Abstract. A high-performance, all-sky imaging system has been used to obtain novel data on the morphology and dynamics of short-period (<1 hour) gravity waves at equatorial latitudes. Gravity waves imaged in the upper mesosphere and lower thermosphere were recorded in three nightglow emissions, the near-infrared OH emission, and the visible wavelength OI (557.7 nm) and Na (589.2 nm) emissions spanning the altitude range ---80-100 km. The measurements were made from Alcantara, Brazil (2.3øS, 44.5øW), during the period August-October 1994 as part of the NASA/Instituto Nacional de Pesquisas Espaciais "Guara campaign". Over 50 wave events were imaged from which a statistical study of the characteristics of equatorial gravity waves has been performed. The data were found to divide naturally into two groups. The first group corresponded to extensive, freely propagating (or ducted) gravity waves with observed periods ranging from 3.7 to

Over the past 60 years, ground-based remote sensing measurements of the Earth's mesospheric temperature have been performed using the nighttime hydroxyl (OH) emission, which originates at an altitude of ∼87  km. Several types of instruments have been employed to date: spectrometers, Fabry-Perot or Michelson interferometers, scanning-radiometers, and more recently temperature mappers. Most of them measure the mesospheric temperature in a few sample directions and/or with a limited temporal resolution, restricting their research capabilities to the investigation of larger-scale perturbations such as inertial waves, tides, or planetary waves. The Advanced Mesospheric Temperature Mapper (AMTM) is a novel infrared digital imaging system that measures selected emission lines in the mesospheric OH (3,1) band (at ∼1.5  μm) to create intensity and temperature maps of the mesosphere around 87 km. The data are obtained with an unprecedented spatial (∼0.5  km) and temporal (typically 30″) resolution over a large 120° field of view, allowing detailed measurements of wave propagation and dissipation at the ∼87  km level, even in the presence of strong aurora or under full moon conditions. This paper describes the AMTM characteristics, compares measured temperatures with values obtained by a collocated Na lidar instrument, and presents several examples of temperature maps and nightly keogram representations to illustrate the excellent capabilities of this new instrument.

Improved ground-based Fourier transform infrared spectroscopic measurements of the OH Meinel (M) Av= 2 and 3 nightglow emissions revealed unexpectedly intense transitions from high rotational levels. The rotational development in the P branches of the OH M (3,1), (6,3), and (7,4) bands has been followed to the PI(N") transitions with N"= 10, 12, and 13, respectively. These measurements indicate that -10% of the OH(X, v' = 7) column rotational population is typically in rotational levels with N' _>7, in sharp contrast with the corresponding local thermodynamic equilibrium estimate of-1%. The excess high-N' population also persists in the lower vibrational states of OH(X), as evidenced by the readily detectable high-P 1 (N") transitions in the (3,1) and (6,3) bands and by the presence of observable R branch heads in the (4,2) and (5,3) bands. The R heads in the (4,2) and (5,3) bands form at moderately high N' and are typically much more intense than expected on the assumption of complete rotational-translational equilibration throughout the OH M source region. These new results provide strong evidence for incomplete R-T equilibration of OH(X2/'/, 3

Abstract. High-precision (4).

The impacts of gravity wave (GW) on the thermal and dynamic characteristics within the mesosphere/lower thermosphere, especially on the atmospheric instabilities, are still not fully understood. In this paper, we conduct a comprehensive and detailed investigation on one GW breaking event during a collaborative campaign between the Utah State University Na lidar and the Advanced Mesospheric Temperature Mapper (AMTM) on 9 September 2012. The AMTM provides direct evidence of the GW breaking as well as the horizontal parameters of the GWs involved, while the Na lidar's full diurnal cycle observations are utilized to uncover the roles of tide and GWs in generating a dynamical instability layer. By studying the changes of the OH layer peak altitude, we located the wave breaking altitude as well as the significance of a 2 h wave that are essential to this instability formation. By reconstructing the mean fields, tidal and GW variations during the wave breaking event, we find that the large-amplitude GWs significantly changed the Brunt-Vaisala frequency square and the horizontal wind shear when superimposed on the tidal wind, producing a transient dynamic unstable region between 84 km and 87 km around 11:00 UT that caused a subsequent small-scale GW breaking. IntroductionThe gravity wave (GW) forcing and its related spectra within the mesosphere/lower thermosphere (MLT) are the key parameters for the understanding of energy and momentum transfers between the lower and middle atmosphere and the ionosphere. They have been known to drive the circulation and generate the counter intuitive cold summer and warm winter in the mesopause region [Garcia and Solomon, 1985], along with some irregularities in the ionosphere [Liu and Vadas, 2013]. Yet after decades of investigations, their characteristics and effects on the upper atmosphere are still not fully understood due to the GW's random spatial scales with their periods varying from a few minutes to several hours. Mainly generated in the troposphere by orographic, convection, and jet-front system, the GWs propagate upward with growing amplitude to compensate for the decrease of the air density due to conservation of the wave energy density during their propagation, before they reach the critical levels or become unstable and break . It is also possible that the GW breaking can generate secondary waves within the breaking region [Vadas et al., 2003;Smith et al., 2013], affecting the atmosphere above MLT region. The wave breaking process deposits momentum into the mean flow field, causing mean flow acceleration in the wave propagation direction; changes the thermal structure; and generates turbulence around the breaking region.Ground-based experimental studies have been playing the significant role in studying the GW dynamics and the associated atmospheric instabilities during the recent decades. However, single instrument only partially resolves the complex wave breaking process and the instability phenomenon. For example, the airglow measurement techniques like OH ima...

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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).

Improved spectral measurements of the OH Meinel Δv = 3 night airglow emissions have revealed unexpectedly intense transitions from high rotational levels. The example selected for this communication involves the previously unreported P(N″ = 13) transitions in the OH M (7, 4) band. Under the extant conditions, the column emission rates associated with these new features exceeded by factors ∼104 those expected on the basis of the assumption of rotational‐kinetic equilibrium for the v′ = 7 rotational manifold. We present the key observations and discuss some of the implications.