Stratospheric Gravity Wave Scales And Fluxes During Deepwave

Abstract:

The Gulfstream V research aircraft flights for the Deepwave project in June and July 2014 included 97 legs over the Southern Alps of New Zealand and 150 legs over the Tasman Sea and Southern Ocean, mostly in the low stratosphere at 12.1 km altitude. Improved instrument calibration, redundant sensors, longer flight legs, energy flux estimation and wavelet and filter analysis revealed new gravity wave properties. Over the sea, flight level wave amplitudes mostly fall below our detection threshold. Over terrain, disturbances show characteristic mountain wave attributes of positive vertical energy flux (EFz), negative momentum flux and upwind horizontal energy flux. In some cases, the fluxes change rapidly within an eight hour flight, even though environmental conditions are unchanged, suggesting high sensitivity or auto-oscillation. The extreme EFz=22W/m2.
Most intriguing is the variety of disturbance scales found over New Zealand. In many cases, the dominant wavelength of the flux carrying mountain waves is long; between 60 and 150km. In the strong cases however, with EFz>4W/m2, the wave “downshifts” to an intermediate wavelength between 20 to 60 km. Two other disturbance scales are seen. In all cases, the vertical wind variance at flight level is dominated by short “fluxless” waves with wavelengths in the 8 to 15km range. These waves propagate upwind but they carry no vertical momentum or energy fluxes. These fluxless waves may be secondary ducted waves riding on the tropopause inversion. In four cases, mountain waves stagnate the wind and turbulent wave breaking is found with a dominant wavelength of 500m.