Atmospheric waves above New Zealand

When air blown across a sea or a plain encounters a mountain range, it’s pushed upward into the cooler air above. The difference in buoyancy between the two air masses sets up a standing gravity wave—a mountain wave—leeward of the range. Mountain waves, in turn, engender other gravity waves that lift energy and momentum into the stratosphere and mesosphere. (See Backscatter in Physics Today, June 2006, page 96 .) To characterize those waves, Bernd Kaifler of the German Aerospace Center and his collaborators developed a compact, mobile light and detection ranging (lidar) experiment. Between June and November 2014, they installed it in one of the world’s strongest sources of mountain waves: New Zealand’s Southern Alps. (The accompanying photo shows waves above the site.) The experiment sent light pulses upward into the atmosphere and measured the echoes’ travel time, which yields the altitude, and their intensity, which is proportional to the local atmospheric density. Thanks to the lidar’s power, the experiment could quantify gravity waves up to 80 km, which corresponds to the top of the mesosphere. As expected, the gravity waves were most active during southern winter, when the prevailing westerly winds are strongest. Yet even during the summer, the gravity waves pervaded the stratosphere and mesosphere. The biggest surprise came when the researchers correlated the speed of the surface winds with the altitude of the gravity waves: Moderate, not strong, winds are the most effective at sending gravity waves into the upper mesosphere. That finding overturns the previous assumption of a linear relationship that was built into climate models. (B. Kaifler et al., Geophys. Res. Lett. 42, 9488, 2015, doi:10.1002/2015GL066465 .)