Characteristics of gravity waves observed by Rayleigh lidar in the middle atmosphere above New Zealand

The Temperature Lidar for Middle Atmosphere research (TELMA) made atmospheric soundings at Lauder, New Zealand, over a period of 5 months in southern winter 2014. Strong gravity wave (GW) activity occurred in sporadic intervals lasting a few days. Large-amplitude mountain waves were detected during periods with high tropospheric wind speeds perpendicular to the Southern Alps. During the strongest event (29 July to 1 August) mountain wave amplitudes in excess of 15 K occurred at 40 km altitude, resulting in GW potential energy density enhancements exceeding one order of magnitude. The quasi-stationary large-amplitude waves often break and dissipate in the stratosphere and do not reach mesospheric altitudes. Secondary GWs radiated in this process account for a significant shift in the wave spectrum toward smaller scales. In addition, a substantial number of small-scale secondary GWs are generated as a result of propagating primary GWs interacting with tides. In the mesosphere, largest mountain wave amplitudes occur during conditions of weak to moderate tropospheric forcing (wind speeds in the range 2-12 m/s) and sufficiently strong stratospheric wind speeds (> 10 m/s) to allow vertical propagation.