Horizontal propagation of large‐amplitude mountain waves into the polar night jet

Abstract

We analyze a large‐amplitude mountain wave event, which was observed by a ground‐based lidar above New Zealand between 31 July and 1 August 2014. Besides the lidar observations, European Centre for Medium‐Range Weather Forecasts (ECMWF) data, satellite observations, and ray tracing simulations are utilized in this study. It is found that the propagation of mountain waves into the middle atmosphere is influenced by two different processes at different stages of the event. At the beginning of the event, instabilities in a weak wind layer cause wave breaking in the lower stratosphere. During the course of the event the mountain waves propagate to higher altitudes and are refracted southward toward the polar night jet due to the strong meridional shear of the zonal wind. As the waves propagate out of the observational volume, the ground‐based lidar observes no mountain waves in the mesosphere. Ray tracing simulations indicate that the mountain waves propagated to mesospheric altitudes south of New Zealand where the polar night jet advected the waves eastward. These results underline the importance of considering horizontal propagation of gravity waves, e.g., when analyzing locally confined observations of gravity waves.

Directional Absorption of Parameterized Mountain Waves and Its Influence on the Wave Momentum Transport in the Northern Hemisphere

“…Dunkerton () studied the propagation of stationary inertia gravity waves in the stratosphere by means of ray tracing. Owing to the horizontal shear of mean zonal wind, gravity waves were refracted and focused into the polar night jet, which was confirmed by recent observational and numerical studies (e.g., Ehard et al, ; Sato et al, ). Moreover, upgoing gravity waves can be reflected downward at the interface of static stability and/or wind shear discontinuities (e.g., Klemp & Lilly, ; Teixeira et al, ).…”

Infrasound and Gravity Waves Over the Andes Observed by a Pressure Sensor on Board a Stratospheric Balloon

“…This trend, from smaller wavelengths to larger wavelengths, when following the downwind balloon trajectory is in agreement with the expected behavior of the mountain gravity waves. In addition, the shift of maximum energy relative to the ridge in the downwind direction (Figure ) can be explained by the upward and downwind propagation of mountain gravity waves (Ehard et al, ; Iwasaki et al, ). This energy increase cannot be justified by the slight decrease of balloon altitude after the Andes, because only 4% amplitude increase of pressure variations are expected.…”

Effects of Horizontal Wind Structure on a Gravity Wave Event in the Middle Atmosphere Over Syowa (69°S, 40°E), the Antarctic

“…Since the GWs observed by the RR lidar have a long wave period, that is, longer than 2 hr, they can travel a long horizontal distance during their vertical propagation. On the other hand, the GWs in the higher latitudes are refracted to lower latitudes, that is, such waves refracted toward the PNJ (Dunkerton, 1984;Ehard et al, 2017;Sato et al, 2009). The nightly mean wind and temperature fields acquired from MERRA for each observation duration on 8-21 August 2014 and August 2015 were used as the background for the ray-tracing procedure.…”