Stratospheric Trailing Gravity Waves from New Zealand

Abstract

Gravity waves are frequently observed in the stratosphere, trailing long distances from mid- to high-latitude topography. Two such trailing-wave events documented over New Zealand are examined using observations, numerical simulations, and ray-tracing analysis to explore and document stratospheric trailing-wave characteristics and formation mechanisms. We find that the trailing waves over New Zealand are orographically generated and regulated by several processes, including interaction between terrain and mountaintop winds, critical-level absorption, and lateral wave refraction. Among these, the interaction between topography and low-level winds determines the perturbation energy distribution over horizontal scales and directions near the wave source, and accordingly, trailing waves are sensitive to terrain features and low-level winds. Terrain-forced wave modes are filtered by absorption associated with directional wind shear and Jones critical levels. The former plays a role in defining wave-beam orientation, and the latter sets an upper limit for the permissible horizontal wavelength of trailing waves. On propagating into the stratosphere, these orographic gravity waves are subject to horizontal refraction associated with the meridional shear in the stratospheric westerlies, which tends to elongate the wave beams pointing toward stronger westerlies and shorten the wave beams on the opposite side.

Far‐Ranging Impact of Mountain Waves Excited Over Greenland on Stratospheric Dehydration and Rehydration

“…The long-lasting flow over Greenland excited inertia-gravity waves with horizontally long modes (λ > 500 km) propagating into the direction of the ambient wind, a process similar to that observed in the lee of the Scandinavian mountains (Dörnbrack et al, 1999(Dörnbrack et al, , 2002. The additional meridional propagation into the PNJ is a dynamical process well-described by Sato et al (2012) and further discussed by Ehard et al (2017) and Jiang et al (2019) for the Southern Hemisphere. Figure 4 displays vertical sections along the baseline sketched in Figure 3 for the 25 January 2005 18 UTC (i.e., 24 hr before our q H2O observation) and 26 January 2005 18 UTC.…”

Numerical Simulation of Mountain Waves over the Southern Andes. Part I: Mountain Wave and Secondary Wave Character, Evolutions, and Breaking

“…This step drastically reduces the number of mesh points required near the surface thus allowing us to cluster the bulk of the points at the upper altitudes where instability and wave breaking occur. From a GW perspective, the main effect of the boundary layer is an adjustment of the effective terrain shape due to its displacement effect, and perhaps local regions of separated flow behind steep leeward ridges (e.g., Jiang et al 2007). These are difficult features to predict without a fine near-surface grid and an accurate turbulence model, however, and thus we have not resorted to any much more approximate techniques in order to account for these effects.…”

“…In addition, the radar studies of de Wit et al (2017) suggest that the large eastward GW momentum flux observed above the stratospheric jet is likely due to secondary waves resulting from MW breaking over the southern Andes. DEEPWAVE airborne observations also provided evidence of trailing waves refracting into the polar vortex in the stratosphere (Jiang et al 2019). Finally, ground-based measurements at the Andes Lidar Observatory at 308S and during DEEPWAVE enabled identification of the instability dynamics accounting for MW breaking in the MLT in two events (Hecht et al 2018;Fritts et al 2019a).…”

SOUTHTRAC-GW: An Airborne Field Campaign to Explore Gravity Wave Dynamics at the World’s Strongest Hotspot

“…DEEPWAVE was also the first comprehensive airborne mission studying GW dynamics up to the mesopause (∼100 km). To mention just a few outstanding results, DEEPWAVE provided insight into the relation between tropospheric forcing and GW activity in the middle atmosphere (Fritts et al 2016(Fritts et al , 2018Kaifler et al 2015;Bramberger et al 2017;Portele et al 2018), the horizontal propagation of OGW into the polar night jet (Ehard et al 2017), secondary wave generation in regions of strong MW breaking (Bossert et al 2017), the effect of the background atmosphere on GW propagation also in the absence of critical level filtering (Kruse et al 2016), the relative contribution of various parts of the GW spectrum to momentum fluxes (Smith et al 2016;Kruse 2017, 2018;Bossert et al 2018), and the general characteristics of both OGWs and NOGWs (Smith et al 2016;Smith and Kruse 2017;Eckermann et al 2016;Pautet et al 2016Pautet et al , 2019Jiang et al 2019). In the following Section 2, we describe the research aircraft and its instruments dedicated to the measurements of GW properties, along with the ground based measurements in the region.…”