The Atmospheric Waves Experiment

Lower ionosphere response to external forcing: A brief review

<p>Atmospheric gravity waves transport energy and momentum trough the different atmospheric layers from the troposphere up to the mesosphere and above. On the one hand this transport has influence on atmospheric circulation patterns and drives for example the meridional circulation in the mesosphere. On the other hand the prevailing wind field selectively influences the vertical propagation conditions of gravity waves of different phase speed and horizontal propagation direction.</p><p>The OH-airglow layer at ca. 86 km altitude (upper mesosphere / lower thermosphere, UMLT) is well-suited for the investigation of atmospheric dynamics, allowing continuous observations of the night-sky throughout the year. Especially, atmospheric gravity waves are prominent features in the data of airglow imaging systems. Furthermore, this altitude region is known to be a region where wave breaking occurs quite often making it particular interesting for quantifying the amount of energy and momentum released due to gravity waves.</p><p>Five years of airglow observations with three FAIM (Fast Airglow Imager) systems in and around the Alpine region are analysed regarding high-frequency gravity waves. Prevailing wind fields and tides from meteor radar wind data and ERA5 data are compared with the propagation direction of these waves and show patterns with high correlation. On seasonal timescales, the gravity waves clearly propagate predominantly to the East in summer and to the West in winter regarding the zonal direction. The meridional direction varies between the different years. On diurnal timescales, we find that atmospheric tides significantly impact the main propagation directions of the gravity waves.</p><p>We further present a case study of a stereoscopic reconstruction using two synchronized airglow-imagers with overlapping field-of-views. This allows deriving the wave amplitude and a 3D visualization of gravity wave patterns within the airglow layer.</p><p>This work received funding from the Bavarian State Ministry of the Environment and Consumer Protection.</p>

The first no‐gap OH airglow all‐sky imager network was established in northern China in February 2012. The network is composed of six all‐sky airglow imagers that make observations of OH airglow gravity waves and cover an area of about 2000 km east and west and about 1400 km south and north. An unusual outbreak of Concentric Gravity Wave (CGW) events were observed by the network nearly every night during the first half of August 2013. These events were coincidentally observed by satellite sensors from Fengyun‐2 (FY‐2), Atmospheric Infrared Sounder (AIRS)/Aqua, and Visible Infrared Imaging Radiometer Suite (VIIRS)/Suomi National Polar‐orbiting Partnership (NPP). Combination of the ground imager network with satellites provides multilevel observations of the CGWs from the stratosphere to the mesopause region. In this paper, two representative CGW events in August 2013 are studied in detail: first is the CGW on the night of 13 August 2013, likely launched by a single thunderstorm. The temporal and spatial analyses indicate that the CGW horizontal wavelengths follow freely propagating waves based on a GW dispersion relation within 300 km from the storm center. In contrast, the more distant observed gravity wave field exhibits a smaller horizontal wavelength of ~20 km, and our analysis strongly suggest this wave field represents a ducted wave. A second event, exhibiting multiple CGWs, was induced by two very strong thunderstorms on 9 August 2013. Multiscale waves with horizontal wavelengths ranging from less than 10 km to 200 km were observed.

We studied atmospheric gravity waves (AGWs) and nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) using a three‐dimensional spectral analysis technique for airglow images at wavelengths of 557.7 nm (altitude: 90–100 km for AGWs) and 630.0 nm (200–300 km for MSTIDs), obtained from Athabasca (ATH), Canada (55°N, 247°E, 2005–2017), and Magadan (MGD), Russia (60°N, 151°E, 2008–2017), over 10–13 years. The AGW propagation direction in summer was from northward to northeastward in ATH and northeastward in MGD with phase speeds of 20–60 m/s. In winter at ATH, they are more omnidirectional with weak preference from northward to southwestward with a speed less than 40 m/s, while another weaker power exists from northeastward to southeastward from 70 to 120 m/s. In winter at MGD, there was no dominant direction in the phase‐velocity spectra with spectral power an order smaller than ATH. We suggest that these AGW characteristics were caused by wind filtering and intensity and locations of tropospheric sources. The MSTIDs at ATH propagated westward in spring and winter and eastward in summer and fall. The MSTIDs at MGD propagated northeastward, eastward, and westward in spring, fall, and winter, respectively, with weaker power than that at ATH. The phase speeds are mostly less than 50 m/s except for fall. The propagation direction tends to change from southwest‐westward in the evening to northeast‐eastward after the midnight at ATH and from south‐southwestward in the evening to north‐northeastward after the midnight at MGD. We discuss possible reasons for these MSTID characteristics at high latitudes based on Perkins and E‐F coupling instabilities, high‐latitude plasma convection, and thermospheric neutral winds.

Case study of convective instability observed in airglow images over the Northeast of Brazil

Colder temperatures have been shown to increase hospitalization and mortality rates in adults with chronic obstructive pulmonary disease (COPD) and cardiac disease. Seasonal influences on exacerbation rates in adults with severe COPD but without significant cardiovascular disease are unclear. In addition, regional variations in COPD exacerbations in North America have not yet been explored. In this study, we sought to determine the seasonal and regional variability in exacerbation rates in those with COPD but without significant cardiovascular risk factors. We studied adults without cardiovascular risk factors from STATCOPE (Simvastatin in the Prevention of COPD Exacerbations) and placebo arm of MACRO (Azithromycin for the Prevention of Exacerbations of COPD) studies. Forty-five study sites were divided into climate regions in Canada and the United States; seasons were defined as winter, spring, summer, and fall. The primary outcome was the rate of COPD exacerbation. Secondary outcomes included time to first exacerbation, severity of exacerbations, all-cause mortality, and antibiotic and steroid use. We analyzed 1,175 subjects with a mean age of 63.3 ± 8.6 years, forced expiratory volume in 1 second of 41.5 ± 17.1% predicted, and 53.6 ± 29.4 pack-years of smoking history. The COPD exacerbation rate was higher in winter (0.13 exacerbations/person-month) than in spring, summer, and fall (0.11, 0.079, and 0.10 exacerbations/person-month, respectively) (P < 0.001). Summer had the highest proportion of severe exacerbations (40.5%) compared with spring, fall, and winter (32.6%, 34.7%, and 33.1%, respectively) (P = 0.004). Mortality was highest in spring and winter (34% and 30%, respectively). There was significant regional variability in the time to first exacerbation, with the Southeast and West having longer median times to first exacerbation (350 and 342 d, respectively, compared with 184 d in other regions) (P < 0.001). Significant seasonal and regional variability exist in the rate and severity of exacerbations and overall mortality in adults with COPD without cardiovascular disease.

Retrieval of the three-dimensional wave structure of gravity waves from multi-position airglow measurements

Amplitude growth rate of monochromatic gravity waves were estimated and compared in this study by using multiple instrument measurements carried out in Brazil. Wave dynamic parameters (vertical wavelength, horizontal wavelength, period, and wave amplitude) were obtained from sodium density profiles of lidar observations in São José dos Campos (23°S, 46°W), while multiple airglow layer imaging provided brightness fluctuations amplitudes and parameters of waves over Cachoeira Paulista (23°S, 45°W). Vertical scales of gravity waves in lidar measurements were found to range from 2 to 10 km, and their growth rates are consistent with dissipative waves. These same characteristics are also observed in waves detected through airglow images, but their vertical scales are larger (λz>15 km). Despite the fact that both instruments present complementary information, the general saturated/damped behavior is consistent with diffusive filtering processes imposing limits to the amplitude growth of the observed waves, i.e., the atmospheric diffusivity acts on the wave in order to reduce its amplitude.

We report rare simultaneous observations of columniform sprites and associated gravity waves (GWs) using the Transient Luminous Events (TLEs) camera and All-sky imager at Prayagraj (25.5° N, 81.9° E, geomag. lat. ~ 16.5° N), India. On 30 May 2014, a Mesoscale Convective System generated a group of sprites over the north horizon that reached the upper mesosphere. Just before this event, GWs (period ~ 14 min) were seen in OH broadband airglow (emission peak ~ 87 km) imaging that propagated in the direction of the sprite occurrence and dissipated in the background atmosphere thereby generating turbulence. About 9–14 min after the sprite event, another set of GWs (period ~ 11 min) was observed in OH imaging that arrived from the direction of the TLEs. At this site, we also record Very Low Frequency navigational transmitter signal JJI (22.2 kHz) from Japan. The amplitude of the JJI signal showed the presence of GWs with ~ 12.2 min periodicities and ~ 18 min period. The GWs of similar features were observed in the ionospheric Total Electron Content variations recorded at a nearby GPS site. The results presented here are important to understand the physical coupling of the troposphere with the lower and upper ionosphere through GWs.

The upper mesospheric neutral winds and temperatures have been derived from continuous meteor radar (MR) measurements over Sodankyla, Finland, in 2008–2014. Under conditions of low solar activity pronounced sudden mesospheric coolings linked to the major stratospheric warming (SSW) in 2009 and a medium SSW in 2010 are observed while there is no observed thermal signature of the major SSW in 2013 occurred during the solar maximum. Mesosphere‐ionosphere anomalies observed simultaneously by the MR, the Aura satellite, and the rapid‐run ionosonde during a period of major SSW include the following features. The mesospheric temperature minimum occurs 1 day ahead of the stratospheric maximum, and the mesospheric cooling is almost of the same value as the stratospheric warming (~50 K), the former decay faster than the latter. In the course of SSW, a strong mesospheric wind shear of ~70 m/s/km occurs. The wind turns clockwise (anticlockwise) from north‐eastward (south‐eastward) to south‐westward (north‐westward) above (below) 90 km. As the mesospheric temperature reaches its minimum, the gravity waves (GW) in the ionosphere with periods of 10–60 min decay abruptly while the GWs with longer periods are not affected. The effect is explained by selective filtering and/or increased turbulence near the mesopause.

Grease ice is an agglomeration of disk-shaped ice crystals, named frazil ice, which forms in turbulent waters of the Polar Oceans and in rivers as well. It has been recognized that the property of grease ice that it damps surface gravity waves could be explained in terms of the effective viscosity of the ice slurry. This paper is devoted to the study of the dynamics of a suspension of disk-shaped particles in a gravity wave field. For dilute suspensions, depending on the strength and frequency of the external wave flow, two orientation regimes of the particles are predicted: a preferential orientation regime with the particles rotating in coherent fashion with the wave field, and a random orientation regime in which the particles oscillate around their initial orientation while diffusing under the effect of Brownian motion. For both motion regimes, the effective viscosity has been derived as a function of the wave frequency, wave amplitude and aspect ratio of the particles. Model predictions have been compared to wave attenuation data in frazil ice layers grown in wave tanks.

It is pointed out that observations of periodic nightglow structures give excellent information on atmospheric gravity waves in the mesosphere and lower thermosphere. The periods, the horizontal wavelengths and the phase speeds of the waves can be determined from airglow images and, using several cameras, the approximate altitude of the luminous layer can also be determined by triangulation. In this paper the possibility of applying tomographic methods for reconstructing the airglow structures is investigated using numerical simulations. A ground-based chain of cameras is assumed, two-dimensional airglow models in the vertical plane above the chain are constructed, and simulated data are calculated by integrating the models along a great number of rays with different elevation angles for each camera. After addition of random noise, these data are then inverted to obtain reconstructions of the models. A tomographic analysis package originally designed for satellite radiotomography is used in the inversion. The package is based on a formulation of stochastic inversion which allows the input of a priori information to the solver in terms of regularization variances. The reconstruction is carried out in two stages. In the first inversion, constant regularization variances are used within a wide altitude range. The results are used in determining the approximate altitude range of the airglow structures. Then, in the second inversion, constant non-zero regularization variances are used inside this region and zero variances outside it. With this method reliable reconstructions of the models are obtained. The number of cameras as well as their separations are varied in order to find out the limitations of the method.Key words. Tomography · Airglow · Mesopause · Gravity waves

Airglow imager and dynasonde/IDI radar wind measurements at Halley Station, Antarctica (76°S, 27°W) have been used to estimate the diurnal variation of the vertical fluxes of horizontal momentum carried by high‐frequency atmospheric gravity waves. The cross‐correlation coefficients between the vertical and horizontal wind perturbations were calculated from the sodium airglow imager data collected during four consecutive nights of near total darkness during July of 2000. These were combined with wind‐velocity variances from coincident radar measurements to estimate the upper limit of the vertical flux of horizontal momentum during three‐hour intervals throughout the period. The resulting momentum flux showed a marked semi‐diurnal oscillation in the zonal and meridional components. Calculations of the momentum flux through the Na airglow show variations in period and phase consistent with the observations, implying that tidal propagation and modulated gravity‐wave forcing may both affect observed wind variations.

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.

Airglow imaging instrumentation has been developed to provide quality imagery of airglow in the visible and near IR wavelengths. The lower thermosphere airglow layers emit between 85 and 102 km altitude. The layers are structured with nonuniformity in the horizontal dimension as a result of atmospheric gravity waves (AGWs) passing through the layer and disturbing the nominal recombination processes producing intensity and temperature modulations. Imagers have been developed to measure the AGW-produced airglow nonuniformities. The instrumentation combines large format, low noise CCDs with large aperture optics for improved S/N images. In particular, the large dynamic range of the detectors provides information from the low intensity zenith sky and the bright, van Rhijn enhanced horizon simultaneously in all-sky fields. The imagers have been used effectively to identify AGW structure from a number of ground based facilities as well as a recent NSF sponsored aircraft campaign. Imagery from the OH Meinel bands and OI 5577 angstrom are presented. Discussions are also presented regarding Na 5896 angstrom, and O<SUB>2</SUB> atmospheric (0,1) band at 8650 angstrom emissions.