Gravity wave excitation during the coastal transition of an extreme katabatic flow in Antarctica
- 1École Polytechnique Fédérale de Lausanne, Laboratoire de Télédetection Environnementale, Lausanne, Switzerland (etienne.vignon@epfl.ch)
- 2UGA, CNRS, Institut des Géosciences de l’Environnement (IGE), Grenoble, France
- 3Australian Antarctic Division, Hobart, Tasmania, Australia
The offshore extent of Antarctic katabatic winds exert a strong control on sea ice production and the formation of polynyas. In this study, we combine ground-based remotely-sensed and meteorological measurements at Dumont d’Urville (DDU) station, satellite images and simulations with the WRF model to analyze a major katabatic wind event in Adélie Land. Once developed over the slope of the ice sheet, the katabatic flow experiences an abrupt transition near the coastal edge. The transition consists in a sharp increase in the boundary layer depth, a sudden decrease in wind speed and a decrease in Froude number from 3.5 to 0.3. This so-called ‘katabatic jump’ visually manifests as a turbulent ‘wall’ of blowing snow in which updrafts exceed 5 m s −1 . The wall reaches heights of 1000 m and its horizontal extent along the coast is more than 400 km. By destabilizing the boundary-layer downstream, the jump favors the trapping of a gravity wave train with an horizontal wavelength of 10.5 km. The trapped gravity waves exert a drag that significantly slows down the low-level outflow. Moreover, atmospheric rotors form below the first wave crests. The wind speed record measured at DDU in 2017 (58.5 m s −1 ) is due to the vertical advection of momentum by a rotor. A statistical analysis of observations at DDU reveals that katabatic jumps and low-level trapped gravity waves occur frequently over coastal Adélie Land. It emphasizes the important role of such phenomena in the coastal Antarctic dynamics.
How to cite: Vignon, É., Picard, G., Durán-Alarcón, C., Alexander, S. P., Gallée, H., and Berne, A.: Gravity wave excitation during the coastal transition of an extreme katabatic flow in Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3043, https://doi.org/10.5194/egusphere-egu2020-3043, 2020.
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Hi Etienne
I missed the chance to comment on your presentation in live chat. Is your katabatic jumps associated with the change in the terrain steepness? Katabatic flows increase in depth when going from steeper to less steep terrain (Smith and Skyllingstaad 2005 ) but to the bluff-body boundary layer separation if the terrain is increasingly steeper (Sachsperger et al. 2016 ) . Sachsperger actually have an analytical solution for both wavelength and amplitude of the trapped waves. You could compare your results to their prediction.
Ivana
Dear Ivana,
thanks a lot for your interest and your question.
The formation of a katabatic jump is often due to several factors but the shape of the topography is indeed a key prerequisite. The ice sheet near Dumont d'Urville station shows a very sharp increase in slope near the coast (concave topography), favoring the deepening of the katabatic layer as well as jump developments. Thanks for the Smith and Skyllingstad reference. There is an additional paper that discusses this aspect for the Antarctic context (Yu et al. 2007).
Regarding the amplitude and wavelength of trapped waves, I did not compare the observations with the analytical solutions from Sachsperger et al. 2016 but that's something that could indeed be interesting (well, except that in my context, the topography is not a nice bell-shaped mountain and defining the non-dimensional mountain height might be not straightforward). Thanks a lot for the suggestion. As a follow-up study, we plan to look at jumps and waves in different Antarctic regions with different topographies and meterological contexts. This will probably lead us to look more closely at analytical models to explain possible differences between locations.
Thanks again,
Étienne
Yu et al. 2007, Influence of topography and large-
scale forcing on the occurrence of katabatic flow jumps in
Antarctica: Idealized simulations. Adv. Atmos. Sci., 24,
819–832, https://doi.org/10.1007/s00376-007-0819-x.
Thanks Etienne for the additional references. Very interesting study!
Yes, your profile is not very Witch of Agnesi :-) But maybe as a first guess you could sue the height of the drop. Looking forward to more of your results!
Ivana
Hi Etienne,
I missed the live chat this morning due to technical problems, but I find your presentation really interesting. You mention that you would like to expand your analysis to other Antarctic coastal regions - it would be great if you could quantify the overall impact of gravity wave momentum fluxes on the atmosphere in the coastal region. As you seem to be aware, we have some evidence from observations and models that gravity waves are frequently generated by hydraulic jumps in the vicinity of Halley station, where they probably play a role in the rapid deceleration of katabatic winds that means that the katabatic flow never reaches Halley. It would be very interesting to see how much of the Antarctic coast is like DDU (hydraulic jump and gravity wave generation occuring close to the coast) and how much is like Halley (HJ and GW generation further inland).
Dear John,
thank a lot for your interest in our presentation. I fully agree with you that quantifying the effect of GW on the deceleration of the katabatic flow at the Antarctic scale would be great. Initially, I wanted to do that with WRF simulations. However, trapped GW have a wavelength of a few km and I would need a very high resolution (~1km) to correctly reproduce them. Running a 1-km resolution simulation at the Antarctic scale for several years would be computationally too expensive...
That is why, as a first step, we would like to analyse long-term wind time series and satellite images at different spots along the Antarctic coast where we do know that katabatic jump and GW occur (Halley, DDU but also Syowa for instance). At Syowa, we know that GW can be generated further inland, as near Halley (please see Appendix C in our JAS paper: doi:10.1175/JAS-D-19-0264.1).
This is something I (and co-authors) wanted to investigate already a few months ago but I did not find the time to do it.... I hope to start the analysis soon.
Thanks again for your interest and your comment,
Étienne