OH Airglow Images Research Articles

A “Boreing” Night of Observations of the Upper Mesosphere and Lower Thermosphere Over the Andes Lidar Observatory

AbstractA very high‐spatial resolution (∼21–23 m pixel at 85 km altitude) OH airglow imager at the Andes Lidar Observatory at Cerro Pachón, Chile observed considerable ducted wave activity on the night of 29–30 October 2016. This instrument was collocated with a Na wind‐temperature lidar that provided data revealing the occurrence of strong ducts. A large field of view OH and greenline airglow imager showed waves present over a vertical extent consistent with the altitudes of the ducting features identified in the lidar profiles. While waves that appeared to be ducted were seen in all imagers throughout the observation interval, the wave train seen in the OH images at earlier times had a distinct leading nonsinusoidal phase followed by several, lower‐amplitude, more sinusoidal phases, suggesting a likely bore. The leading phase exhibited significant dissipation via small‐scale secondary instabilities suggesting vortex rings that progressed rapidly to smaller scales and turbulence (the latter not fully resolved) thereafter. The motions of these small‐scale features were consistent with their location in the duct at or below ∼83–84 km. Bore dissipation caused a momentum flux divergence and a local acceleration of the mean flow within the duct along the direction of the initial bore propagation. A number of these features are consistent with mesospheric bores observed or modeled in previous studies.

Observation of Gravity Wave Vertical Propagation through a Mesospheric Inversion Layer

The impact of a mesospheric temperature inversion on the vertical propagation of gravity waves has been investigated using OH airglow images and ground-based Rayleigh lidar measurements carried out in December 2017 at the Haute-Provence Observatory (OHP, France, 44N). These measurements provide complementary information that allows the vertical propagation of gravity waves to be followed. An intense mesospheric inversion layer (MIL) observed near 60 km of altitude with the lidar disappeared in the middle of the night, offering a unique opportunity to evaluate its impact on gravity wave (GW) propagation observed above the inversion with airglow cameras. With these two instruments, a wave with a 150 min period was observed and was also identified in meteorological analyses. The gravity waves’ potential energy vertical profile clearly shows the GW energy lost below the inversion altitude and a large increase of gravity wave energy above the inversion in OH airglow images with waves exhibiting higher frequency. MILs are known to cause instabilities at its top part, and this is probably the reason for the enhanced gravity waves observed above.

A case study of a ducted gravity wave event over northern Germany using simultaneous airglow imaging and wind-field observations

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 northern Germany. The airglow imaging observations at multiple altitudes in the mesosphere and lower thermosphere region reveal that a prominent upward-propagating wave structure appeared in O(1S) and O2 airglow images. However, the same wave structure was observed to be very faint in OH airglow images, despite OH being usually one of the brightest airglow emissions. In order to investigate this rare phenomenon, the altitude profile of the vertical wavenumber was derived based on colocated meteor radar wind-field and SABER temperature profiles close to the event location. The results indicate the presence of a thermal duct layer in the altitude range of 85–91 km in the southwest region of Kühlungsborn, Germany. Utilizing these instrumental data sets, we present evidence to show how a leaky duct layer partially inhibited the wave progression in the OH airglow emission 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.

Mesospheric Short‐Period Gravity Waves in the Antarctic Peninsula Observed in All‐Sky Airglow Images and Their Possible Source Locations

AbstractThis study presents an analysis of OH airglow images observed from an all‐sky camera (ASC) at King Sejong Station (KSS), Antarctic for the 2012–2016 period. The two‐dimensional power spectra of short‐period gravity waves (<1 hr) as a function of phase velocities are obtained using the M‐transform method that employs the time sequence of ASC images. The amplitudes of the power spectral densities show that the mesospheric wave activity is the largest during winter (May, June, and July) and is the smallest in fall (February, March, and April). Wind‐blocking diagrams are constructed on the same two‐dimensional domain as in the two‐dimensional spectra using horizontal winds obtained from MERRA‐2 reanalysis at z = 0–80 km and from KSS meteor radar data at z = 80–90 km. Climatologically, the spectral regions of slowly propagating gravity waves (<30 m s−1) are overlaid by the wind‐blocking areas, which suggests the filtering of gravity waves with small phase speeds by winds below the upper stratosphere. Eastward propagating gravity waves in winter and intense south‐eastward waves in spring (October) are found to be unfiltered by the stratospheric winds. It is also found from the spectral analysis that these unfiltered gravity waves can originate from the upper stratosphere or the lower mesosphere, and not from the troposphere, which suggests the possibility of ASC observation of the secondary gravity waves generated near the stratopause.

Kelvin‐Helmholtz Billow Interactions and Instabilities in the Mesosphere Over the Andes Lidar Observatory: 1. Observations

AbstractA very high spatial resolution (∼25 m pixel at 90 km altitude) OH airglow imager was installed at the Andes Lidar Observatory on Cerro Pachón, Chile, in February 2016. This instrument was collocated with a Na wind‐temperature lidar. On 1 March 2016, the lidar data showed that the atmosphere was dynamically unstable before 0100 UT and thus conducive to the formation of Kelvin‐Helmholtz instabilities (KHIs). The imager revealed the presence of a KHI and an apparent atmospheric gravity wave (AGW) propagating approximately perpendicular to the plane of primary KHI motions. The AGW appears to have induced modulations of the shear layer leading to misalignments of the emerging KHI billows. These enabled strong KHI billow interactions, as they achieved large amplitudes and a rapid transition to turbulence thereafter. The interactions manifested themselves as vortex tube and knot features that were earlier identified in laboratory studies, as discussed in Thorpe (1987, https://doi.org/10.1029/JC092iC05p05231; 2002, https://doi.org/10.1002/qj.200212858307) and inferred to be widespread in the atmosphere based on features seen in tropospheric clouds but which have never been identified in previous upper atmospheric observations. This study presents the first high‐resolution airglow imaging observation of these KHI interaction dynamics that drive rapid transitions to turbulence and suggest the potential importance of these dynamics in the mesosphere and at other altitudes. A companion paper (Fritts et al., 2020, https://doi.org/10.1029/2020JD033412) modeling these dynamics confirms that the vortex tubes and knots yield more rapid and significantly enhanced turbulence relative to the internal instabilities of individual KHI billows.

First OH Airglow Observation of Mesospheric Gravity Waves Over European Russia Region

AbstractFor the first time, we perform a study of mesospheric gravity waves (GWs) for four different seasons of 1 year in the latitudinal band from 45°N to 75°N using an OH all‐sky airglow imager over Kazan (55.8°N, 49.2°E), Russia, during the period of August 2015 to July 2016. Our observational study fills a huge airglow imaging observation gap in Europe and Russia region. In total, 125 GW events and 28 ripple events were determined by OH airglow images in 98 clear nights. The observed GWs showed a strong preference of propagation toward northeast in all seasons, which was significantly different from airglow imager observations at other latitudes that the propagation directions were seasonal dependent. The middle atmosphere wind field is used to explain the lack of low phase speed GWs since these GWs were falling into the blocking region due to the filtering effects. Deep tropospheric convections derived from the European Centre for Medium‐Range Weather Forecasts reanalysis data are determined near Caucasus Mountains region, which suggests that the convections are the dominant source of the GWs in spring, summer, and autumn seasons. This finding extends our knowledge that convection might also be an important source of GWs in the higher latitudes. In winter the generation mechanism of the GWs are considered to be jet stream systems. In addition, the occurrence frequency of ripple is much lower than other stations. This study provides some constraints on the range of GW parameters in GW parameterization in general circulation models in Europe and Russia region.

Mesospheric front observations by the OH airglow imager carried out at Ferraz Station on King George Island, Antarctic Peninsula, in 2011

Abstract. The main goals of this work are to characterize and investigate the potential wave sources of four mesospheric fronts identified in the hydroxyl near-infrared (OH-NIR) airglow images, obtained with an all-sky airglow imager installed at Comandante Ferraz Antarctic Station (EACF, as per its Portuguese acronym) located on King George Island in the Antarctic Peninsula. We identified and analyzed four mesospheric fronts in 2011 over King George Island. In addition, we investigate the atmospheric background environment between 80 and 100 km altitude and discuss the ducts and propagation conditions for these waves. For that, we used wind data obtained from a meteor radar operated at EACF and temperature data obtained from the TIMED/SABER satellite. The vertical wavenumber squared, m2, was calculated for each of the four waves. Even though no clearly defined duct (indicated by positive values of m2 sandwiched between layers above and below with m2 < 0) was found in any of the events, favorable propagation conditions for horizontal propagation of the fronts were found in three cases. In the fourth case, the wave front did not find any duct support and it appeared to dissipate near the zenith, transferring energy and momentum to the medium and, consequently, accelerating the wind in the wave propagation direction (near to south) above the OH peak (88–92 km). The likely wave sources for these four cases were investigated by using meteorological satellite images and in two cases we could find that strong instabilities were potential sources, i.e., a cyclonic activity and a large convective cloud cell. In the other two cases it was not possible to associate troposphere sources as potential candidates for the generation of such wave fronts observed in the mesosphere and secondary wave sources were attributed to these cases. Keywords. Atmospheric composition and structure (airglow and aurora) – meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)

Characteristics of ripple structures revealed in OH airglow images

AbstractSmall‐scale ripple structures observed in OH airglow images are most likely induced by either dynamic instability due to large wind shear or convective instability due to superadiabatic lapse rate. Using the data set taken in the mesopause region with an OH all‐sky imager at Yucca Ridge Field Station, Colorado (40.7°N, 104.9°W), from September 2003 to December 2005, we study the characteristics and seasonal variations of ripple structures. By analyzing the simultaneous background wind and temperature observed by the nearby sodium temperature/wind lidar at Fort Collins, Colorado (40.6°N, 105°W), and a nearby medium‐frequency radar at Platteville, Colorado (40.2°N, 105.8°W), we are able to statistically study the possible relation between ripples and the background atmosphere conditions. Characteristics and seasonal variations of ripples are presented in detail in this study. The occurrence frequency of ripples exhibits clear seasonal variability, with peak in autumn. The occurrence of ripples shows a local time dependence, which is most likely associated with the solar tides. The lifetime and spatial scale of these ripples are typically 5–20 min and 5–10 km, respectively, and most of the ripples move preferentially either southward or northward. However, more than half of the observed ripples do not advect with background flow; they have higher Richardson numbers than those ripples that advect with background flow. It is possible that they are not instability features but wave structures that are hard to be distinguished from the real instability features.

Statistical analysis of mesospheric gravity waves over King Sejong Station, Antarctica (62.2°S, 58.8°W)

We have investigated the characteristics of mesospheric short period (<1h) gravity waves which were observed with all-sky images of OH Meinel band and OI 557nm airglows over King Sejong Station (KSS) (62.22°S, 58.78°W) during a period of 2008–2015. By applying 2-dimensional FFT to time differenced images, we derived horizontal wavelengths, phase speeds, and propagating directions (188 and 173 quasi-monochromatic waves from OH and OI airglow images, respectively). The majority of the observed waves propagated predominantly westward, implying that eastward waves were filtered out by strong eastward stratospheric winds. In order to obtain the intrinsic properties of the observed waves, we utilized winds simultaneously measured by KSS Meteor Radar and temperatures from Aura Microwave Limb Sounder (MLS). More than half the waves propagated horizontally, as waves were in Doppler duct or evanescent in the vertical direction. This might be due to strong eastward background wind field in the mesosphere over KSS. For freely propagating waves, the vertical wavelengths were in the interquartile range of 9–33km with a median value of 15km. The vertical wavelengths are shorter than those observed at Halley station (76°S, 27°W) where the majority of the observed waves were freely propagating. The difference in the wave propagating characteristics between KSS and Halley station suggests that gravity waves may affect mesospheric dynamics in this part of the Antarctic Peninsula more strongly than over the Antarctic continent. Furthermore, strong wind shear over KSS played an important role in changing the vertical wavenumbers as the waves propagated upward between two airglow layers (87 and 96km).

Characteristics of mesospheric gravity waves over the southeastern Tibetan Plateau region

AbstractThe Tibetan Plateau (TP), known as “Third Pole” of the Earth, has important influences on global climates and local weather. An important objective in present study is to investigate how orographic features of the TP affect the geographical distributions of gravity wave (GW) sources. Three‐year OH airglow images (November 2011 to October 2014) from Qujing (25.6°N, 103.7°E) were used to study the characteristics of GWs over the southeastern TP region. Along with the almost concurrent and collocated meteor radar wind measurements and temperature data from SABER/TIMED satellite, the propagation conditions of three types of GWs (freely propagating, ducted, or evanescent) were estimated. Most of GWs exhibited ducted or evanescent characteristics. Almost all GWs propagate southeastward in winter. The GW propagation directions in winter are significantly different from other airglow imager observations at northern middle latitudes. Wind data and convective precipitation fields from the European Centre for Medium‐Range Weather Forecasts reanalysis data are used to study the sources of GWs on the edge of the TP. Using backward ray‐tracing analysis, we find that most of the mesospheric freely propagating GWs are located in or near the large wind shear intensity region (~10 km–~17 km) on the southeastern edge of the TP in spring and winter. The averaged value of momentum flux is 11.6 ± 5.2 m2/s2 in winter and 7.5 ± 3.1 m2/s2 in summer. This work will provide valuable information for the GW parameterization schemes in general circulation models in TP region.

Gravity wave characteristics in the mesopause region revealed from OH airglow imager observations over Northern Colorado

AbstractUsing 5 years of all‐sky OH airglow imager data over Yucca Ridge Field Station, CO (40.7°N, 104.9°W), from September 2003 to September 2008, we extract and deduce quasi‐monochromatic gravity wave (GW) characteristics in the mesopause region. The intrinsic periods are clustered between approximately 4 and 10 min, and many of them are unstable and evanescent. GW occurrence frequency exhibits a clear semiannual variation with equinoctial minima, which is likely related to the seasonal variation of background wind. The anomalous propagation direction in January 2006, with strong southward before major warming starting in 21 January and weak southward propagation afterward, was most likely affected by stratospheric sudden warming. The momentum fluxes show strongly anticorrelated with the tides, with ~180° out of phase in the zonal component. While in the meridional component, the easterly maximum occurred approximately 2–6 h after maximum easterly tidal wind. However, the anticorrelations are both weakest during the summer. The dissipating and breaking of small‐scale and high‐frequency GW's components could have a potential impact on the general circulation in the mesopause region.

Simultaneous observations of convective gravity waves from a ground‐based airglow imager and the AIRS satellite experiment

We report the first joint observations of convectively generated gravity waves (GWs) using an OH airglow imager in Colorado and the Atmospheric Infrared Sounder (AIRS) onboard the Aqua satellite. Convective GWs, appearing as concentric rings, are observed over the western Great Plain regions of North America in the evening of 3 June 2008 in the airglow images. Inspecting both weather radars and AIRS radiances at 8.1 µm, strong convective clouds are found near the center of the concentric rings. The AIRS data at 4.3 µm show semicircular GWs with horizontal wavelengths of 60–80 km at 0900 UT, whereas the airglow imager observed circular GWs with horizontal wavelengths of ~44 km and airglow emission perturbation of ~6% at the same geographic location at 0910 UT. Large‐scale GWs (horizontal wavelengths greater than 100 km) emanating northwestward can be seen in both AIRS data and airglow images at different times. The imager observed small‐scale ripples associated with unstable concentric GWs in the mesopause in the early evening. Given that the brightness temperature perturbation of the GWs in the AIRS data is about 0.16 K and assuming that the GWs propagate without dissipation from the stratosphere to the upper mesosphere, the expected airglow emission perturbation caused by the GWs would be 4%–17%. Ray tracing simulations are performed to demonstrate that the GWs seen in AIRS and in the imager were likely excited by the same convective system.

Evidence of gravity wave ducting in the mesopause region from airglow network observations

AbstractWe present observational evidence of gravity wave ducting and show, for the first time, that the ducting occurs over large horizontal distances in the mesopause region. An optical network of four all‐sky imagers in Japan identified two‐dimensional patterns of small‐scale gravity wave bands in OH airglow images on 13 June 2004. The wave signatures clearly showed northward propagation of more than 1800 km over a wide range of latitudes. The horizontal wavelength, horizontal phase speed, and wave period were estimated from the airglow data as 33.4 km, 42.8 m s−1, and 13.4 min, respectively. The wave structure lasted for the whole 5‐h airglow observation period. Simultaneous MF radar wind data and TIMED/SABER measurements suggested that the wave was trapped and ducted at the airglow height. The ducting likely contributed to the remarkable coherence of the wave as it propagated northward.

Statistical characteristics of high frequency gravity waves observed by OH airglow imaging from Tirunelveli (8.7°N)

The importance of high frequency gravity waves in the upper mesospheric region is now well recognized. In this study, we examine the characteristics of high frequency gravity waves observed in OH emission by an all-sky airglow imager operated from Tirunelveli (8.7°N, 77.8°E) during 2007. Background wind information was obtained from co-located MF radar from which the intrinsic wave parameters were estimated. The study reveals predominance of meridionally propagating waves. The phase speeds of the waves were found to be relatively higher during equinoxes compared to solstices. Nearly half of the observed waves showed vertically evanescent behavior. We probed the relative importance of various terms in the gravity wave dispersion relation. We found that the contribution from the wind shear and curvature terms are not significant for statistical studies of high frequency wave events. Nevertheless, they may be important for case studies of individual wave events. Moreover, the analysis revealed the importance of wave reflections arising out of oppositely directed background winds for the short horizontal wavelength gravity waves. This could explain the observed predominance in the propagation towards the meridional directions.

Mesospheric concentric gravity waves generated by multiple convective storms over the North American Great Plain

We report on six continuous hours of OH airglow imager observations (at z ∼ 87 km) of convectively generated gravity waves (GWs) near Fort Collins, Colorado, on the evening of 08 September 2005. These GWs appeared as nearly concentric rings, and had epicenters near the locations of deep convection in three thunderstorms in Colorado, Nebraska and South Dakota. Using GOES satellite and weather radar observations, we show that the GWs closely follow the thunderstorms. Using the background wind from a nearby radar, the intrinsic wave parameters and vertical wavelengths are calculated. The temperature perturbations are estimated to be ∼ 1–3% for GWs with horizontal wavelengths λh∼ 20–40 km and horizontal phase speeds ∼40–60 m/s. The horizontal wavelengths of GWs from a convective cluster decreased in time from 30 to 15 km. We employ convective plume and ray‐trace models to simulate the GW‐induced OH intensity perturbations from convective plumes, clusters and complexes. We find that the results using the background model wind (radiosonde/TIME‐GCM) agree well with the late‐time observations, when the images are dominated by southwestward, short‐wavelength, high‐frequency GWs. These late‐time GWs propagate against the background wind, and haveλh ∼ 30–40 km and periods of τ ∼ 20–30 min. The OH intensity perturbations are enhanced because the vertical wavelengths λz increased, increased, and the vertical velocity perturbations w′ decreased (because the GWs were near their reflection levels). We also find that these short‐wavelength GWs were created ∼5 h earlier by an extremely energetic, deep convective plume in South Dakota, thereby showing that small‐scale, convective GWs directly link the troposphere and mesopause region.

Gravity wave characteristics from OH airglow imager over Maui

[1] The gravity wave (GW) characteristics over Maui from 2002 to 2007 are deduced from OH airglow imager. The dominant horizontal wavelength falls between 15 and 30 km. Intrinsic phase speeds are mainly in the range of 50–100 m s−1. Observed phase speeds are mainly in the range of 20–70 m s−1. Intrinsic wave periods are mostly below 30 min with the majority clusters around 5–15 min. For intrinsic wave period larger than 5 min, as wave period increases, the number of wave events decreases. Observed wave periods fall in the range of 8–20 min. The vertical wavelength distribution peaks between 20 and 30 km and falls rapidly as wavelength decreases. GWs propagating against (along) background wind are Doppler shifted to a higher (lower) frequency, larger (smaller) vertical wavelength, and decreased (enhanced) chance of observation. During all seasons Doppler shifting is part of the explanation as to why the majority of observed GWs propagate against the background wind. The preferentially poleward or equatorward propagation of GWs during summer or winter observed by airglow imagers at Maui can be explained by the effects of critical layer filtering, ducting, and Doppler shifting. In March, when critical layer filtering effects are not important, Doppler shifting and ducting play dominant roles in determining preferred zonal propagation direction. In December, ducting and wave source location predominantly determine wave propagation direction. Momentum flux deduced from OH imager is also highly anticorrelated with background wind on both seasonal and diurnal time frames. Momentum flux is highly modulated by tidal wind. Waves with shorter vertical wavelengths have much smaller power in GW spectra. OH imager may capture the majority of momentum flux of high-frequency GW spectra through the airglow layer.

Statistical characteristics of gravity wave activities observed by an OH airglow imager at Xinglong, in northern China

Abstract. An all-sky airglow imager (ASAI) was installed at Xinglong, in northern China (40.2° N, 117.4° E) in November 2009 to study the morphology of atmospheric gravity waves (AGWs) in the mesosphere and lower thermosphere (MLT) region. Using one year of OH airglow imager data from December 2009 to November 2010, the characteristics of short-period AGWs are investigated and a yearlong AGW climatology in northern China is first ever reported. AGW occurrence frequency in summer and winter is higher than that in equinoctial months. Observed bands mainly have horizontal wavelengths from 10 to 35 km, observed periods from 4 to 14 min and observed horizontal phase speeds in the range of 30 to 60 m s−1. Most of the bands propagate in the meridional direction. The propagation directions of the bands show a strong southwestward preference in winter, while almost all bands propagate northeastward in summer. Although the wind filtering in the middle atmosphere may control AGW propagations in the zonal direction, the non-uniform distribution of wave sources in the lower atmosphere may contribute to the anisotropy in the meridional direction in different seasons. Additionally, as an indication of local instability, the characteristics of ripples are also analyzed. It also shows seasonal variations, occurring more often in summer and winter and mainly moving westward in summer and eastward in winter.

Gravity wave amplitudes and momentum fluxes inferred from OH airglow intensities and meteor radar winds during SpreadFEx

Abstract. We show in this report the momentum flux content input in the mesosphere due to relatively fast and small scale gravity waves (GWs) observed through OH airglow images. The acquisition of OH NIR images was carried out in Brazil at Brasilia (14.8° S, 47.6° W) and Cariri (7.4° S, 36.5° W) from September 2005 to November 2005 during the SpreadFEx Campaign. Horizontal wind information from meteor radar was available in Cariri only. Our findings showed strong wave activity in both sites, mainly in Cariri. High wave directionality was also observed in both sites during SpreadFEx, which have been observed by other investigators using different analysis' techniques and different types of data during the campaign. We discuss also the possibility of plasma bubble seeding by gravity waves presenting spatial and temporal scales estimated with our novel analysis technique during the SpreadFEx campaign.

Characteristics of equatorial gravity waves derived from mesospheric airglow imaging observations

Abstract. We present the characteristics of small-scale (&lt;100 km) gravity waves in the equatorial mesopause region derived from OH airglow imaging observations at Kototabang (100.3° E, 0.2° S), Indonesia, from 2002 to 2005. We adopted a method that could automatically detect gravity waves in the airglow images using two-dimensional cross power spectra of gravity waves. The propagation directions of the waves were likely controlled by zonal filtering due to stratospheric mean winds that show a quasi-biennial oscillation (QBO) and the presence of many wave sources in the troposphere.

Atmospheric gravity waves identified by ground-based observations of the intensity and rotational temperature of OH airglow

Spectroscopic observations of OH airglow undertaken on May 2, 2006 at Uji, Japan reveal variations in intensity and rotational temperature related to the passage of an atmospheric gravity wave. The variations exhibit a period of approximately 1 h and magnitudes of 2–6% in intensity and 0.5–2% in rotational temperature. The vertical wavelength and intrinsic frequency of the atmospheric gravity wave were determined from the horizontal wavelength derived by an OH airglow imager, the background horizontal wind velocity obtained by the middle and upper atmosphere (MU) radar, and the dispersion relationship. The observed variations are consistent with the values calculated using the model of Liu, A.Z., Swenson, G.R. [2003. A modeling study of O 2 and OH airglow perturbations induced by atmospheric gravity waves. J. Geophys. Res. 108 (No. D4), 4151. doi:10.1029/2002JD002474].