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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]

Weak cross‐mountain flow over the New Zealand South Island on 21 June 2014 during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) led to large‐amplitude mountain waves in the mesosphere and lower thermosphere. The mesosphere and lower thermosphere responses were observed by ground‐based instruments in the lee of the Southern Alps supporting DEEPWAVE, including an Advanced Mesosphere Temperature Mapper, a Rayleigh lidar, an All‐Sky Imager, and a Fabry‐Perot Interferometer. The character of the mountain wave responses at horizontal scales of ~30–90 km reveals strong "sawtooth" variations in the temperature field suggesting large vertical and horizontal displacements leading to mountain wave overturning. The observations also reveal multiple examples of apparent instability structures within the mountain wave field that arose accompanying large amplitudes and exhibited various forms, scales, and evolutions. This paper employs detailed data analyses and results of numerical modeling of gravity wave instability dynamics to interpret these mountain wave dynamics, their instability forms, scales, and expected environmental influences. Results demonstrate apparently general instability pathways for breaking of large‐amplitude gravity waves in environments without and with mean shear. A close link is also found between large‐amplitude gravity waves and the dominant instability scales that may yield additional abilities to quantify gravity wave characteristics and effects. Key Points: Weak mountain waves at low altitudes can achieve breaking amplitudes in the mesosphere under suitable propagation conditionsMountain wave breaking occurs via 3‐D instability dynamics predicted by high‐resolution modeling in various environmentsMountain wave instability dynamics evolve on short time scales and impose significant intermittency in wave amplitudes and momentum fluxes [ABSTRACT FROM AUTHOR]

The Utah State University (USU) campus (41.7°N, 111.8°W) hosts a unique upper atmospheric observatory that houses both a high‐power, large‐aperture Rayleigh lidar and a Na lidar. For the first time, we will present 19 nights of coordinated temperature measurements from the two lidars, overlapping in the 80–110 km observational range, over one annual cycle (summer 2014 to summer 2015). This overlap has been achieved through upgrades to the existing USU Rayleigh lidar that increased its observational altitude from 45–95 to 70–115 km and by relocating the Colorado State Na lidar to the USU campus. Previous climatological comparisons between Rayleigh and Na lidar temperatures have suggested that significant temperature differences exist between the two techniques. This new comparison aims to further these previous studies by using simultaneous, common‐volume observations. The present comparison showed the best agreement between 85 and 95 km, with a temperature difference, averaged over the whole data set, of about 1.1 ± 0.5 K. Larger differences occurred above and below these altitudes with the Rayleigh temperatures being colder by about 3.5 ± 0.5 K at 82 km and warmer by up to 9.1 ± 3.5 K above 95 km. Key Points: First simultaneous temperature comparisons from collocated Rayleigh and Na lidars were made over one annual cycleGood agreement was found in the two lidars' temperatures from 85 to 95 kmSignificant temperature differences were found above 95 km and below 85 km [ABSTRACT FROM AUTHOR]