Large‐Amplitude Mountain Waves in the Mesosphere Observed on 21 June 2014 During DEEPWAVE: 1. Wave Development, Scales, Momentum Fluxes, and Environmental Sensitivity.

A remarkable, large‐amplitude, mountain wave (MW) breaking event was observed on the night of 21 June 2014 by ground‐based optical instruments operated on the New Zealand South Island during the Deep Propagating Gravity Wave Experiment (DEEPWAVE). Concurrent measurements of the MW structures, amplitudes, and background environment were made using an Advanced Mesospheric Temperature Mapper, a Rayleigh Lidar, an All‐Sky Imager, and a Fabry‐Perot Interferometer. The MW event was observed primarily in the OH airglow emission layer at an altitude of ~82 km, over an ~2‐hr interval (~10:30–12:30 UT), during strong eastward winds at the OH altitude and above, which weakened with time. The MWs displayed dominant horizontal wavelengths ranging from ~40 to 70 km and temperature perturbation amplitudes as large as ~35 K. The waves were characterized by an unusual, "saw‐tooth" pattern in the larger‐scale temperature field exhibiting narrow cold phases separating much broader warm phases with increasing temperatures toward the east, indicative of strong overturning and instability development. Estimates of the momentum fluxes during this event revealed a distinct periodicity (~25 min) with three well‐defined peaks ranging from ~600 to 800 m2/s2, among the largest ever inferred at these altitudes. These results suggest that MW forcing at small horizontal scales (<100 km) can play large roles in the momentum budget of the mesopause region when forcing and propagation conditions allow them to reach mesospheric altitudes with large amplitudes. A detailed analysis of the instability dynamics accompanying this breaking MW event is presented in a companion paper, Fritts et al. (2019, https://doi.org/10.1029/2019jd030899). Key Points: Multi‐instrument characterization of a mesospheric mountain wave exhibit a unique, large‐amplitude "saw‐tooth" wave breaking signatureUnexpected large‐amplitude mesospheric mountain waves accompany weak winds over low orography due to favorable propagation conditionsVery large mesospheric mountain wave momentum fluxes ~400‐800 m2/s2 are sustained for multiple wave periods [ABSTRACT FROM AUTHOR]