Measuring internal solitary wave parameters based on VIIRS/DNB data

Gadian

Hoffmann³

et al. 2021

Atmos. Chem. Phys.

Self Cite

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.

Section: Numerical Modelling: Local-area Simulations Overmentioning

confidence: 82%

Section: Numerical Modelling: Local-area Simulations Overmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Gadian

Hoffmann³

et al. 2021

Atmos. Chem. Phys.

Self Cite

“…It has mountains ranges nearly 3 km high, lies in a region of very strong tropospheric winds and is an intense source of stratospheric gravity wave activity (e.g. Alexander and Grimsdell, 2013;Hoffmann et al, 2013Hoffmann et al, , 2016Hindley et al, 2016;Jackson et al, 2018;Garfinkel and Oman, 2018). Figure 5a shows AIRS-measured temperature perturbations over the island in the region specified by dashed lines in Fig.…”

Section: Case Study 2: 3-d Gravity Wave Measurements Over South Georgmentioning

confidence: 99%

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

Smith²,

Hoffmann

et al. 2019

Atmos. Chem. Phys.

Self Cite

Abstract. Atmospheric gravity waves play a key role in the transfer of energy and momentum between layers of the Earth's atmosphere. However, nearly all general circulation models (GCMs) seriously under-represent the momentum fluxes of gravity waves at latitudes near 60∘ S, which can lead to significant biases. A prominent example of this is the “cold pole problem”, where modelled winter stratospheres are unrealistically cold. There is thus a need for large-scale measurements of gravity wave fluxes near 60∘ S, and indeed globally, to test and constrain GCMs. Such measurements are notoriously difficult, because they require 3-D observations of wave properties if the fluxes are to be estimated without using significant limiting assumptions. Here we use 3-D satellite measurements of stratospheric gravity waves from NASA's Atmospheric Infrared Sounder (AIRS) Aqua instrument. We present the first extended application of a 3-D Stockwell transform (3DST) method to determine localised gravity wave amplitudes, wavelengths and directions of propagation around the entire region of the Southern Ocean near 60∘ S during austral winter 2010. We first validate our method using a synthetic wavefield and two case studies of real gravity waves over the southern Andes and the island of South Georgia. A new technique to overcome wave amplitude attenuation problems in previous methods is also presented. We then characterise large-scale gravity wave occurrence frequencies, directional momentum fluxes and short-timescale intermittency over the entire Southern Ocean. Our results show that highest wave occurrence frequencies, amplitudes and momentum fluxes are observed in the stratosphere over the mountains of the southern Andes and Antarctic Peninsula. However, we find that around 60 %–80 % of total zonal-mean momentum flux is located over the open Southern Ocean during June–August, where a large “belt” of increased wave occurrence frequencies, amplitudes and fluxes is observed. Our results also suggest significant short-timescale variability of fluxes from both orographic and non-orographic sources in the region. A particularly striking result is a widespread convergence of gravity wave momentum fluxes towards latitudes around 60∘ S from the north and south. We propose that this convergence, which is observed at nearly all longitudes during winter, could account for a significant part of the under-represented flux in GCMs at these latitudes.

“…Based upon Stockwell et al (1996), this provides estimates of the dominant wave amplitudes, wavelengths and directions at every location in a 3-D data volume. This method has been applied to 3-D AIRS data and other geophysical data sets in several recent studies (Hindley et al, 2016(Hindley et al, , 2019Hu, Ma, Yan, Hindley, Xu, & Jiang, 2019;Hu, Ma, Yan, Hindley, & Zhao, 2019;Wright et al, 2017).…”

Section: Measuring 3-d Gw Propertiesmentioning

confidence: 99%

An 18‐Year Climatology of Directional Stratospheric Gravity Wave Momentum Flux From 3‐D Satellite Observations

Geophysical Research Letters

Wright

Hoffmann

et al. 2020

Atmospheric gravity waves (GWs) are key drivers of the atmospheric circulation, but their representation in general circulation models (GCMs) is challenging, leading to significant biases in middle atmospheric circulations. Unresolved GW momentum transport in GCMs must be parameterized, but global directional GW observations are needed to constrain this. Here we present an 18-year climatology of directional stratospheric GW momentum flux (GWMF) from global AIRS/Aqua 3-D satellite observations during 2002 to 2019. Striking hemispheric asymmetries are found at high latitudes, including dramatic reductions and reversals of GWMF during sudden stratospheric warmings. During Southern Hemisphere winter, a lateral convergence of GWMF toward 60°S is found that has no Northern Hemisphere counterpart. In the tropics, we find that zonal GWMF in AIRS measurements is strongly modulated by the semiannual oscillation (SAO) but not the quasi-biennial oscillation (QBO). Our results provide guidance for future GW parameterizations needed to resolve long-standing biases in GCMs. Plain Language Summary Gravity waves (GWs) are traveling waves that can occur in geophysical fluid environments subject to the gravitational force. In the Earth's atmosphere, GWs transport momentum that helps to drive the atmospheric circulation, especially in the middle atmosphere. But for numerical weather and climate models, accurately simulating GWs has proven very challenging because of a lack of GW observations, leading to significant model biases. Here we present an 18-year climatology of GW observations near 40-km altitude for 2002 to 2019. For the first time, we present multidecadal global measurements of directional GW momentum flux derived from 3-D satellite observations from National Aeronautics and Space Administration's (NASA's) AIRS instrument. We find that during Southern Hemisphere winter, GWs travel laterally toward 60°S each year. This is significant because most models underestimate GW momentum near 60°S and do not typically include lateral propagation in their parameterizations. In the tropical stratosphere, we find no modulation of GWs in AIRS observations by the quasi-biennial oscillation (QBO). This is consistent with the hypothesis that GW-QBO interactions occur at shorter vertical wavelengths, which are invisible to AIRS. Our results will help to guide future GW parameterizations, leading to more accurate atmospheric simulations and ultimately better forecasts of weather and climate.