AS1.14

Effects of realistic propagation of gravity waves (GWs) on distribution of GW pseudomomentum fluxes are explored using a global ray-tracing model for the 2009 sudden stratospheric warming (SSW) event. Four-dimensional (4D; x–z and t) and two-dimensional (2D; z and t) results are compared for various parameterized pseudomomentum fluxes. In ray-tracing equations, refraction due to horizontal wind shear and curvature effects are found important and comparable to one another in magnitude. In the 4D, westward pseudomomentum fluxes are enhanced in the upper troposphere and northern stratosphere due to refraction and curvature effects around fluctuating jet flows. In the northern polar upper mesosphere and lower thermosphere, eastward pseudomomentum fluxes are increased in the 4D. GWs are found to propagate more to the upper atmosphere in the 4D, since horizontal propagation and change in wave numbers due to refraction and curvature effects can make it more possible that GWs elude critical level filtering and saturation in the lower atmosphere. GW focusing effects occur around jet cores, and ray-tube effects appear where the polar stratospheric jets vary substantially in space and time. Enhancement of the structure of zonal wavenumber 2 in pseudomomentum fluxes in the middle stratosphere begins from the early stage of the SSW evolution. An increase in pseudomomentum fluxes in the upper atmosphere is present even after the onset in the 4D. Significantly enhanced pseudomomentum fluxes, when the polar vortex is disturbed, are related to GWs with small intrinsic group velocity (wave capture), and they would change nonlocally nearby large-scale vortex structures without substantially changing local mean flows.

How to cite: Song, I.-S., Lee, C., Chun, H.-Y., Kim, J.-H., Jee, G., Song, B.-G., and Bacmeister, J.: Propagation of gravity waves and its effects on pseudomomentum flux in a sudden stratospheric warming event, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1909, https://doi.org/10.5194/egusphere-egu21-1909, 2021.

Recent observations from the ERA5 reanalysis have revealed wave contributions from a wide range of spatial and temporal scales to the momentum budget of the equatorial stratosphere. Although it is generally accepted that the wave forcing at the equator drives the quasi-biennial oscillation (QBO) of equatorial winds, the individual contribution of each type of wave is still poorly understood. Here, we seek to disentangle the role of different wave types in the momentum budget of an idealized stratosphere. Numerical simulations with increasing spatial resolution are used to infer the sensitivity of the wave spectrum and mean flow oscillation to resolved instabilities. At higher resolution, Kelvin-Helmholtz generated small-scale gravity waves are combined to the background low frequency wave forcing and accelerate the period of mean-flow reversals due to an increased momentum transfer from the wave to the mean flow. This mechanism is confirmed using a simplified one-dimensional model for which the wave properties are specified.

How to cite: Chartrand, X., Nadeau, L.-P., and Venaille, A.: Small-scale gravity waves influence on an idealized quasi-biennial oscillation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14030, https://doi.org/10.5194/egusphere-egu21-14030, 2021.

An interaction between Kelvin waves and gravity waves (GWs) in the tropical stratosphere is investigated using the global weather-forecasting model ICON with a horizontal grid spacing of ~160 km. To represent GWs in ICON, the Multi-Scale Gravity Wave Model (MS-GWaM) is used as a subgrid-scale parameterization, which is a prognostic model that explicitly calculates the evolution of GW action density in phase space. The simulation is initialized on a day in the QBO phase of the easterly maximum at ~20 hPa, so that Kelvin waves can propagate vertically throughout the lower stratosphere during the simulation. We show that Kelvin waves with zonal-wind amplitudes of about 10 m s^-1 can largely affect the distribution of GW drag, by disturbing the local wind shear. Moreover, due to the zonal asymmetry in the activity of tropospheric convection, which is the source of GWs in the tropics, this effect of Kelvin waves can also influence the zonal mean of GW drag. The effect seems to be large when a strong convective system, from which large-amplitude GWs are generated, propagates eastward in the troposphere together with a phase of stratospheric Kelvin wave aloft. In our case, such an interaction causes a zonal-mean GW drag of ~0.26 m s^-1 d^-1 at ~20 hPa for a week during an early phase of the easterly-to-westerly transition of the QBO. The result emphasizes the importance of a correct representation of large-scale waves as well as subgrid-scale GWs in QBO simulations.

How to cite: Kim, Y.-H. and Achatz, U.: Interaction between equatorial stratospheric Kelvin waves and gravity waves in a QBO phase, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5535, https://doi.org/10.5194/egusphere-egu21-5535, 2021.

Comprehensive global climate simulations are still conducted in fairly low resolution. Current general circulation models therefore rely on gravity wave parameterisations to simulate atmospheric dynamics correctly. Among other parameters, the surface wind determines gravity wave launching in orographic gravity wave parameterisations. However, the mountainous terrain in regions where orographic gravity waves occur suggests larger surface wind variability on unresolved topography than the model grid box wind can provide. To account for this variability, we here present a stochastic modification of the low-level wind direction when it is used in the orographic gravity wave scheme of the EMAC (ECHAM MESSy Atmospheric Chemistry) model. For our first application, we implemented a random normal function to evoke a modest deviation of the wind direction at each time step when it is used in the subgrid scale scheme.

An EMAC simulation shows that this gravity wave modification locally leads to significant changes of orographic gravity wave drag, but this does not result in significant annual or seasonal differences in temperatures or winds. However, the Arctic polar vortex is stretched and its center shifts in February. Moreover, we find a shift in the Antarctic polar vortex breakdown date, resulting in a significant zonal mean temperature change in October and possibly in an alleviation of the EMAC low bias in Antarctic polar vortex strength. In this presentation, we discuss our results, the method and possible further developments like allowing gusts in the modified scheme.

How to cite: Eichinger, R., Sacha, P., Kuchar, A., Pisoft, P., and Garny, H.: Enhanced wind variability in the orographic gravity wave parameterisation and its influence on dynamics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2133, https://doi.org/10.5194/egusphere-egu21-2133, 2021.

We present a novel, single-column gravity wave parameterization (GWP) that uses machine learning to emulate a physics-based GWP. An artificial neural network (ANN) is trained with output from an idealized atmospheric model and tested in an offline environment, illustrating that an ANN can learn the salient features of gravity wave momentum transport directly from resolved flow variables. We demonstrate that when trained on the westward phase of the Quasi-Biennial Oscillation, the ANN can skillfully generate the momentum fluxes associated with the eastward phase. We also show that the meridional and zonal wind components are the only flow variables necessary to predict horizontal momentum fluxes with a globally and temporally averaged R^2 value over 0.8. State-of-the-art GWPs are severely limited by computational constraints and a scarcity of observations for validation. This work constitutes a significant step towards obtaining observationally validated, computationally efficient GWPs in global climate models.

How to cite: Espinosa, Z., Sheshadri, A., Cain, G., Gerber, E., and DallaSanta, K.: Machine Learning Emulation of Parameterized Gravity Wave Momentum Fluxes in an Atmospheric Global Climate Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1398, https://doi.org/10.5194/egusphere-egu21-1398, 2021.

Internal gravity waves are a well-known mechanism of energy redistribution in stratified fluids such as the atmosphere. They may propagate from their generation region, typically in the Troposphere, up to high altitudes. During their lifetime internal waves couple to the atmospheric background through various processes. Among the most important interactions are the exertion of wave drag on the horizontal mean-flow, the heat generation upon wave breaking, or the mixing of atmospheric tracers such as aerosols or greenhouse gases.

Many of the known internal gravity wave properties and interactions are covered by linear or weakly nonlinear theories. However, for the consideration of some of the crucial effects, like a reciprocal wave-mean-flow interaction including the exertion of wave drag on the mean-flow, strongly nonlinear systems are required. That is, there is no assumption on the wave amplitude relative to the mean-flow strength such that they may be of the same order.

Here, we exploit a strongly nonlinear Boussinesq theory to analyze the stability of a stationary internal gravity wave which is refracted at the vertical edge of a horizontal jet. Thereby we assume that the incident wave is horizontally periodic, non-hydrostatic, and vertically modulated. Performing a linear stability analysis in the vicinity of the jet edge we find necessary and sufficient criteria for instabilities to grow. In particular, the refracted wave becomes unstable if its incident amplitude is large enough and both mean-flow horizontal winds, below and above the edge of the jet, do not exceed particular upper bounds.

How to cite: Voelker, G. S. and Schlutow, M.: On strongly nonlinear gravity waves in a vertically sheared atmosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4878, https://doi.org/10.5194/egusphere-egu21-4878, 2021.

With an aim of understanding the role of internal waves to oceanic mixing, various mechanisms have been cited as a possible explanation for how they transfer energy across the wavenumber and frequency spectra and eventually contribute to small-scale turbulence. Triadic Resonance Instability (TRI) has become increasingly recognised as potentially one of these mechanisms. This talk will summarise both experimental work and theoretical modelling (using numerical solutions of a weakly non-linear system) that examines the long-term temporal and spatial evolution of this instability for a finite-width internal wave beam. Experiments have been conducted using a new generation of wave maker, featuring a flexible horizontal boundary driven by an array of independently controlled actuators. We present experimental results exploring the role that a finite width wave beam has on the evolution of TRI. Experimentally, we find that the approach to a saturated equilibrium state for the three triadic waves is not monotonic, rather their amplitudes continue to oscillate without reaching a steady equilibrium. Further theoretical modelling then suggests that part of this variability is due to multiple resonant frequencies interacting with each other, as opposed to a simple triad system. We show how a spectrum of these resonant frequencies in the flow ‘beat’ to cause interference patterns which manifest throughout the instability as slow amplitude modulations.

How to cite: Grayson, K., Dalziel, S., and Lawrie, A.: Experimental and Weakly Non-linear Investigation into the Long-term Spatial and Temporal Development of Triadic Resonance Instability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3247, https://doi.org/10.5194/egusphere-egu21-3247, 2021.

The Surface Water and Ocean Topography (SWOT) mission is the next generation of satellite altimetry, set to launch in early 2022. It will be the first of its kind to provide global sea surface height (SSH) measurements fine enough to begin resolving the submesoscale. In this newly resolvable regime, “slow” flows (jets, vortices…) interact with internal waves by redistributing wave energy to other wave-vectors and frequencies. This introduces the challenge of distinguishing “slow” flows from waves, which is of key importance for inferring ocean circulation, from SSH measurements. I run numerical simulations of the one layer rotating shallow water equations to model the interaction between a single internal tide mode and vortices in (cyclo)geostrophic balance to characterize scattering and map its relevant parameter space. Preliminary results show wave scattering by vortices with Rossby numbers ranging from 0.1-4 that are not explained by the standard methods (frozen-field approximation, ray tracing…) which have been successful in the mesoscale. We find that the Rossby number, the Burger number, and the ratio of the length and velocity scales of the wave and vortex are all necessary to characterize the interaction in submesoscale regimes. Harmonic analysis is used to highlight the direction of the scattered wave energy.

How to cite: Uncu, J. and Grisouard, N.: Wave-Vortex Interactions in the Submesoscale, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16381, https://doi.org/10.5194/egusphere-egu21-16381, 2021.

The presence of large-amplitude internal waves in the Andaman Sea have been observed since 1965 but their temporal variability is yet to be understood. Therefore, in-situ observations from March 2017 to February 2018 are used to study the temporal variability and vertical structure of internal tides. The kinetic energy of semidiurnal internal tides dominates that of diurnal internal tides by a factor of 4. The internal tides at semidiurnal frequency are relatively stronger in summer and autumn, whereas at the diurnal frequency they are stronger during summer and winter. Density stratification seems to be playing a more significant role in controlling the temporal variability of internal tides when compared with the astronomical tides. Moreover, the stratification near the surface is controlled by salinity variations, whereas the temperature variations control the sub-surface stratification. This leads to the occurrence of a strong double pycnocline during autumn and winter. The first-mode semidiurnal internal tides are more significant in all the seasons except during autumn. The semidiurnal internal tides are more coherent than the diurnal internal tides. Strong background currents due to mesoscale eddies are observed during periods of high incoherent internal tides. Therefore, the variations in background stratification and currents due to the presence of mesoscale eddies could be causing incoherent internal tides in this region.

How to cite: Yadidya, B., Rao, A. D., and Latha, G.: Seasonal variations in the Andaman Sea’s internal tides, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14439, https://doi.org/10.5194/egusphere-egu21-14439, 2021.

Wind generated near-inertial waves (NIWs) are a major source of energy for deep-ocean mixing by transmitting wind energy from the ocean surface into the interior. Recently, it has been established that the NIW energy transmission to ocean depths is significantly modulated by background mesoscale vorticity. Thus, understanding NIW energetics in the presence of mesoscale eddies on a global scale is crucial.

We study the generation, propagation and dissipation of NIWs in global 1/25^o Hybrid Coordinate Ocean Model (HYCOM) simulations with realistic tidal forcing. The model has 41 layers with uniform vertical coordinates in the mixed layer and isopycnal coordinates in the ocean interior. The model is forced by 1/3hr wind from the NAVGEM atmospheric model. We analyze one month of model data for May-June 2019. The 3D HYCOM fields are projected on vertical normal modes to compute the wind input, wave kinetic energy (KE), flux divergence and dissipation per mode.

We find that the globally integrated wind input in surface near-inertial motions is 0.21 TW for the 30-day period and is consistent with previous studies. The sum of the wind input to the first 5 modes accounts to only 31% of the total wind input while the sum of the NIW kinetic energy in the first 5 modes adds up to 60% of the total NIW KE. The difference in the fraction of the total between the wind input and NIW KE (31% and 60%) suggests that a significant portion of wind-induced near-inertial motions is dissipated close to the surface without being projected onto modes. We also find that NIW horizontal fluxes diverge from areas with cyclonic vorticity and converge in areas with anticyclonic vorticity, i.e., anticyclonic eddies are a sink for NIW energy in the global ocean.

The residual NIW KE that does not project onto modes is found to be largely trapped in anticyclonic eddies. In a next step, we will study the fate of this energy, which most likely propagate downward as beam-like features with large wave numbers. We will compute the near-inertial wave energy balance for fixed subsurface layers and consider the energy exchange between these layers to understand the vertical structure of NIW energy dissipation. We find that the downward NIW radiation to the ocean interior at 500 m depth is 19% of the surface near-inertial wind input for the 30-day period.

How to cite: Raja, K., Buijsman, M., Siyanbola, O., Solano, M., Shriver, J., and Arbic, B.: Near-inertial waves modulated by background flow in realistic global ocean simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14207, https://doi.org/10.5194/egusphere-egu21-14207, 2021.

The Arctic Ocean contains a warm layer originating from the Atlantic Ocean below the pycnocline which has a thermohaline staircase structure that inhibits vertical mixing. If this heat were to rise to the surface, the rate of sea ice loss would increase dramatically. Wind stress and ice floes generate internal waves which can cause vertical mixing. As the ice cover in the Arctic continues to decline, it will be important to predict how these changing internal waves propagate through such stratification profiles. Here, we investigate how density staircases enhance or limit downward near-inertial wave propagation. We use direct numerical simulations to solve the Boussinesq equations of motion using spectral methods. We simulate the propagation of internal waves through a vertically stratified fluid which includes one or more steps (i.e., mixed layers). We find that we reproduce the results of laboratory experiments showing transmission and reflection of internal waves from one or two mixed layers. We then extend our parameter regime to simulate the propagation of internal waves through a more realistic stratification profile tending toward that of the Arctic pycnocline.

How to cite: Schee, M. and Grisouard, N.: Idealized Numerical Modelling of Internal Waves Through Density Staircases, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6464, https://doi.org/10.5194/egusphere-egu21-6464, 2021.

We investigate the influence of a barotropic geostrophic current on
internal tide (IT) generation over a shelf slope.
The current $V_g(x)$ is modeled as an idealized Gaussian function centered at
$x_0$ with width $x_r$ and maximum velocity $V_{max}$.
The bathymetry is modelled as a linear slope with smoothed corners.
We calculate the total barotropic-to-baroclinic energy conversion $C =
\int \overbar{C} \,dx = \int \int \rho' g W \,dx\, dz$. 
$\overbar{C}(x,t)$ can be either positive or negative. Positive (negative) conversion means energy is
converted from barotropic to baroclinic (baroclinic to barotropic)
waves. 
The main conclusions are: 1) $V_g(x)$ changes the effective
frequency $f_{eff}$. This has a direct impact on the slope of the IT
characteristics and the slope criticality, which affects the total
conversion rate;
2) Since $(V_g)_x$ is not a constant value, $f_{eff}$ varies along the
slope. This has a significant effect on the IT beam generation
location and its propagation path. If the current is strong enough so
that $f_{eff}$ is greater than the barotropic tidal frequency $\sigma_T$, a blocking
region is formed where the conversion vanishes and IT propagation is blocked;
3) Changes of sign in $\bar{C}(x,t)$ correspond to the locations where
IT beams reflect from the boundaries. As a result, the total conversion rate $C$ is
also strongly affected by the IT beam pattern.
In conclusion, the total conversion rate $C$ is affected by a
combination of three factors: slope criticality, size and location of the blocking
region and the IT beam patterm, all of which can be varied by changing
the strength, width and location of the geostrophic current $V_g(x)$.

How to cite: He, Y. and Lamb, K.: The influence of a geostrophic current on the internal tide generation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1002, https://doi.org/10.5194/egusphere-egu21-1002, 2021.

We model internal tides generated by the interaction of a barotropic tide with variable topography. For the barotropic part, an asymptotic solution valid over the variable topography is considered. The resulting non-uniform ambient flow is used as a prescribed barotropic forcing for the baroclinic equations (linearized, non-hydrostatic, Euler equations within the Boussinesq approximation).

The internal-tide generation problem is reformulated by means of a Coupled-Mode System (CMS) based on the decomposition of the baroclinic stream function in terms of vertical basis functions that consistently satisfy the bottom boundary condition. The proposed CMS is solved numerically with a finite difference scheme and shows good convergence properties, providing efficient calculations of internal tides due to 2D topographies of arbitrary height and slope. We consider several seamounts and shelf profiles and perform calculations for a wide range of heights and slopes. Our results are compared against existing analytical estimates on the far-field energy flux in order to examine the limit of validity of common simplifications (Weak Topography Approximation, Knife edge). For subcritical cases, local extrema of the energy flux exist for different heights. Non-radiating topographies are also identified for some profiles of large enough heights. For supercritical cases, the energy flux is in general an increasing function with increasing height and criticality, and does not compare well against analytical results for very steep idealized topographies. The effect of the adjusted barotropic tide in the energy flux and the local properties of the baroclinic field is investigated through comparisons with other semi-analytical methods based on a uniform barotropic tide (Green’s function approach).  A method for estimating the sea-surface signature of internal tides is also provided.

How to cite: Papoutsellis, C., Mercier, M., and Grisouard, N.: Internal tide generation due to topographically adjusted barotropic tide, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8707, https://doi.org/10.5194/egusphere-egu21-8707, 2021.

As the first Doppler wind lidar in space, ADM-Aeolus provides us with a unique opportunity to study the propagation of gravity waves (GWs) from the surface to the tropopause and UTLS. Existing space-based measurements of GWs in this altitude range are spatially limited and, where available, use temperature as a proxy for wind perturbations. Thus, space-borne wind lidars such as Aeolus have the potential to transform our understanding of these critically-important dynamical processes. Here, we present the first observations of GWs in Aeolus data. We analyse a case study of a large orographic GW over the Southern Andes in July 2019 which is clearly visible in the horizontal wind. This example demonstrates the capability of Aeolus to measure the phase structure of GWs from near the surface up into the stratosphere. We validate these results against temperature-based observations from the AIRS nadir sounder and CORAL lidar, and also against ERA5 wind and temperature. There is close agreement in phase structure between Aeolus and the validation datasets, and with a near-identical observed vertical wavelength and spatial location. This case study suggests that data from Aeolus, and similar next-generation space-borne wind lidars, could play a critical role in constraining future model GW parameterisations, with the potential to significantly broaden our understanding of atmospheric dynamics.

How to cite: Banyard, T., Wright, C., Hindley, N., Halloran, G., Krisch, I., Kaifler, B., and Hoffmann, L.: Atmospheric Gravity Waves in ADM-Aeolus Wind Lidar Observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7911, https://doi.org/10.5194/egusphere-egu21-7911, 2021.

In order to improve global atmospheric modelling, the trend is towards including source-specific gravity waves (GWs) in general circulation models. In a case study, we search for the source of a GW observed over Greenland on 10 March 2016 using the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) onboard the German research aircraft HALO. GLORIA is a remote sensing instrument where the measured infrared radiances are converted into a 3D temperature field through tomography. 
We observe a GW packet between 10 and 13km that covers ∼1/3 of the Greenland mainland. GLORIA observations indicate a horizontal (vertical) wavelength of 330km (2km) and a temperature amplitude of 4.5K. Slanted phase fronts indicate intrinsic propagation against the jet but the GW packet propagates (ground-based) with the wind. To find the GW source, 3D GLORIA observations, GROGRAT raytracer, ERA5 data, and an ECMWF numerical experiment are used. The numerical experiment with a smoothed topography indicates virtually no GWs suggesting that the GW field in the full model is caused by the orography. However, these are not mountain waves. A favourable area for spontaneous GW emission is identified within the jet exit region by the cross-stream ageostrophic wind speed, which indicates when the flow is not in geostrophic balance. Backtracing experiments (using GROGRAT) trace into the jet and imbalance regions. The difference between the full and the smooth-topography experiment is the change in wind components by the compression of air above Greenland. These accelerations and decelerations in the jet cause the jet to become out of geostrophic balance, which excites GWs by spontaneous adjustment. We present, to the best of our knowledge, the first observational evidence of GWs by this topography-jet mechanism.

How to cite: Geldenhuys, M., Preusse, P., Krisch, I., Zülicke, C., Ungermann, J., Ern, M., Friedl-Vallon, F., and Riese, M.: A new mechanism for spontaneous imbalance exciting large-area gravity waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5591, https://doi.org/10.5194/egusphere-egu21-5591, 2021.

During the SOUTHTRAC-GW (Southern hemisphere Transport, Dynamics and Chemistry – Gravity Waves) field campaign, gravity waves above the Southern Andes mountains, the Drake passage and the Antarctic Peninsula were probed with airborne instruments onboard the HALO research aircraft. The Airborne Lidar for Middle Atmosphere research (ALIMA) detected particularly strong mountain waves in excess of 25 K amplitude in cross-mountain legs above the Southern Andes of research flight ST08 on 12 September 2019. The mountain waves propagated well into the mesosphere up to 65 km altitude with possible generation of smaller-scale secondary waves during wave breaking above 65 km. A superposition of mountain waves with horizontal wavelengths in the range 15-200 km and vertical wavelengths 7-24 km dominated the wave field between 18 and 65 km altitude. Vertical wavelengths predicted by the hydrostatic equation and horizontal wind from the European Center for Medium-Range Weather Forecasts’ Integrated Forecasting System are in good agreement with observed vertical wavelengths. We apply wavelet analysis to the measured temperature field along the flight track in order to identify and separate dominant scales, and estimate their relative contributions to the total gravity wave momentum flux as well as the local and zonal-mean gravity wave drag. Furthermore, we compare our observations to results obtained by Fourier ray analysis of the terrain of the Southern Andes. The Fourier model allows the investigation of the 3d-wave field and trapped waves which are not well sampled by the ALIMA instrument because of the relative alignment between the wave fronts and the flight track. These sampling biases are quantified from virtual flights through the model domain at multiple angles and taken into account in the estimation of the total momentum flux derived from ALIMA observations. The combination of high-resolution observations and model data reveals the significance of this and similar mountain wave events in the Southern Andes region for the atmospheric dynamics at ~60° S.

How to cite: Kaifler, N., Kaifler, B., Dörnbrack, A., Gisinger, S., Mixa, T., and Rapp, M.: Multi-scale mountain waves observed with the ALIMA lidar during SOUTHTRAC-GW above the southern Andes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13504, https://doi.org/10.5194/egusphere-egu21-13504, 2021.

The combination of the airborne GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) and ALIMA (Airborne LIdar for Middle Atmosphere research) instruments allows for probing of temperature perturbations associated with gravity waves within the range from the troposphere up to the mesosphere. Both instruments were part of the scientific payload of the German HALO (High Altitude and LOng Range Research Aircraft) during the SouthTRAC-GW (Southern hemisphere Transport, Dynamics, and Chemistry - Gravity Waves) mission, aiming at probing gravity waves in the hotspot region around South America and the Antarctic peninsula. For the research flight on 16 September 2019, complex temperature perturbations attributed to internal gravity waves were forecasted well above the Atlantic to the south-west of Buenos Aires, Argentina. The forecasted temperature perturbations were located in a region where the polar front jet stream met with the subtropical jet, with the polar night jet above. We present temperature perturbations observed by GLORIA and ALIMA during the discussed flight and compare the data with ECMWF IFS (European Centre for Medium-Range Weather Forecasts – Integrated Forecasting System) high-resolution deterministic forecasts, aiming at validating the IFS data and identifying sources of the observed wave patterns.

How to cite: Woiwode, W., Dörnbrack, A., Friedl-Vallon, F., Geldenhuys, M., Giez, A., Gulde, T., Höpfner, M., Johansson, S., Kaifler, B., Kleinert, A., Krasauskas, L., Kretschmer, E., Maucher, G., Neubert, T., Nordmeyer, H., Piesch, C., Preusse, P., Rapp, M., Riese, M., and Ungermann, J.: Observations of internal gravity waves in vicinity of jet streams during SouthTRAC flight on 16 September 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15102, https://doi.org/10.5194/egusphere-egu21-15102, 2021.

How to cite: Dörnbrack, A.: Stratospheric mountain waves trailing across northern Europe, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1635, https://doi.org/10.5194/egusphere-egu21-1635, 2021.

We present results of seven years of gravity waves (GW) observations between 2012 and 2018. The measurements were conducted by ground-based lidars in Kühlungsborn (54°N, 12°E) and at ALOMAR (69°N, 16°E). Our analysis technique includes different types of filtering which allow for selection of different ranges from the entire GW-spectrum. We studied wave properties as a function of altitude and location and summarized the results in monthly and seasonally mean profiles. Complementary data is taken from the satellite-based SABER instrument. Additionally, we consistently applied our analysis technique to the reanalyses data from MERRA-2 and ERA-5.

A seasonal cycle of gravity wave potential energy density (GWPED) with maximum values in winter is present at both stations in nearly all lidar/SABER measurements and in reanalysis data. For SABER and for lidar the winter to summer ratios are a factor of about 3. The winter to summer ratios are nearly identical at both stations. GWPEDs from reanalysis are smaller compared to lidar. The difference increases with altitude in winter and reaches almost two orders of magnitude around 70 km.

GWPEDs per volume decreases with height differently for the winter and summer seasons, irrespective of filtering method and location. In summer for altitudes above roughly 50 km, GWPED is nearly constant or even increases with height. This feature is very pronounced at ALOMAR and to a lesser extent also at Kühlungsborn. This behavior is seen by both, lidar and SABER. The observed variation of GWPED with height can not be explained by conservation of wave action alone.

The GWPED at Kühlungsborn is significantly larger compared to ALOMAR. This observation is opposite to simple scenarios which take into account the potential impact of background winds on GW filtering and Doppler shifts of vertical wavelengths and periods.

We present results of observations and analyses and suggest geophysical explanations of our findings.

How to cite: Strelnikova, I., Baumgarten, G., Baumgarten, K., Ern, M., Gerding, M., and Lübken, F.-J.: General characteristics of Gravity Wave Potential Energy Density at 54 ºN and 69 ºN, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9116, https://doi.org/10.5194/egusphere-egu21-9116, 2021.

The stable boundary layer, especially the very stable boundary layer, (vSBL) is a fundamental challenge for boundary layer meteorology as assumptions such as ergodicity and local scaling do not apply. The violation of these commonly-employed theories is associated with the presence of submeso-scale structures, which span spatial scales between tens of meters and kilometers and temporal scales from tens of seconds up to an hour. The nature of these structures is largely unknown but they are suspected to encompass a wide-range of flow modes, including meandering of the horizontal wind direction, thermal submeso fronts, complex and unknown non-stationary modes, and relevant to this work, various wave modes. Progress on submeso-turbulence interactions requires distributed observations with fine enough resolution to separate between the submeso and turbulent scales.

To that end we present results from FlyFOX in which fiber optic distributed sensing (FODS) was deployed along a tethered balloon. FODS yields spatially continuous observations of air temperature with fine spatial (0.25m – 0.5m) and temporal (1s-10s) resolutions along fiber optic cables that can span kilometers. In this case FlyFOX spanned between 0.5m and 200m height. FlyFOX was deployed in a broad mountain valley in the Ficthelgebirge mountains, Germany in which intense cold air pooling commonly occurs.

Using FlyFOX we simultaneously characterize the spatial and temporal spectra of the boundary layer through morning transitions, revealing that the vSBL has a unique spectral enhancement between 80s-640s and 8m-64m relative to weakly-stable and neutral conditions. These scales correspond to a gap in the observational capabilities of existing methods, which FlyFOX fills.

Corresponding to this observational gap, we demonstrate the existence of “sublayer striations”, thin (5m-20m) but persistent layers (duration up to an hour) of exceptionally stable air separated by layers of near-neutral stability. Using wavelet coherence for different time scales, gravity waves were found to be unable to penetrate into the sublayer striations and instead ducted in the neutral air between striations. During periods with overall lower static stability, these sublayer striations did not occur and waves acted across the entire depth of the SBL from ~120m down to ~0.5m and can be tracked propagating along the surface at 1m height using a near surface DTS array. These sublayer striations thereby acted to decouple the upper boundary layer from the surface layer in this mountain valley. FlyFOX and FODS provide an observational breakthrough for the study of vertical coupling and wave activity in the vSBL by closing an observational gap and facilitating observations of atmospheric properties from the turbulent to submeso scales.

How to cite: Lapo, K., Fritz, A., Freundorfer, A., Muppa, S. K., and Thomas, C. K.: Revealing the role of missing scales in boundary layer observations in gravity wave propagation using the Flying Fiber Optic eXperiment (FlyFOX), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10672, https://doi.org/10.5194/egusphere-egu21-10672, 2021.

Field observations of nonlinear atmospheric gravity waves are sparse and involved due to many challenges for the instrumentation. Due to these complications of field measurements, laboratory experiments are an indispensable tool.

As of today, all laboratory experiments on gravity waves have in common that they were performed with water as the working fluid. Due to flow similarities, most of the features observed in the water tanks are equally valid for the atmosphere. However, one particular property of air cannot be emulated by water: compressibility. Especially for the dynamics of nonlinear waves, compressibility plays a significant role.

We propose a laboratory experiment by means of a rapidly rotating gas centrifuge. The centrifugal forces act on the gas like the gravitational pull causing a stratified compressible working fluid. In this device, atmosphere-like gravity waves would be observable under controlled and replicable conditions for the first time.

We show that the waves in a centrifuge would theoretically behave like their atmospheric counterparts; they exhibit the same dispersion and polarization relations. Futhermore, spinning the centrifuge with the right frequency, there is a clear scale separation between acoustic and gravity waves. In addition to the centrifugal force, the Coriolis force acts in the same plane potentially spoiling the similarities. However, the influence of the Coriolis force on the wave is negligibly small.

How to cite: Schlutow, M.: Atmospheric gravity waves in a gas centrifuge, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9681, https://doi.org/10.5194/egusphere-egu21-9681, 2021.