Gravity Wave Breaking over the Central Alps: Role of Complex Terrain

Abstract

The characteristics of gravity waves excited by the complex terrain of the central Alps during the intensive observational period (IOP) 8 of the Mesoscale Alpine Programme (MAP) is studied through the analysis of aircraft in situ measurements, GPS dropsondes, radiosondes, airborne lidar data, and numerical simulations. Mountain wave breaking occurred over the central Alps on 21 October 1999, associated with wind shear, wind turning, and a critical level with Richardson number less than unity just above the flight level (ϳ5.7 km) of the research aircraft NCAR Electra. The Electra flew two repeated transverses across the Ö tztaler Alpen, during which localized turbulence was sampled. The observed maximum vertical motion was 9 m s Ϫ1 , corresponding to a turbulent kinetic energy (TKE) maximum of 10.5 m 2 s Ϫ2. Spectrum analysis indicates an inertia subrange up to 5-km wavelength and multiple energy-containing spikes corresponding to a wide range of wavelengths. Manual analysis of GPS dropsonde data indicates the presence of strong flow descent and a downslope windstorm over the lee slope of the Ö tztaler Alpen. Farther downstream, a transition occurs across a deep hydraulic jump associated with the ascent of isentropes and local wind reversal. During the first transverse, the turbulent region is convectively unstable as indicated by a positive sensible heat flux within the turbulent portion of the segment. The TKE derived from the flight-level data indicates multiple narrow spikes, which match the patterns shown in the diagnosed buoyancy production rate of TKE. The turbulence is nonisotropic with the major TKE contribution from the-wind component. The convectively unstable zone is advected downstream during the second transverse and the turbulence becomes much stronger and more isotropic. The downslope windstorm, flow descent, and transition to turbulence through a hydraulic jump are captured by a real-data Coupled Ocean-Atmosphere Mesoscale Predition System (COAMPS) simulation. Several idealized simulations are performed motivated by the observations of multiscale waves forced by the complex terrain underneath. The simulations indicate that multiscale terrain promotes wave breaking, increases mountain drag, and enhances the downslope winds and TKE generation.

An example of enhanced tephra deposition driven by topographically induced atmospheric turbulence

“…6 A simplified model of the variation in transport distance for different particle sizes falling from a volcanic plume and how this may be modified by lee-wave generation a height of a few kilometres, where the breaking of lee waves is initiated. Lee-wave breaking is often associated with downslope windstorms and hydraulic jumps on the lee side of the topographic obstacle (Wurtele et al 1996;Jiang and Doyle 2004). It results in extreme air turbulence and significant enhancement of vertical atmospheric mixing, and could therefore drive rapid downward acceleration and deposition of particles.…”

“…Under certain conditions, the instabilities initiated by the breaking of lee waves can propagate upwards, even entering the stratosphere (Smith et al 2008). The effects of lee waves have been extensively reported through observations (Jiang and Doyle 2004;Doyle et al 2005;Smith et al 2007;Lane et al 2009;Jiang et al 2010). Some early theoretical studies (Long 1953;Miles and Huppert 1968) elucidated the nonlinear mechanisms for an early stage of the wave breaking processes.…”

“…Some early theoretical studies (Long 1953;Miles and Huppert 1968) elucidated the nonlinear mechanisms for an early stage of the wave breaking processes. Understanding of these mechanisms has been significantly enriched through numerical experiments (Laprise and Peltier 1989;Lin and Wang 1996;Jiang et al 2004;McHugh and Sharman 2013). For a highly simplified case of uniform stratification and wind profile, a Mountain Froude number (cf.…”

Simulating wind channelling over Frobisher Bay and its interaction with downslope winds during the 7–8 November 2006 wind event

“…evidence of a strengthening downslope flow below a critical level, which is topped by a weakly stable layer characterized by an increasing reverse wind shear and directional shear that is associated with wave breaking is provided in Jiang and Doyle (2004, Fig 3). A near-surface wind with a speed profile resembling that of a low-level jet can be seen developing below 950 hPa.…”

Trapped mountain waves during a light aircraft accident

“…Thus, some kind of instability or additional process is required to induce a cascade from the relatively long wave scale, down to sub-kilometre scales. Examples of these processes include trapped lee waves and rotors (Doyle and Durran 2002), wave breaking (Lilly 1978, Jiang and Doyle 2004, Lane et al 2009), eddy shedding associated with mechanical effects (Clark et al 1997), and shear instabilities that form within wakes (Lane et al 2006). See Sharman et al (2012) for a brief review.…”