Observation and simulation of mountain wave turbulence above Iceland: Turbulence intensification due to wave interference

Abstract

The High‐Altitude LOng Range research aircraft (HALO) encountered strong turbulence above Iceland at 13.8 km altitude on 13 October 2016. The generation of turbulence along the flight path is studied through numerical simulations in combination with the aircraft insitu observations. From the insitu observations, maximum energy dissipation rate values (cube root of the energy dissipation rate) of 0.39 m2/3 ·s−1 are obtained, which correspond to moderate to severe turbulence for a medium‐weight aircraft such as HALO. The turbulent region is characterized by observed large‐amplitude vertical wind fluctuations which coincide locally with a stagnation of the horizontal flow. The strong turbulence occurred downstream of and between the two Icelandic mountains Hofsjökull and Langjökull. High‐resolution numerical simulations, with realistic and idealized topography, show that the flow above these two nearby mountains is responsible for the observed turbulence. Vertically propagating hydrostatic mountain waves disperse horizontally in the region downstream and between Hofsjökull and Langjökull. There, both waves interfere and their superposition leads to enhanced amplitudes and, eventually, to convective instabilities. By comparing simulations with only one of the mountains to the simulation with both mountains, we infer that the wave interference can locally amplify the turbulence intensity by a factor of five and double the vertical extent of the turbulent region.

High‐Resolution Aircraft Observations of Turbulence and Waves in the Free Atmosphere and Comparison With Global Model Predictions

“…During the DEEPWAVE campaign (Fritts et al, 2016), measurements were made with the NOAA/NCAR GV over New Zealand and most flights were mountain-crossing transects to study gravity waves excited by flow over the Southern Alps. The NAWDEX campaign (Schäfler et al, 2018) took place over the North Atlantic and there were few mountain crossings (Bramberger et al, 2020;Wilms et al, 2020). The SOUTHTRAC campaign is the subject of this paper.…”

“…(2020), Wilms et al. (2020). Therefore, the second objective is to provide wave energy and momentum fluxes from the flight level observations to investigate if increased gravity wave fluxes also imply larger ϵ values.…”

“…Thus, the first objective of this study is to present probability density functions of ϵ for the different segments of the research flights and to ask how large the eddy dissipation rate can be and what are its typical values in the free atmosphere. A number of studies dealing with radiosonde and airborne turbulence observations near the tropopause mention the coexistence with internal gravity waves, for example, Turner (1955), Bellenger et al (2017), their possible role as triggers of instabilities in layers of the atmosphere (Panofsky et al, 1968), or even the potential of breaking gravity waves in generating turbulent patches, see for example, Dörnbrack (1998), Bramberger et al (2020), Wilms et al (2020). Therefore, the second objective is to provide wave energy and momentum fluxes from the flight level observations to investigate if increased gravity wave fluxes also imply larger ϵ values.…”

Not All Clear Air Turbulence Is Kolmogorov—The Fine‐Scale Nature of Atmospheric Turbulence

“…Other important sources of CAT include loss of geostrophic balance in highly curved jet streams (Williams et al., 2003, 2005, 2008), and airflow over mountains, which excites vertically propagating gravity waves (GWs) that become unstable and break (Bramberger et al., 2018; Doyle et al., 2005; Lilly, 1978; McCann, 2001). In particular, GWs‐CAT‐related events are believed to be more severe (Bramberger et al., 2020; Schmid & Dörnbrack, 1999; Wilms et al., 2020) and to cover larger horizontal and vertical ranges.…”

“…severe (Bramberger et al, 2020;Schmid & Dörnbrack, 1999;Wilms et al, 2020) and to cover larger horizontal and vertical ranges.…”