Deep Propagation of Mountain Waves Observed During the DEEPWAVE Field Program

An airborne and ground-based field program called DEEPWAVE was performed in New Zealand during June and July 2014. DEEPWAVE measurements provided the first opportunity to observe gravity waves (GWs) arising from various tropospheric sources and propagating throughout the atmosphere to regions of dissipation as high as the mesosphere and lower thermosphere (MLT). Quantification of GW amplitudes, scales, and phase structures was enabled by observations that were nearly continuous in altitude and often spanned broad horizontal extents.

The measurements were made possible by new Na resonance and UV Rayleigh lidars and an Advanced Mesosphere Temperature Mapper (AMTM) specifically built for measurements aboard the NSF/NCAR GV research aircraft. Additional ground-based optical instruments quantified stratosphere and MLT mean and GW structures whenever skies were clear.

This talk will focus on deep propagation of mountain waves (MWs). Our observations yielded many surprises and important new insights into these phenomena. Flight-level measurements often revealed relatively weak MW forcing and an apparent distinction between vertically-propagating MWs having larger horizontal wavelengths >30 km, and trapped lee waves having wavelengths <30 km. But no such distinction was observed in the MLT, where the new lidar and AMTM instruments revealed large MW amplitudes at horizontal wavelengths of ~10-100 km. Large MLT MW responses were also observed on nights when the GV did not fly because of expected weak MW forcing. Both larger- and smaller-scale MWs achieved T’ ~10-20 K at ~87 km, implying horizontal velocity perturbations of u’~20-50 m/s and surprisingly large peak momentum fluxes. These results suggest that MWs routinely account for localized, but significant momentum transport to altitudes as high as allowed by the intervening winds.