Gravity Wave Excitation during the Coastal Transition of an Extreme Katabatic Flow in Antarctica

Abstract

The offshore extent of Antarctic katabatic winds exerts a strong control on the production of sea ice and the formation of polynyas. In this study, we make use of a combination of ground-based remotely sensed and meteorological measurements at Dumont d’Urville (DDU) station, satellite images, and simulations with the Weather Research and Forecasting Model to analyze a major katabatic wind event in Adélie Land. Once well developed over the slope of the ice sheet, the katabatic flow experiences an abrupt transition near the coastal edge consisting of 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 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 a horizontal wavelength of 10.5 km—that develops in a few hours. The trapped gravity waves exert a drag that considerably 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.

Introducing CRYOWRF v1.0: Multiscale atmospheric flow simulations with advanced snow cover modelling

“…As reported by Vignon et al (2020) using station measurements, there is a relatively quiescent period before the 9th of August 2017 with stable wind speed, wind direction and air temperature recorded at DDU. The situation dramatically changes on the 9th when the katabatic flow rapidly strengthens and very high wind speeds are reported at DDU in the first half of the 10th.…”

“…At 1200 hrs on the 10th, there is a sudden drop in the wind speed, following which the wind becomes gusty but on average remains slower than the previous day. Vignon et al (2020) showed that the abrupt deceleration of the katabatic flow is due to excitation of gravity waves and the enhanced friction induced. nested domains with different resolutions, particularly for wind speeds and direction.…”

“…The case study is directly inspired by the recent work of Vignon et al (2020) who present a detailed analysis on the genesis, dynamics and implications of the hydraulic jumps and the corresponding gravity waves in the region in proximity of DDU.…”

“…As mentioned earlier, CRYOWRF is compatible with the nesting capability of WRF. Finally, unlike, Vignon et al (2020), who used standard WRF, CRYOWRF includes the blowing snow model, which allows for studying the impact of hydraulic jump formation on blowing snow dynamics and simulate the formation of the aforementioned 'snow walls' for the first time.…”

Radiosonde Observations of a Wintertime Meridional Convergence of Gravity Waves Around 60°S in the Lower Stratosphere

“…Conversely, Dumont D'Urville is located on Petrel Island and approximately 5 km from the main ice sheet. It is subject to strong katabatic winds, which flow from the Antarctic interior, which can also generate gravity waves (Vignon et al, 2020). It is also expected, from previous studies (Moffat-Griffin et al, 2011, 2013, that those stations close to the Drake Passage will see greater levels of gravity wave activity as this is where the main gravity wave "hotspot" is located.…”