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  1. Abstract. An intriguing and rare gravity wave event was recorded on the night of 25 April 2017 using a multiwavelength all-sky airglow imager over northernGermany. The airglow imaging observations at multiple altitudes in themesosphere and lower thermosphere region reveal that a prominent upward-propagating wave structure appeared in O(1S) and O2 airglowimages. However, the same wave structure was observed to be very faint in OH airglow images, despite OH being usually one of the brightest airglowemissions. In order to investigate this rare phenomenon, the altitudeprofile of the vertical wavenumber was derived based on colocated meteorradar wind-field and SABER temperature profiles close to the event location.The results indicate the presence of a thermal duct layer in the altituderange of 85–91 km in the southwest region of Kühlungsborn, Germany.Utilizing these instrumental data sets, we present evidence to show how aleaky duct layer partially inhibited the wave progression in the OH airglowemission layer. The coincidental appearance of this duct layer is responsible for the observed faint wave front in the OH airglow images compared O(1S) and O2 airglow images during the course of the night over northern Germany. 
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  2. Abstract

    The cloud imaging and particle size (CIPS) instrument onboard the Aeronomy of Ice in the Mesosphere satellite provides images of gravity waves (GWs) near the stratopause and lowermost mesosphere (altitudes of 50–55 km). GW identification is based on Rayleigh Albedo Anomaly (RAA) variances, which are derived from GW‐induced fluctuations in Rayleigh scattering at 265 nm. Based on 3 years of CIPS RAA variance data from 2019 to 2022, we report for the first time the seasonal distribution of GWs entering the mesosphere with high (7.5 km) horizontal resolution on a near‐global scale. Seasonally averaged GW variances clearly show spatial and temporal patterns of GW activity, mainly due to the seasonal variation of primary GW sources such as convection, the polar vortices and flow over mountains. Measurements of stratospheric GWs derived from Atmospheric InfraRed Sounder (AIRS) observations of 4.3 μm brightness temperature perturbations within the same 3‐year time range are compared to the CIPS results. The comparisons show that locations of GW hotspots are similar in the CIPS and AIRS observations. Variability in GW variances and the monthly changes in background zonal wind suggest a strong GW‐wind correlation. This study demonstrates the utility of the CIPS GW variance data set for statistical investigations of GWs in the lowermost mesosphere, as well as provides a reference for location/time selection for GW case studies.

     
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  3. Abstract

    The behaviors of the nitric oxide (NO) cooling in the lower thermosphere during the 14 December 2020 solar eclipse are studied using Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) measurements and WACCM‐X simulations. We found that NO cooling rate decreases during the solar eclipse in both SABER measurements and WACCM‐X simulations. The maximum decrease of the NO cooling is 40% in SABER measurements and 25% in WACCM‐X simulations. The NO cooling process is initiated almost entirely through the collisions with atomic oxygen (O) which depends linearly on NO and O densities and non‐linearly on the neutral temperature. During the eclipse, the NO concentration and temperature decreases are larger than that of O concentration. Consequently, the eclipse‐time NO concentration and temperature decreases are the major drivers of the NO cooling rate decrease. The decreases of the temperature and the NO concentration contribute comparably to the eclipse‐time NO cooling rate decrease.

     
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  4. Abstract

    In the mesosphere and lower thermosphere (MLT) region, residual circulations driven by gravity wave breaking and dissipation significantly impact constituent distribution and the height and temperature of the mesopause. The distribution of CO2can be used as a proxy for the residual circulations. Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) CO2volume mixing ratio (VMR) and temperature measurements from 2003 to 2020 are used to study the monthly climatology of MLT residual circulations and the mesopause height. Our analyses show that (a) mesopause height strongly correlates with the CO2VMR vertical gradient during solstices; (b) mesopause height has a discontinuity at midlatitude in the summer hemisphere, with a lower mesopause height at mid‐to‐high latitudes as a result of adiabatic cooling driven by strong adiabatic upwelling; (c) the residual circulations have strong seasonal variations at mid‐to‐high latitudes, but they are more uniform at low latitudes; and (d) the interannual variability of the residual circulations and mesopause height is larger in the Southern Hemisphere (SH; 4–5 km) than in the Northern Hemisphere (NH; 0.5–1 km).

     
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  5. Abstract

    We use Whole Atmosphere Community Climate Model simulations made under various climate change scenarios to study the evolution of the global‐mean temperature trend in the late twentieth century and the twenty‐first century. Results are compared with available satellite observations, including new trend estimates derived from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on NASA's TIMED spacecraft. Modeled and observed trends are shown to be consistent throughout the entire middle atmosphere, from near the tropopause (~16 km) to the lower thermosphere (~95 km) in the period covered by the satellite data. Simulations are extended into the twenty‐first century to document the evolution of the global‐mean temperature trend profile. We find, consistent with previous studies, a marked change in the trend profile at the turn of the twenty‐first century, which is driven by the recovery of stratospheric ozone following the adoption of the Montreal Protocol. In the twenty‐first century, the trend profile becomes more uniform with altitude, but its overall shape and magnitude are conditioned by the scenario adopted for future emissions of greenhouse gases. Our results suggest that the vertical profile of temperature trends in the middle atmosphere will remain an important signature of global climate change, and they underscore the importance of global, continuous monitoring of this region of the atmosphere.

     
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  6. We aim to extract a universal law that governs the gravity wave manifestation in polar mesospheric clouds (PMCs). Gravity wave morphology and the clarity level of display vary throughout the wave population manifested by the PMC albedo data. Higher clarity refers to more distinct exhibition of the features, which often correspond to larger variances and a better-organized nature. A gravity wave tracking algorithm based on the continuous Morlet wavelet transform is applied to the PMC albedo data at 83 km altitude taken by the Aeronomy of Ice in the Mesosphere (AIM) Cloud Imaging and Particle Size (CIPS) instrument to obtain a large ensemble of the gravity wave detections. The horizontal wavelengths in the range of  ∼ 20–60 km are the focus of the study. It shows that the albedo (wave) power statistically increases as the background gets brighter. We resample the wave detections to conform to a normal distribution to examine the wave morphology and display clarity beyond the cloud brightness impact. Sample cases are selected at the two tails and the peak of the normal distribution to represent the full set of wave detections. For these cases the albedo power spectra follow exponential decay toward smaller scales. The high-albedo-power category has the most rapid decay (i.e., exponent  =  −3.2) and corresponds to the most distinct wave display. The wave display becomes increasingly blurrier for the medium- and low-power categories, which hold the monotonically decreasing spectral exponents of −2.9 and −2.5, respectively. The majority of waves are straight waves whose clarity levels can collapse between the different brightness levels, but in the brighter background the wave signatures seem to exhibit mildly turbulent-like behavior. 
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  7. Abstract

    Analyzing Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations from 2003 to 2018, the interannual variability of 2–5d eastward propagating planetary waves is found to correlate positively with zonal‐mean zonal winds averaged over 67.5°±10°S but negatively with the quasi‐biennial oscillation (QBO) index in austral winter. The composite‐mean wave amplitudes are ~20% larger in QBOe than in QBOw. On statistical average, the poleward flank strengthening and the equatorward flank weakening of polar night jet (PNJ) during QBOe form a dipole‐cell pattern. In contrast, only a single negative cell is seen in the Northern Hemisphere zonal‐mean zonal winds (January) previously explained by the Holton‐Tan theory. Such difference implies an interhemispheric asymmetry and other processes needed to explain the additional positive cell in Antarctica. Mechanistic modeling illustrates that the stronger PNJ generates eastward propagating planetary waves with larger growth rates (stronger waves) in QBOe than QBOw, explaining the QBO‐like signal in the Antarctic planetary waves.

     
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  8. Abstract

    In order to understand the characteristics of long‐lasting “C‐type” structure in the Sodium (Na) lidargram, six cases from different observational locations have been analyzed. The Na lidargram, collected from low‐, middle‐, and high‐latitude sites, show long lifetime of the C‐type structures which is believed to be the manifestation of Kelvin‐Helmholtz (KH) billows in the Mesosphere and Lower Thermosphere (MLT) region. In order to explore the characteristics of the long‐lasting C‐type structures, the altitude profile of square of Brunt‐Väisälä frequency in the MLT region has been derived using the temperature profile collected from the Na lidar instruments and the SABER instrument onboard TIMED satellite. It is found to be positive in the C‐type structure region for all the six cases which indicates that the regions are convectively stable. Simultaneous wind measurements, which allowed us to calculate the Richardson numbers and Reynolds numbers for three cases, suggest that the regions where the C‐type structure appeared were dynamically stable and nonturbulent. This paper brings out a hypothesis wherein the low temperature can increase the magnitude of the Prandtl number and convectively stable atmospheric region can cause the magnitude of Reynolds number to decrease. As a consequence, the remnant of previously generated KH billows in nearly “frozen‐in” condition can be advected through this conducive region to a different location by the background wind where they can sustain for a long time without much deformation. These long‐lived KH billows in the MLT region will eventually manifest the long‐lasting C‐type structures in the Na lidargram.

     
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  9. Abstract

    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).

     
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