWs at short horizontal wavelengths (λH≈30–40 km) directly over the island in AIRS measurements that show excellent agreement with the model sampled as AIRS. This suggests that orographic GWs in the full-resolution model with T′≈45 K and λH≈30–40 km can occur in reality. Our study demonstrates that not only can high-resolution local-area models simulate realistic stratospheric GWs over small mountainous islands but the application of satellite sampling and resolution to these models can also be a highly effective method for their validation.

A High‐Resolution Whole‐Atmosphere Model With Resolved Gravity Waves and Specified Large‐Scale Dynamics in the Troposphere and Stratosphere

“…Opposite or different phases between the model and AIRS results have also been shown in a recent paper by Hindley et al. (2020) who investigated GW events during the wintertime in the region of the island of South Georgia using a regional model with very high resolution and driven by reanalysis at its lateral boundaries. Also note that the HIAMCM shows medium‐scale GWs in the stratosphere over northern Europe at 33 km (Figure 8a) and at lower altitudes (Figures 8d and 8e) that are neither captured by MERRA‐2 nor by AIRS.…”

Radar observations of winds, waves and tides in the mesosphere and lower thermosphere over South Georgia island (54°S, 36°W) and comparison to WACCM simulations

“…It is expected from recent studies (e.g. Wright et al, 2017;Hindley et al, 2019Hindley et al, , 2020Hindley et al, , 2021 that the stratosphere exhibits a strong westward tendency in GW momentum flux over the Southern Ocean during winter for upwardly propagating GWs. If these westward GWs propagate up to the mesosphere, we would expect to see a strong zonal directional tendency in Fig.…”

“…In a recent numerical modelling study over South Georgia, Hindley et al (2021) found that in some cases, mountain waves from South Georgia can reach altitudes near the stratopause at around 75 km if eastward wind conditions were favourable throughout the stratosphere. In one example (their Sect.…”

“…The radar made near-continuous measurements of neutral winds in the MLT from February 2016 to November 2020. South Georgia is located near to the global GW "hot spot" of activity in the stratosphere of the southern Andes and Antarctic Peninsula (Hindley et al, 2015;Hoffmann et al, 2013;Hindley et al, 2020), and is also an intense source of wintertime GW activity itself (Hoffmann et al, 2014;Hindley et al, 2021). Further, recent observations and modelling have indicated significant generation and propagation of 2GWs in the mesosphere and thermosphere in the region (Becker and Vadas, 2018;Kogure et al, 2020;Lund et al, 2020;Fritts et al, 2021).…”

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

“…A means of validating the parametrization scheme directly across a range of grid spacings is therefore required. Gravity waves simulated using high‐resolution simulations have been shown to agree well with satellite‐derived observations, in terms of timing and amplitude (Hindley et al ., 2020), and they provide a dataset for validating parametrization schemes that is sampled uniformly in both space and time. Here, we exploit the seamless modelling framework of the Met Office Unified Model by performing simulations, both regional and global, at horizontal grid spacings ranging from 2–130 km.…”