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Altitude vs. latitude cross-section of the (a) zonal and (b) meridional background wind provide by the Horizontal Wind Model 93 (HWM93) (Hedin et al., 1991). The following inputs were used in the model run: year: 2009, doy: 180 (June 29), latitude: 30.3°S, longitude: 70.7°W, local solar time: 20.2 h (00:00 UT), F107: 150, and Ap: 4.
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Abstract Data of high frequency gravity wave propagation direction from globally distributed stations indicate a meridional preference of mesospheric gravity waves to be globally oriented toward the summer pole. This orientation is opposite to the mean residual circulation (from summer to winter pole) at mesospheric altitudes. We discuss here a num...
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... The complicated residual circulation might not only directly control the gradient of composition by advective processes, but also modulate the dynamics of tidal and GWs in the upper atmosphere as the magnitude of the zonally-averaged meridional wind shown by the TIDI observations is comparable with the mean zonal wind (Forbes & Garrett, 1979;Vargas et al., 2015). The shallow lower-thermospheric circulation is likely to impact the thermosphere and ionosphere in multiple ways. ...
In this study, the mechanism driving the narrow lower‐thermospheric winter‐to‐summer meridional circulation is thoroughly investigated for the first time using the Specified Dynamics configuration runs of the Whole Atmosphere Community Climate Model eXtended (SD‐WACCMX) simulations and the TIMED Doppler Interferometer (TIDI) observations. The mean meridional circulation in the SD‐WACCMX is qualitatively consistent with the TIDI measurements, though the magnitude in the SD‐WACCMX is about 50% weaker. The lower‐thermospheric winter‐to‐summer circulation is mainly driven by the resolved wave forcing, including the tides and internally generated inertia gravity waves (GWs). The momentum forcing from the parameterized sub‐grid scale GWs is not as significant as the resolved wave forcing in driving the lower‐thermospheric meridional circulation. The GW parameterization scheme in the SD‐WACCMX only includes GWs with phase velocities in the range of ±45 m/s, which might result in most of the parameterized sub‐grid GWs dissipating and breaking in the mesosphere and hardly impacting the lower thermosphere. Only including slow GWs in the SD‐WACCMX parameterization could potentially lead to the underestimation of the meridional wind in the model. Analysis also indicates the lower‐thermospheric meridional circulation is stronger in the summer hemisphere, which is attributed to the hemispheric asymmetry in the resolved wave momentum forcing. This study underlines the importance of the whole atmosphere coupling through wave propagation and dissipation. This understanding can guide the model development with an accurate representation of underlying physical processes in the mesosphere and lower thermosphere which drives the lower‐thermospheric circulation as well as the overall dynamics of this region.
... One fundamental aspect of the MLT is its strong response to the forcing due to atmospheric GWs, which results in upwelling in the middle atmosphere over the summertime pole and downwelling over the winter pole (Soloman and Garcia, 1987;Vargas et al., 2015). These adiabatic cooling and heating conditions drive the thermal structure of the atmosphere away from that expected under radiative equilibrium, leading to a global-scale pole-to-pole residual circulation from the summer pole to the winter pole in the MLT (Houghton, 1978;Holton, 1983;Becker, 2012). ...
The mesosphere and lower thermosphere (MLT) is a dynamic layer of the earth's atmosphere. This region marks the interface at which neutral atmosphere dynamics begin to influence the upper atmosphere and ionosphere. However, our understanding of this region and our ability to accurately simulate it in global circulation models (GCMs) is limited by a lack of observations, especially in remote locations. To this end, a meteor radar was deployed from 2016 to 2020 on the remote mountainous island of South Georgia (54∘ S, 36∘ W) in the Southern Ocean. In this study we use these new measurements to characterise the fundamental dynamics of the MLT above South Georgia including large-scale winds, solar tides, planetary waves (PWs), and mesoscale gravity waves (GWs). We first present an improved method for time–height localisation of radar wind measurements and characterise the large-scale MLT winds. We then determine the amplitudes and phases of the diurnal (24 h), semidiurnal (12 h), terdiurnal (8 h), and quardiurnal (6 h) solar tides at this latitude. We find very large amplitudes up to 30 m s−1 for the quasi 2 d PW in summer and, combining our measurements with the meteor SAAMER radar in Argentina, show that the dominant modes of the quasi 5, 10, and 16 d PWs are westward 1 and 2. We investigate and compare wind variance due to both large-scale “resolved” GWs and small-scale “sub-volume” GWs in the MLT and characterise their seasonal cycles. Last, we use our radar observations and satellite temperature observations from the Microwave Limb Sounder to test a climatological simulation of the Whole Atmosphere Community Climate Model (WACCM). We find that WACCM exhibits a summertime mesopause near 80 km altitude that is around 10 K warmer and 10 km lower in altitude than observed. Above 95 km altitude, summertime meridional winds in WACCM reverse to poleward, but this not observed in radar observations in this altitude range. More significantly, we find that wintertime zonal winds between 85 to 105 km altitude are eastward up to 40 m s−1 in radar observations, but in WACCM they are westward up to 20 m s−1. We propose that this large discrepancy may be linked to the impacts of secondary GWs (2GWs) on the residual circulation, which are not included in most global models, including WACCM. These radar measurements can therefore provide vital constraints that can guide the development of GCMs as they extend upwards into this important region of the atmosphere.
... Again, as the bow wave travels eastward over a zonal wind of ∼20 m/s (eastward wind), the intrinsic phase speed is estimated in 84 m/s, and the intrinsic period is 29 min. The bow wave is fast compared with regularly observed waves over ALO that usually show horizontal phase speeds of <60 m/s (e.g., Vargas et al., 2015) but usually with shorter wavelengths. However, the bow wave is still in the gravity wave branch since the estimated sound speed in the MLT for the eclipse night was 276 m/s. ...
This article presents the results of a week of observations around the 2 July 2019, total Chilean eclipse. The eclipse occurred between 19:22 and 21:46 UTC, with complete sun disc obscuration at 20:38–20:40 UTC (16:38–16:40 LT) over the Andes Lidar Observatory (ALO) at (30.3°S, 70.7°W). Observations were carried out using ALO instrumentation with the goal to observe possible eclipse‐induced effects on the mesosphere and lower thermosphere region (MLT; 75–105 km altitude). To complement our data set, we have also utilized TIMED/SABER temperatures and ionosonde electron density measurements taken at the University of La Serena's Juan Soldado Observatory. Observed events include an unusual fast, bow‐shaped gravity wave structure in airglow images, mesosphere temperature mapper brightness as well as in lidar temperature with 150 km horizontal wavelength 24 min observed period, and vertical wavelength of 25 km. Also, a strong zonal wind shear above 100 km in meteor radar scans as well as the occurrence of a sporadic E layer around 100 km from ionosonde measurements. Finally, variations in temperature and density and the presence of a descending sporadic sodium layer near 98 km were seen in lidar data. We discuss the effects of the eclipse in the MLT, which can shed light on a sparse set of measurements during this type of event. Our results point out several effects of eclipse‐associated changes in the atmosphere below and above but not directly within the MLT.
... One fundamental aspect of the MLT is its strong response to the forcing due to atmospheric GWs, which results in upwelling 40 in the middle atmosphere over the summertime pole and downwelling over the winter pole (Soloman and R., 1987;Vargas et al., 2015). These adiabatic cooling and heating conditions drive the thermal structure of the atmosphere away from that expected under radiative equilibrium, leading to a global-scale pole-to-pole residual circulation from the summer pole to the winter pole (Houghton, 1978;Holton, 1983;Becker, 2012). ...
The mesosphere and lower thermosphere (MLT) is a dynamic layer of the earth’s atmosphere. This region marks the interface at which neutral atmosphere dynamics begin to influence the ionosphere and space weather. However, our understanding of this region and our ability to accurately simulate it in global circulation models (GCMs) is limited by a lack of observations, especially in remote locations. To this end, a meteor radar was deployed on the remote mountainous island of South Georgia (54° S, 36° W) in the Southern Ocean from 2016 to 2020. The goal of this study is to use these new measurements to characterise the fundamental dynamics of the MLT above South Georgia including large-scale winds, solar tides, planetary waves (PWs) and mesoscale gravity waves (GWs). We first present an improved method for time-height localisation of radar wind measurements and characterise the large-scale MLT winds. We then explore the amplitudes and phases of the diurnal (24 h), semidiurnal (12 h) and terdiurnal (8 h) solar tides at this latitude. We also explore PW activity and find very large amplitudes up to 30 ms−1 for the quasi-2 day wave in summer and show that the dominant modes of the quasi-5, 10 and 16 day waves are westward W1 and W2. We investigate wind variance due to GWs in the MLT and use a new method to show an east-west tendency of GW variance of up to 20 % during summer and a weaker north-south tendency of 0–5 % during winter. This is contrary to the expected tendency of GW directions in the winter stratosphere below, which is a strong suggestion of secondary GW (2GW) observations in the MLT. Lastly, comparison of radar winds to a climatological Whole Atmosphere Community Climate Model (WACCM) simulation reveals a simulated summertime mesopause and zonal wind shear that occur at altitudes around 10 km lower than observed, and southward winds during winter above 90 km altitude in the model that are not seen in observations. Further, wintertime zonal winds above 85 km altitude are eastward in radar observations but in WACCM they are found to weaken and reverse to westward. Recent studies have linked this discrepancy to the impact of 2GWs on the residual circulation which are not included in WACCM. These measurements therefore provide vital constraints that can guide the development of GCMs as they extend upwards into this important region of the atmosphere.
... The resulting dynamical forcing causes a global-scale circulation within the mesosphere with strong upwelling within the summer polar region and downwelling within the winter polar region (Houghton, 1978;Holton, 1984). Adiabatic cooling and heating connected to this circulation cause thermal conditions within the mesosphere to deviate greatly from radiative equilibrium (Solomon et al., 1987;Vargas et al., 2015). ...
... Also, it is believed 70 % of the momentum is carried by short-period waves (< 1 h) (Vincent, 1984). Estimations of F D (wave drag) in the meridional direction from airglow measurements unveiled accelerations of 3 m s −1 d −1 (Vargas et al., 2015), which is significant given that the meridional wind magnitude is weak (∼ 20 m s −1 or less at midlatitudes), while in the zonal wind the wave F D = 15-60 m s −1 d −1 (Vincent and Fritts, 1987). ...
We describe in this study the analysis of small and large horizontal-scale gravity waves from datasets composed of images from multiple mesospheric airglow emissions as well as multistatic specular meteor radar (MSMR) winds collected in early November 2018, during the SIMONe–2018 (Spread-spectrum Interferometric Multi-static meteor radar Observing Network) campaign. These ground-based measurements are supported by temperature and neutral density profiles from TIMED/SABER (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry) satellite in orbits near Kühlungsborn, northern Germany (54.1∘ N, 11.8∘ E). The scientific goals here include the characterization of gravity waves and their interaction with the mean flow in the mesosphere and lower thermosphere and their relationship to dynamical conditions in the lower and upper atmosphere. We have obtained intrinsic parameters of small- and large-scale gravity waves and characterized their impact in the mesosphere via momentum flux (FM) and momentum flux divergence (FD) estimations. We have verified that a small percentage of the detected wave events is responsible for most of FM measured during the campaign from oscillations seen in the airglow brightness and MSMR winds taken over 45 h during four nights of clear-sky observations. From the analysis of small-scale gravity waves (λh 3 m2 s−2 (11 % of the events) transport 50 % of the total measured FM. Likewise, wave events of FM > 10 m2 s−2 (2 % of the events) transport 20 % of the total. The examination of large-scale waves (λh > 725 km) seen simultaneously in airglow keograms and MSMR winds revealed amplitudes > 35 %, which translates into FM = 21.2–29.6 m2 s−2. In terms of gravity-wave–mean-flow interactions, these large FM waves could cause decelerations of FD = 22–41 m s−1 d−1 (small-scale waves) and FD = 38–43 m s−1 d−1 (large-scale waves) if breaking or dissipating within short distances in the mesosphere and lower thermosphere region.
... It is widely accepted that the gravity waves affect the mesospheric dynamics greatly and have an important impact on the mesospheric circulation (Ma et al., 2018;Vargas et al., 2015). Li et al. (2005) have found that the mean wind acceleration and the propagation of gravity wave were in the same direction, suggesting that the broken gravity waves release momentum flux and generate drag force on the background atmosphere. ...
Based on the long‐term observations of the Wuhan and Beijing Mesosphere‐Stratosphere‐Troposphere (MST) radars, the statistic characteristics of the mesospheric vertical winds at midlatitudes are investigated for the first time. The vertical wind data come from the Wuhan MST radar (29.51°N, 114.13°E) during 2012–2017 and from the Beijing MST radar (39.75°N, 116.96°E) during 2012–2014. The subdaily, seasonal, and annual variations of the mesospheric vertical winds at Wuhan and Beijing are presented. No obvious subdaily and seasonal variations are found and the annual variations of the mean vertical winds are downward in most of time. The mean vertical winds at Wuhan become smaller in summer and turn to upward at some heights, while those of Beijing only become smaller in summer. The difference of summer vertical winds over Wuhan and Beijing is closely related to the gravity wave activity. The Wuhan MST radar is located to the east side of the Qinghai‐Tibet plateau and also in the East Asian monsoon region, thus the mesospheric winds over Wuhan may experience more influence of gravity waves compared with those over Beijing. The possible bias sources in the vertical wind measurement of the two MST radars are carefully analyzed and the presented wind velocities are proved to be credible. The measured velocity magnitude is similar to other Very High Frequency radar observations, but much larger than that of the WACCM simulation. The Stokes drift, multicellular circulation structures, the wave influence on the radar echoes are raised as the potential sources for this discrepancy.
... This behaviour for mesospheric gravity waves is not limited to the two Alpine stations OPN and SBO, but it is well known. For example Tang et al. (2014) and Vargas et al. (2015) compared several airglow observations of many research groups around the globe and find a meridional propagation towards the summer pole for many stations. The zonal component of the eastward propagation during summer and westward propagation during winter is also dominant at many stations. ...
... However, there are alternative hypotheses concerning the seasonal dependency of meridional wave propagation. Vargas et al. (2015) lay out that neither the meridional circulation, which is too weak from their point of view, nor tides can explain this seasonal behaviour on a global scale. They suggest an interaction of the seasonally dependent (and strong) zonal wind with the lower thermosphere duct as described by Walterscheid et al. (1999). ...
Between December 2013 and August 2017 the instrument FAIM (Fast Airglow
IMager) observed the OH airglow emission at two Alpine stations. A year of
measurements was performed at Oberpfaffenhofen, Germany (48.09∘ N,
11.28∘ E) and 2 years at Sonnblick, Austria (47.05∘ N,
12.96∘ E). Both stations are part of the network for the detection
of mesospheric change (NDMC). The temporal resolution is two frames per
second and the field-of-view is 55 km × 60 km and
75 km × 90 km at the OH layer altitude of 87 km with a spatial
resolution of 200 and 280 m per pixel, respectively. This resulted in two
dense data sets allowing precise derivation of horizontal gravity wave
parameters. The analysis is based on a two-dimensional fast Fourier transform
with fully automatic peak extraction. By combining the information of
consecutive images, time-dependent parameters such as the horizontal phase
speed are extracted. The instrument is mainly sensitive to high-frequency
small- and medium-scale gravity waves. A clear seasonal dependency concerning
the meridional propagation direction is found for these waves in summer in
the direction to the summer pole. The zonal direction of propagation is
eastwards in summer and westwards in winter. Investigations of the data set
revealed an intra-diurnal variability, which may be related to tides. The
observed horizontal phase speed and the number of wave events per
observation hour are higher in summer than in winter.
... This behaviour for mesospheric gravity waves is not limited to the two Alpine stations OPN and SBO, but it is well known. For example Tang et al. (2014) and Vargas et al. (2015) compared several airglow observations of many research groups around the globe and find a meridional propagation towards the summer pole for many stations. The zonal component of the eastward propagation during summer and westward propagation during winter is also dominant at many stations. ...
... in Vargas et al. (2015). ...
Between December 2013 and August 2017 the instrument FAIM (Fast Airglow IMager) observed the OH airglow emission at two Alpine stations. One year of measurements was performed at Oberpfaffenhofen, Germany (48.09°N, 11.28°E) and two years at Sonnblick, Austria (47.05°N, 12.96°E). Both stations are part of the Network for the detection of mesospheric change (NDMC). The temporal resolution is two frames per second and the field of view is 55km × 60km and 75km × 90km at the OH layer altitude of 87km with a spatial resolution of 200m and 280m per pixel, respectively. This results in two dense datasets allowing precise derivation of horizontal gravity wave parameters. The analysis is based on a two-dimensional Fast Fourier Transform with fully automatic peak extraction. By combining the information of consecutive images time-dependent parameters such as the horizontal phase speed are extracted. The instrument is mainly sensitive to high-frequency small- and medium-scale gravity waves. A clear seasonal dependency concerning the meridional propagation direction is found for these waves in summer in direction to the summer pole. The zonal direction of propagation is eastwards in summer and westwards in winter. Investigations of the data set revealed an intra-diurnal variability, which may be related to tides. The observed horizontal phase speed and the number of wave events per observation hour are higher in summer than in winter.
... For instance, a wave amplitude decreasing with altitude would indicate dissipation and wave forcing in the region. Wave forcing estimated from the flux divergence measurements represents an important quantity that can be made only by observing the wave perturbation in multiple nightglow layers simultaneously in both of them (e.g., Vargas et al., 2007Vargas et al., , 2009Vargas et al., , 2015. ...
... where D = dū/dt and F = w u are the flux divergence and the momentum flux towards the wave orientation θ =θ ± σ θ , respectively, andρ is the background density. Measurements of D from imagery are achieved only by recording images of two nightglow layers simultaneously as demonstrated in Vargas et al. (2015). A comprehensive modeling of D for several vertical wave scales and dissipation scenarios is found in Vargas et al. (2007). ...
Measurements of dynamic parameters of atmospheric gravity waves, mainly the vertical wavelength, the momentum flux and the momentum flux diver- gence, are affected by large uncertainties crudely documented in the scientific literature. By using methods of error analysis, we have quantified these un- certainties for frequently observed temporal and spatial wave scales. The results show uncertainties of ∼10% , ∼35%, and ∼65%, at least, in the verti- cal wavelength, momentum flux, and flux divergence, respectively. The large uncertainties in the momentum flux and flux divergence are dominated by uncertainties in the Brunt-Va ̈isa ̈l ̈a frequency and in spatial separation of the nightglow layers, respectively. The measured uncertainties in fundamental wave parameters such as the wave amplitude, intrinsic period, horizontal wavelength, and wave orientation are ∼10% or less and estimated directly from our nightglow image data set. Other key environmental quantities such as the scale height and the Brunt-Va ̈isa ̈l ̈a frequency, frequently considered as constants in gravity wave parameter estimations schemes, are actually quite variable, presenting uncertainties of ∼4% and ∼9%, respectively, ac- cording to the several solar activity and seasonal atmosphere scenarios from the NRLMSISE-00 model simulated here.
... O( 1 S) greenline and O 2 atmospheric band observations have also been used to characterize dynamical activity in the upper atmosphere and to determine wave parameters (e.g. Taylor et al., 1995;Schubert et al., 1999;Medeiros et al., 2005;Vargas et al., 2015). Thus, accurate knowledge of the chemical reactions and rate coefficients is essential to obtain atomic oxygen densities, to simulate airglow emission profiles and intensities, and to extract wave information from observations. ...
The branching ratios ε and α in the three-body recombination reaction for O(¹S) greenline and O2(0,0) atmospheric band airglow chemistry represent the fraction of O2 that branch into the b¹∑g⁺ and c¹∑u⁻ electronic states, respectively. In the present work, the empirical values of these branching ratios have been deduced using a numerical optimization approach. They were obtained using the optimization scheme known as the Covariance Matrix Adaptation Evolution Strategy (CMA-ES) with our MACD-00 model and simultaneous volume emission rate (VER) measurements of the O(¹S) greenline and O2(0,0) atmospheric band emissions. The CMA-ES was employed as the optimization algorithm that would match the O(¹S) and O2(0,0) VER profiles simulated by the MACD-00 model to observations made by OXYGEN/S35, S310.10, NASA Flight 4.339, ETON flights P229H and P230H, OASIS, SOAP/WINE, MULTIFOT, and WINDII. We found that most of the values deduced for ε were in the [0.1, 0.3] range, while most of the values of α were in the [0.01, 0.03] range. Excluding the outliers, the average branching ratio values involving the production of O2(b¹∑g⁺) and O2(c¹∑u⁻) were determined to be ε = 0.15 ± 0.02 and α = 0.018 ± 0.004, respectively. Overall, the simulations showed good agreement with the observations albeit with some discrepancies in the peak altitudes and shape of the profiles, possibly due to small perturbations in the observed VER profiles that are not considered in our simulations.
