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Gravity waves are a major source of the middle atmospheric short-term variability. The Hunga Tonga-Hunga Ha‘apai volcanic eruption provided a unique opportunity to study gravity wave propagation around the globe from a well-defined source. The eruption triggered several atmospheric signatures including a lamb wave (troposphere/stratosphere/mesosphere) and a package of gravity waves. Here we present results of gravity wave signatures found in mesospheric winds leveraging multi-static meteor radar networks such as the Nordic Meteor Radar Cluster and CONDOR. We were able to identify the eastward and westward propagating gravity waves. Furthermore, it was possible to estimate the intrinsic wave properties such as a horizontal wavelength of approximately 1600-2000 km and an intrinsic phase speed of 200 m/s.

Sudden stratospheric warmings (SSWs) have significant impacts on the Arctic ozone. In this study, MERRA-2 provides the characteristics of the zonal-mean zonal wind and temperature influenced by the planetary waves during major SSWs. We present an analysis of ozone variations in the stratosphere over Ny-Ålesund, Svalbard (79°N, 12°E) based on the ground-based microwave radiometer GROMOS-C during the major SSW events that occurred from 2015 to 2022. The results are compared with Aura-MLS observations and MERRA-2 simulations. GROMOS-C captures the high variability of stratospheric ozone fluctuations during SSWs at polar latitudes very well. The stratospheric ozone dramatically increases after SSW onset day, which lasts up to two months. The polar vortex is disturbed or weakened by SSW resulting in the meridional transport of ozone from the mid-latitude into the polar regions. Therefore, this study assists in understanding the relationship between the interannual variability of stratospheric ozone and the occurrence of SSWs and has significant implications for stratospheric ozone trends in the northern polar regions.

Atmospheric Gravity Waves (AGWs) forced in the lower atmosphere are known to have a significant impact on the mesosphere and lower thermosphere (MLT) region. In the ionosphere, they can generate Medium-Scale Traveling Ionospheric Disturbances (MSTIDs). These disturbances roughly occur on time scales of 15−80 min and are therefore often parametrized rather than directly resolved in ionosphere models. The energy and momentum transport by AGW-TIDs strongly depends on their wave parameters. Measurements of AGW-TIDs in the MLT region and determination of the wave parameters (vertical and horizontal wavelength, wave period and propagation direction) are therefore an essential step to improve ionosphere modelling. However, measurements that provide a good resolution in the vertical dimension (≲ 10 km) and time (≲ 10 min) as well as a large enough coverage in the horizontal dimension (≳ 300 × 300 km) are difficult at MLT altitudes. We show, that combined measurements of the EISCAT VHF incoherent scatter radar and the Nordic Meteor Radar Cluster allow to determine the wave parameters of AGW-TIDs across the whole MLT region. Fourier filter methods are used to separate wave modes by wavelength, period and propagation direction. The extracted wave modes are fitted with wave functions in time-altitude and horizontal cross sections which gives the wave parameters. The coverage regions of the two applied instruments are separated only by approximately 10 km in altitude, which allows to identify a single wave mode in both measurements. We present the developed techniques on the example of a strongly pronounced AGW-TID measured on July 7, 2020. As a first application, two measurement campaigns have been conducted in early September and mid-October 2022 to study possible changes in AGW-TID parameters due to the MLT fall transition occurring around equinox. Another possible application of our method is to infer thermospheric neutral winds from the observed waves. We demonstrate this process under the assumption of the anelastic dissipative gravity wave dispersion relation.

Continuous temperature measurements in the stratosphere (12-50 km) and the mesosphere (50-80 km) are crucial for the deeperunderstanding of the physical processes in the middle atmosphere and our understanding of the vertical coupling between thedifferent atmospheric layers. Several studies have shown the importance of atmospheric waves such as planetary waves, tides,and gravity waves, their propagation and breaking at these altitudes, and its effect on the global circulation.Investigating these effects requires long-term measurements with high temporal resolution and altitude coverage. Satellite datacovers the required altitude range but provides limited temporal resolution due to its fixed orbital geometry. Active measurementtechniques such as LIDAR are usually limited to nighttime and only a few instruments have daytime capability and thereforeare unsuitable for continuous observations. Ground-based microwave radiometry provides a robust observational method thatis independent of the daytime, almost independent of the weather conditions, and that permits to perform continuous soundingsfrom 20-60 km altitude.TEMPERA (TEMPErature RAdiometer) is a ground-based radiometer developed at the University of Bern in 2013. It measuresmicrowave radiation spectra from atmospheric oxygen in a range between 52 GHz and 53 GHz. Atmospheric temperature profiles can be retrieved from these spectra. In the last 9 years, the accuracy and performance of this instrument were continuouslyimproved. The latest version of TEMPERA has a temporal resolution of one measurement per 30 min and temperature profilescan be retrieved up to an altitude of about 50 km.The reason for the altitude limitation is the Zeeman effect, which occurs due to the interaction of the atmospheric oxygen withthe Earths magnetic field. The polarisation of atmospheric radiation affected by the Zeeman effect depends on the orientationof the propagation direction to the magnetic field. Therefore the altitude range for temperature retrievals could be furtherimproved by decomposing the measured radiation in its polarisation components. In addition, the inclusion of the Zeemaneffect in the retrieval algorithm provides the ability to retrieve the Earths magnetic field from measurements of atmosphericmicrowave emissions.The microwave group from the Institute of Applied Physics of the University of Bern, is currently developing a temperatureradiometer (TEMPERA-C), which is based on the former instrument (TEMPERA), but allows a fully polarymetric analysis ofthe atmospheric emission spectra. In my talk I will present the technical details of TEMPERA-C as for example the challengesin the calibration process. Furthermore I will present calibrated measurements of circular polarized atmospheric emissionspectra as well as temperature retrievals and discuss the effect of the Earth’s magnetic field on these measurements.

This study investigates the response of the semidiurnal tide (SDT) to the 2013 major sudden stratospheric warming (SSW) event using meteor radar wind observations and mechanistic tidal model simulations. In the model, the background atmosphere is constrained to meteorological fields from the Navy Global Environmental Model - High Altitude analysis system. The solar (thermal) and lunar (gravitational) SDT components are forced by incorporating hourly global temperature tendency fields from the ERA5 forecast model, and by specifying the M2 and N2 lunar gravitational potentials, respectively. The simulated SDT response is compared against meteor wind observations from the CMOR (43.3◦N, 80.8◦W), Collm (51.3◦N, 13.0◦E), and Kiruna (67.5◦N, 20.1◦E) radars, showing close agreement with the observed amplitude and phase variability. Numerical experiments investigate the individual roles of the solar and lunar SDT components in shaping the net SDT response. Further experiments isolate the impact of changing propagation conditions through the zonal mean background atmosphere, non-linear wave-wave interactions, and the SSW-induced stratospheric ozone redistribution. Results indicate that between 80-97 km altitude in the northern hemisphere mid-to-high latitudes the net SDT response is driven by the solar SDT component, which itself is shaped by changing propagation conditions through the zonal mean background atmosphere and by non-linear wave-wave interactions. In addition, it is demonstrated that as a result of the rapidly varying solar SDT during the SSW the contribution of the lunar SDT to the total measured tidal field can be significantly overestimated.

Atmospheric Gravity Waves and Traveling Ionospheric Disturbances can be observed in the neutral atmosphere and the ionosphere at a wide range of spatial and temporal scales. Especially at medium scales, these oscillations are often not resolved in general circulation models and are parameterized. We show that ionospheric disturbances forced by upward propagating atmospheric gravity waves can be simultaneously observed with the EISCAT Very High Frequency incoherent scatter radar and the Nordic Meteor Radar Cluster. From combined multi-static measurements, both vertical and horizontal wave parameters can be determined by applying a specially developed Fourier filter analysis method. This method is demonstrated using the example of a strongly pronounced wave mode that occurred during the EISCAT experiment on 7 July 2020. Leveraging the developed technique, we show that the wave characteristics of Traveling Ionospheric Disturbances are notably impacted by the fall transition of the mesosphere/lower thermosphere. We also demonstrate the application of using the determined wave parameters to infer the thermospheric neutral wind velocities. Applying the dissipative anelastic gravity wave dispersion relation, we obtain vertical wind profiles in the lower thermosphere.

We present continuous ozone and water vapor measurements with the two ground-based radiometers GROMOS-C and MIAWARA-C at Ny-Ålesund, Svalbard (79° N, 12° E), that started in September 2015. Leveraging GROMOS-C and MIAWARA-C measurements, MERRA-2, and Aura-MLS datasets, we analyze the long-term behavior and interannual differences of ozone and water vapor and compile climatologies of both trace gases that describe the annual variation of ozone and water vapor at polar latitudes. A climatological comparison of the measurements from our ground-based radiometers with reanalysis and satellite data was performed. Overall differences between GROMOS-C and Aura-MLS ozone climatology are on the order of 10–15 % depending on the altitudes. For the water vapor climatology, MIAWARA-C shows the best agreement with Aura-MLS on average within 5 % throughout the upper stratosphere and mesosphere. The comparison to MERRA-2 yields an agreement that reveals discrepancies larger than 50 % above 0.2 hPa depending on the implemented radiative transfer schemes and other model physics. Furthermore, we perform a conjugate latitude comparison by defining a virtual station in the southern hemisphere at the geographic coordinate (79° S, 12° E) to investigate interhemispheric differences in the atmospheric compositions. Both trace gases show much more pronounced interannual and seasonal variability in the northern hemisphere than in the southern hemisphere. We estimate the effective water vapor transport vertical velocities corresponding to upwelling and downwelling periods driven by the residual circulation. In the northern hemisphere, the water vapor ascent rate is 3.42 ± 1.89 mm s−1 from MIAWARA-C and 4.64 ± 1.83 mm s−1 from Aura-MLS, and the descent rate is 4.98 ± 1.08 mm s−1 from MIAWARA-C and 5.40 ± 1.54 mm s−1 from Aura-MLS. The water vapor ascent and descent rates in the southern hemisphere are 5.22 ± 0.76 mm s−1 and 2.61 ± 1.44 mm s−1 from Aura-MLS, respectively. The water vapor transport vertical velocities analysis further reveals a higher variability in the northern hemisphere and is suitable to monitor and characterize the evolution of the northern and southern polar dynamics linked to the polar vortex as a function of time and altitude.

We present long-term density trends of the Earth's upper atmosphere at altitudes between 71 and 116 km, based on atmospheric occultations of the Crab Nebula observed with X-ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR, and Hitomi. The combination of the five satellites provides a time period of 28 years from 1994 to 2022. To suppress seasonal and latitudinal variations, we concentrate on the data taken in autumn (49 < doy < 111) and spring (235 < doy < 297) in the northern hemisphere with latitudes of 0°–40°. With this constraint, local times are automatically limited either around noon or midnight. We obtain four sets (two seasons × two local times) of density trends at each altitude layer. We take into account variations due to a linear trend and the 11-year solar cycle using linear regression techniques. Because we do not see significant differences among the four trends, we combine them to provide a single vertical profile of trend slopes. We find a negative density trend of roughly −5%/decade at every altitude. This is in reasonable agreement with inferences from settling rate of the upper atmosphere. In the 100–110-km altitude, we found an exceptionally high density decline of about −12%/decade. This peak may be the first observational evidence for strong cooling due to water vapor and ozone near 110 km, which was first identified in a numerical simulation by Akmaev et al. (2006, https://doi.org/10.1016/j.jastp.2006.03.008). Further observations and numerical simulations with suitable input parameters are needed to establish this feature., 地球温暖化に伴う超高層大気の収縮をX線天文衛星で解明 --逆転の発想！捨てられた天体観測データを大気観測に転用--. 京都大学プレスリリース. 2023-02-24.

The Hunga Tonga–Hunga Ha′apai volcano eruption was a unique event that caused many atmospheric phenomena around the globe. In this study, we investigate the atmospheric gravity waves in the mesosphere/lower-thermosphere (MLT) launched by the volcanic explosion in the Pacific, leveraging multistatic meteor radar observations from the Chilean Observation Network De Meteor Radars (CONDOR) and the Nordic Meteor Radar Cluster in Fennoscandia. MLT winds are computed using a recently developed 3DVAR+DIV algorithm. We found eastward- and westward-traveling gravity waves in the CONDOR zonal and meridional wind measurements, which arrived 12 and 48 h after the eruption, and we found one in the Nordic Meteor Radar Cluster that arrived 27.5 h after the volcanic detonation. We obtained observed phase speeds for the eastward great circle path at both locations of about 250 m s−1, and they were 170–150 m s−1 for the opposite propagation direction. The intrinsic phase speed was estimated to be 200–212 m s−1. Furthermore, we identified a potential lamb wave signature in the MLT winds using 5 min resolved 3DVAR+DIV retrievals.

The Hunga Tonga–Hunga Ha′apai volcano eruption was a unique event that caused many atmospheric phenomena around the globe. In this study, we investigate the atmospheric gravity waves in the mesosphere/lower-thermosphere (MLT) launched by the volcanic explosion in the Pacific, leveraging multistatic meteor radar observations from the Chilean Observation Network De Meteor Radars (CONDOR) and the Nordic Meteor Radar Cluster in Fennoscandia. MLT winds are computed using a recently developed 3DVAR+DIV algorithm. We found eastward- and westward-traveling gravity waves in the CONDOR zonal and meridional wind measurements, which arrived 12 and 48 h after the eruption, and we found one in the Nordic Meteor Radar Cluster that arrived 27.5 h after the volcanic detonation. We obtained observed phase speeds for the eastward great circle path at both locations of about 250 m s−1, and they were 170–150 m s−1 for the opposite propagation direction. The intrinsic phase speed was estimated to be 200–212 m s−1. Furthermore, we identified a potential lamb wave signature in the MLT winds using 5 min resolved 3DVAR+DIV retrievals.

Meteor radars have become widely used instruments to study atmospheric dynamics, particularly in the 70 to 110 km altitude region. These systems have been proven to provide reliable and continuous measurements of horizontal winds in the mesosphere and lower thermosphere. Recently, there have been many attempts to utilize specular and/or transverse scatter meteor measurements to estimate vertical winds and vertical wind variability. In this study we investigate potential biases in vertical wind estimation that are intrinsic to the meteor radar observation geometry and scattering mechanism, and we introduce a mathematical debiasing process to mitigate them. This process makes use of a spatiotemporal Laplace filter, which is based on a generalized Tikhonov regularization. Vertical winds obtained from this retrieval algorithm are compared to UA-ICON model data. This comparison reveals good agreement in the statistical moments of the vertical velocity distributions. Furthermore, we present the first observational indications of a forward scatter wind bias. It appears to be caused by the scattering center's apparent motion along the meteor trajectory when the meteoric plasma column is drifted by the wind. The hypothesis is tested by a radiant mapping of two meteor showers. Finally, we introduce a new retrieval algorithm providing a physically and mathematically sound solution to derive vertical winds and wind variability from multistatic meteor radar networks such as the Nordic Meteor Radar Cluster (NORDIC) and the Chilean Observation Network De meteOr Radars (CONDOR). The new retrieval is called 3DVAR+DIV and includes additional diagnostics such as the horizontal divergence and relative vorticity to ensure a physically consistent solution for all 3D winds in spatially resolved domains. Based on this new algorithm we obtained vertical velocities in the range of w = ± 1–2 m s−1 for most of the analyzed data during 2 years of collection, which is consistent with the values reported from general circulation models (GCMs) for this timescale and spatial resolution.

The Draconid meteor shower shows strong bursts of activity at irregular intervals, with nearly no activity in intervening years. Five outbursts of the Draconid meteor shower were observed with specular meteor radars in Canada and Europe between 1999 and 2018. The outbursts generally lasted between 6 and 8 h, and most were not fully visible at a single geographical site, emphasizing the need for observations at multiple longitudes for short-duration shower outbursts. There is at least a factor of two difference in the peak flux as measured on different radars; the initial trail radius effect is undercorrected for Draconid meteors, which are known to be fragile.

Middle atmospheric ozone, water vapour and zonal and meridional wind profiles have been measured with the two ground-based microwave radiometers GROMOS-C and MIAWARA-C. The instruments have been located at the Arctic research base AWIPEV at Ny-Ålesund, Svalbard (79∘ N, 12∘ E), since September 2015. GROMOS-C measures ozone spectra in the four cardinal directions with an elevation angle of 22∘. This means that the probed air masses at an altitude of 3 hPa (37 km) have a horizontal distance of 92 km to Ny-Ålesund. We retrieve four separate ozone profiles along the lines of sight and calculate daily mean horizontal ozone gradients which allow us to investigate the small-scale spatial variability of ozone above Ny-Ålesund. We present the evolution of the ozone gradients at Ny-Ålesund during winter 2018/2019, when a major sudden stratospheric warming (SSW) took place with the central date at 2 January, and link it to the planetary wave activity. We further analyse the SSW and discuss our ozone and water vapour measurements in a global context. At 3 hPa we find a distinct seasonal variation of the ozone gradients. The strong polar vortex during October and March results in a decreasing ozone volume mixing ratio towards the pole. In November the amplitudes of the planetary waves grow until they break in the end of December and an SSW takes place. From November until February ozone increases towards higher latitudes and the magnitude of the ozone gradients is smaller than in October and March. We attribute this to the planetary wave activity of wave numbers 1 and 2 which enabled meridional transport. The MERRA-2 reanalysis and the SD-WACCM model are able to capture the small-scale ozone variability and its seasonal changes.

A newly established multi-static meteor radar network, CONDOR (31.2ºS,70.0ºW), provides the capability to resolve wind and temperature oscillations over a broad range of periods, calculate E-P flux of planetary waves and investigate the short-term variability in the 80-100 km MLT region. In this study we present results of an enhanced westward wavenumber 1 Q6DW activity and its modulation with the amplified diurnal tides and gravity waves (GW) meridional wind variance during a rare minor SH SSW in 2019, using two SH midlatitude meteor radar observations and a recently developed 3DVAR algorithm. This algorithm creates a tomographic reconstruction of the 3D wind field based on optimal estimation technique and Bayesian statistics and is particularly suitable for investigating GW dynamics on regional scales. Furthermore, we present the first results of meteor radar observed Q6DW E-P flux and its comparison with SD-WACCM-X simulated Q6DW E-P flux. The encouraging agreement demonstrated that this SSW-related Q6DW activity had a significant impact on the dynamically coupled MLT region at SH midlatitude.

Multistatic meteor radar observations offer the possibility to investigate the short-term variability at the mesosphere and lower thermosphere on regional scales. Here we present preliminary results of spatially resolved 3D winds and their corresponding horizontal wavelength spectra using the Nordic Meteor Radar Cluster and CONDOR in Chile with a recently developed 3DVAR+div retrieval. The new retrieval provides for the first time a physically consistent solution for the vertical winds that conform the continuity equation. Based on these spectra we can separate the spatial scales that are driven by rotational modes from those dominated by divergent gravity waves. Furthermore, we present the first results of momentum flux spectra derived from these observations on a daily basis.

Data assimilation schemes blend observational data, with limited coverage, with a short term forecast to produce an analysis, which is meant to be the best estimate of the current state of the atmosphere. Appropriately specifying observation error statistics is necessary to obtain an optimal analysis. Observation error can originate from instrument error as well as the error of representation. While representation error is most commonly associated with unresolved scales and processes, this term is often considered to include contributions from pre-processing or quality control and errors associated with the observation operator. With a focus on practical operational implementation, this chapter aims to define the components of observation error, discusses their sources and characteristics, and provides an overview of current methods for estimating observation error statistics. We highlight the implicit assumptions of these methods, as well as their shortcomings. We will detail current operational practice for diagnosing observation error and accounting for correlated observation error. Finally, we provide a practical methodology for using these diagnostics, as well as the associated innovation-based observation impact, to optimize the assimilation of meteor radar observations in the upper atmosphere.

The middle atmospheric circulation is driven by atmospheric waves, which carry energy and momentum from their source to the area of their dissipation and thus providing an energetic coupling between different atmospheric layers. A comprehensive understanding of the wave-wave or wave-mean flow interactions often requires a spatial characterization of these waves. Multistatic meteor radar observations provide an opportunity to investigate the spatial and temporal variability of mesospheric/lower thermospheric winds on regional scales. We apply the 3DVAR+div retrievals to observations from the Nordic Meteor Radar Cluster and the Chilean Observation Network De Meteor Radars (CONDOR). Here we present preliminary results of a new 3DVAR+div retrieval to infer the vertical wind variability using spatially resolved observations. The new retrieval includes the continuity equation in the forward model to ensure physical consistency in the vertical winds. Our preliminary results indicate that the vertical wind variability is about +/-2m/s. The 3DVAR+div algorithm provides spatially resolved winds resolves body forces of breaking gravity waves, which are typically indicated by two counterrotating vortices. Furthermore, we infer horizontal wavelength spectra for all 3 wind components to obtain spectral slopes indicating a transition of the vertical to the divergent mode at scales of about 80-120 km at the mesosphere.

Continuous temperature observations at the stratosphere and lower mesosphere are rare. Radiometry opens the possibility by observing microwave emissions from two oxygen lines to retrieve temperature profiles at all altitudes. In this study, we present observations performed with a temperature radiometer (TEMPERA) at the Meteoswiss station at Payerne for the period from 2014 to 2017. We reanalyzed these observations with a recently developed and improved retrieval algorithm accounting for the Zeeman line splitting in the line center of both oxygen emission lines at 52.5424 and 53.0669 GHz. The new temperature retrievals were validated against MERRA2 reanalysis and the meteorological analysis NAVGEM-HA. The comparison confirmed that the new algorithm yields an increased measurement response up to an altitude of 53–55 km, which extends the altitude coverage by 8–10 km compared to previous retrievals without considering the Zeeman effect. Furthermore, we found correlation coefficients comparing the TEMPERA temperatures with MERRA2 and NAVGEM-HA for monthly mean profiles to be in the range of 0.8–0.96. In addition, mean temperature biases of 1 K and −2 K were found between TEMPERA and both models (MERRA2 and NAVGEM-HA), respectively. We also identified systematic altitude-dependent cold and warm biases compared to both model data sets.

Both high-power large aperture (HPLA) radars and smaller meteor radars readily observe the dense head plasma produced as a meteoroid ablates. However, determining the mass of such meteors based on ...

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.

The Polar Mesospheric Cloud Turbulence (PMC Turbo) experiment was designed to observe and quantify the dynamics of small‐scale gravity waves (GWs) and instabilities leading to turbulence in the upper mesosphere during polar summer using instruments aboard a stratospheric balloon. The PMC Turbo scientific payload comprised seven high‐resolution cameras and a Rayleigh lidar. Overlapping wide and narrow camera field of views from the balloon altitude of ~38 km enabled resolution of features extending from ~20 m to ~100 km at the PMC layer altitude of ~82 km. The Rayleigh lidar provided profiles of temperature below the PMC altitudes and of the PMCs throughout the flight. PMCs were imaged during an ~5.9‐day flight from Esrange, Sweden, to Northern Canada in July 2018. These data reveal sensitivity of the PMCs and the dynamics driving their structure and variability to tropospheric weather and larger‐scale GWs and tides at the PMC altitudes. Initial results reveal strong modulation of PMC presence and brightness by larger‐scale waves, significant variability in the occurrence of GWs and instability dynamics on time scales of hours, and a diversity of small‐scale dynamics leading to instabilities and turbulence at smaller scales. At multiple times, the overall field of view was dominated by extensive and nearly continuous GWs and instabilities at horizontal scales from ~2 to 100 km, suggesting sustained turbulence generation and persistence. At other times, GWs were less pronounced and instabilities were localized and/or weaker, but not absent. An overview of the PMC Turbo experiment motivations, scientific goals, and initial results is presented here.

Ground-based remote sensing of atmospheric parameters is often limited to single station observations of vertical profiles at a certain geographic location. This can be a limiting factor to investigating gravity wave dynamics. In this study we present a new retrieval algorithm for multi-static meteor radar networks to obtain tomographic 3D wind fields within a pre-defined domain area. The algorithm is part of the Agile Software for Gravity wAve Regional Dynamics (ASGARD) called 3DVAR, and based on the optimal estimation technique and Bayesian statistics. The performance of the 3DVAR retrieval is demonstrated using two meteor radar networks, the Nordic Meteor Radar Cluster and the Chilean Observation Network De MeteOr Radars (CONDOR). The optimal estimation implementation provides a statistically sound solution and additional diagnostics from the averaging kernels and measurement response. We present initial scientific results such as body forces of breaking gravity waves leading to two counter-rotating vortices and horizontal wavelength spectra indicating a transition between the vortical κ−3 and divergent κ−5/3 mode at scales of 80–120 km. In addition, we have performed a keogram analysis over extended periods to reflect the latitudinal and temporal impact of a minor sudden stratospheric warming in December 2019. Finally, we demonstrate the applicability of the 3DVAR algorithm to perform large-scale retrievals to derive meteorological wind maps covering a latitude region from Svalbard, north of the European Arctic mainland, to mid-Norway.

There is a growing scientific interest to investigate the forcing from the middle atmosphere dynamics on the thermosphere and ionosphere. This forcing is driven by atmospheric waves at various temporal and spatial scales. In this study, we cross-compare the nudged models Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) and Whole Atmosphere Community Climate Model ExtendedVersion (Specified dynamics) ( WACCM-X(SD)), a free-running version of Upper Atmosphere ICOsahedral Non-hydrostatic (ICON-UA) with six meteor radars located at conjugate polar and mid-latitudes. Mean winds, diurnal and semidiurnal tidal amplitudes and phases were obtained from the radar observations at the mesosphere and lower thermosphere (MLT) and compared to the GAIA, WACCM-X(SD), and ICON-UA data for similar locations applying a harmonized diagnostic.Our results indicate that GAIA zonal and meridional winds show a good agreement to the meteor radars during the winter season on both hemispheres, whereas WACCM-X(SD) and ICON-UA seem to reproduce better the summer zonal wind reversal. However, the mean winds also exhibit some deviation in the seasonal characteristic concerning the meteor radar measurements, which are attributed to the gravity wave parameterizations implemented in the models. All three models tend to reflect the seasonality of diurnal tidal amplitudes, but show some dissimilarities in tidal phases. We also found systematic interhemispheric differences in the seasonal characteristic of semidiurnal amplitudes and phases. The free-running ICON-UA apparently shows most of these interhemispheric differences, whereas WACCM-X(SD) and GAIA trend to have better agreement of the semidiurnal tidal variability in the northern hemisphere.

Long-term and continuous observations of mesospheric/lower thermospheric winds are rare, but they are important to investigate climatological changes at these altitudes on time scales of several years, covering a solar cycle and longer. Such long time series are a natural heritage of the mesosphere/lower thermosphere climate, and they are valuable to compare climate models or long term runs of general circulation models (GCMs). Here we present a climatological comparison of wind observations from six meteor radars at two conjugate latitudes to validate the corresponding mean winds and atmospheric diurnal and semidiurnal tides from three GCMs, namely Ground-to-Topside Model of Atmosphere and Ionosphere for Aeronomy (GAIA), Whole Atmosphere Community Climate Model Extension (Specified Dynamics) (WACCM-X(SD)) and Upper Atmosphere ICOsahedral Non-hydrostatic (UA-ICON) model. Our results indicate that there are interhemispheric differences in the seasonal characteristics of the diurnal and semidiurnal tide. There also are some differences in the mean wind climatologies of the models and the observations. Our results indicate that GAIA shows a reasonable agreement with the meteor radar observations during the winter season, whereas WACCM-X(SD) shows a better agreement with the radars for the hemispheric zonal summer wind reversal, which is more consistent with the meteor radar observations. The free running UA-ICON tends to show similar winds and tides compared to WACCM-X(SD).

In the middle atmosphere, spanning the stratosphere and mesosphere, spring transition is the time period where the zonal circulation reverses from winter westerly to summer easterly which has a strong impact on the vertical wave propagation influencing the tropospheric and ionospheric variability. The spring transition can be rapid in form of a final sudden stratospheric warming (SSW, mainly dynamically driven) or slow (mainly radiatively driven) but also intermediate stages can occur. In most studies spring transitions are classified either by their timing of occurrence or by their vertical structure. However, all these studies focus exclusively on the stratosphere and can give only tendencies under which pre-winter conditions an early or late spring transition takes place and how it takes place. Here we classify the spring transitions regarding their vertical-temporal development beginning in January and spanning the whole middle atmosphere in the core region of the polar vortex. This leads to five classes where the timing of the SSW in the preceding winter and a downward propagating Northern Annular Mode (NAM) plays a crucial role. The results show distinctive differences between the five classes in the months before the spring transition especially in the mesosphere allowing a certain prediction for some of the five spring transition classes which would not be possible considering the stratosphere only.

Observations of upper mesosphere/lower thermosphere (MLT) wind have been performed at Collm (51°N, 13°E) and Kazan (56°N, 49°E), using two SKiYMET all-sky meteor radars with similar configuration. Daily vertical profiles of mean winds and tidal amplitudes have been constructed from hourly horizontal winds. We analyze the response of mean winds and tidal amplitudes to geomagnetic disturbances. To this end we compare winds and amplitudes for very quiet (Ap ≤ 5) and unsettled/disturbed (Ap ≥ 20) geomagnetic conditions. Zonal winds in both the mesosphere and lower thermosphere are weaker during disturbed conditions for both summer and winter. The summer equatorward meridional wind jet is weaker for disturbed geomagnetic conditions. Tendencies over Collm and Kazan for geomagnetic effects on mean winds qualitatively agree during most of the year. For the diurnal tide, amplitudes in summer are smaller in the mesosphere but greater in the lower thermosphere, but no clear tendency is seen for winter. Semidiurnal tidal amplitudes increase during geomagnetic active days in summer and winter. Terdiurnal amplitudes are slightly reduced in the mesosphere during disturbed days, but no clear effect is visible for the lower thermosphere. Overall, while there is a noticeable effect of geomagnetic variability on the mean wind, the effect on tidal amplitudes, except for the semidiurnal tide, is relatively small and partly different over Collm and Kazan.

There is a continuous flux of meteoroids entering the Earth's atmosphere, which are decelerated and heated by collisions with atmospheric molecules, and, depending on the meteoroid kinetic energy, they vaporize and form an ambipolar diffusing plasma trail, which is easily detectable using radar remote sensing. Specular meteor observations are a widely used radar technique to measure winds at the Mesosphere and Lower Thermosphere (MLT). The altitude dependent lifetime (decay time) of the meteor plasma columns provides valuable information about the mean temperature of the atmosphere. Part of the success of these systems is based on the efficient scattering process compared to meteor head echoes.Here we present observations with the Middle Atmosphere Alomar Radar System to detect the faintest observable meteors using the specular geometry, but a focused beam with a beamwidth of 3.6° and the full power of 866kW of the system. We compare our observations to an orbital dynamics model of JFC comets and derive a meteor velocity distribution for the observed population.Further, we performed extensive modeling using a full-wave scattering model based on the model presented in Poulter and Baggaley, 1977. We conducted extensive simulations with the full-wave scattering model to investigate how different plasma distributions would affect the detectability of the meteoric plasma cylinders considering the initial trail radius, diffusion, and electron line density. The obtained reflection coefficients are validated with the triple frequency CMOR (Canadian Meteor Orbit Radar) measurements convolving them with the Fresnel integrals. Our results indicate that the plasma distribution can significantly alter the detectability. Further, the model shows that the observed decay time depends on the polarization of the transmitted wave relative to the meteor trajectory, which also revealed resonance effects for certain critical plasma frequencies.

Investigation of turbulence in the polar mesopause is essential for a better understanding of dynamical or mixing processes in the region. Polar Mesospheric Summer Echoes (PMSEs), occurring at mesopause altitudes during the summer season, are known to be a result of turbulence-induced fluctuations in the refractive index. The presence of ice particles controls and reduce the free-electron diffusivity in D region plasma, which in turn leads to complex, strong radar echoes at very high frequencies.Often, Doppler spectral width of radar measurements are associated with the strength of turbulence in the target area and traditionally used to estimate turbulent kinetic energy dissipation rates, a fundamental parameter of the turbulence processes. Besides the cooling of summer mesopause region induced by GW drag, the turbulence produced by GW breaking contributes to the total energy budget due to release of turbulent kinetic energy to heat. We use PMSE spectral width measurements observed by Middle Atmosphere Alomar Radar System (MAARSY) during summer of 2016 to study their summer temporal mean profiles as well as temporal evolution and connection to the atmospheric turbulence at PMSE altitudes - 80 and 90 km. The current theoretical models suggest that the radar reflectivity should correlate to the strength of the turbulence; however, such a relation is mainly observed for the weaker PMSEs. The mean summer behaviour of estimated turbulent kinetic energy dissipation rates shows an increase from lower altitudes up to 90 km. It should be noticed that spectral width measurements contain additional broadening rather than turbulence, so derived energy dissipation rates are “upper values” than expected from pure turbulence. The results are still slightly lower than those known from climatology obtained from rocket soundings, mostly at altitudes close to the maximum occurrence of PMSE, 86-87 km.We discuss a possible consequence of spectral width measurements under strong PMSEs. In such conditions, the strength of the echo does not correlate with the turbulence intensity, and the observed spectral width is weaker. However, the uniform distribution of spectral width values throughout the echo power is expected from the present theoretical understandings. Based on previous studies, strong PMSEs can also be observed during fossil turbulence. The interpretation of connection the spectral with measurements under fossil turbulence with the turbulence energy dissipation rates and the possibility of using PMSEs for the turbulence studies will be discussed.

Water vapour in the atmosphere is not only a strong greenhouse gas, but also affects many atmospheric processes such as the formation of clouds and precipitation. With increasing temperature, Integrated Water Vapour (IWV) is expected to increase. Analysing how atmospheric water vapour changes in time is therefore important to monitor ongoing climate change. To determine whether IWV increases in Switzerland as expected, we asses IWV trends from a tropospheric water radiometer (TROWARA) in Bern, from a Fourier transform infrared (FTIR) spectrometer at Jungfraujoch and from the Swiss network of ground-based Global Navigation Satellite System (GNSS) stations. In addition, trends are assessed from reanalysis data, using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) and the Modern-Era Retrospecitve Analysis for Research and Applications (MERRA-2). Ground-based GNSS data are well suited for IWV trends due to their high temporal resolution and the spatially dense networks. However, they are highly sensitvie to instrumental changes and care has to be taken when determining GNSS based trends. We therefore use a straightforward trend method to account for jumps in the GNSS data when instrumental changes were performed. Our data show mostly positive IWV trends between 2 and 5% per decade in Switzerland. GNSS trends are significant for some stations and the significance has the tendency to increase with altitude. Further, we found that IWV scales on average to lower tropospheric temperatures as expected, except in winter. However, the correlation between IWV and temperature based on reanalysis data is spatially incoherent. Besides our positive IWV trends, we found a good agreement of radiometer, GNSS and reanalysis data in Bern. Further, we found a dry bias of the FTIR compared to GNSS data at Jungfraujoch, due to the restriction of FTIR to clear-sky conditions. Our results are generally consistent with the positive water vapour feedback in a warming climate. We show that ground-based GNSS networks provide a valuable source for regional climate monitoring with high spatial and temporal resolution, but homogeneously reprocessed data and advanced trend techniques are needed to account for data jumps.

The middle polar atmosphere dynamics is driven by atmospheric waves from the planetary scale to small scale perturbation due to gravity waves. The different atmospheric waves are characterized by their temporal and spatial variability posing challenges to ground-based remote sensing techniques to disentangle and resolve the spatio-temporal ambiguity. Here we present two ground-based remote sensing techniques to resolving spatio-temporal variability at the polar middle atmosphere.Since 2017 the GROMOS-C radiometer measures ozone and winds at NyÅlesund (78.9°N, 11.9°E) on Svalbard. The radiometer employs four beams in the cardinal directions at 22.5° elevation angle to retrieve ozone profiles and winds at altitudes between 30-75 km. the temporal resolution of the ozone retrievals is 30 minutes. Further, we obtain daily mean winds. Due to the high polar latitude the spatial separation between the beams at stratospheric altitudes covers several degrees in longitude to infer spatial gradients in the ozone densities and their perturbation due to planetary waves.Another recently established ground-based remote sensing approach to retrieve the spatial characteristic at the mesosphere and lower thermosphere (MLT) is provided by the Nordic meteor radar cluster consisting of the meteor radars at Tromsø, Alta, Esrange, Sodankylä and on Svalbard. Since October 2019 horizontally resolved winds are obtained using a 3DVAR approach with a temporal resolution of 30 minutes and a vertical resolution of 2 km. Here we present preliminary results to infer horizontal wavelength spectra, the tidal variability as well as gravity activity of the winter season 2019/20.Both datasets are of high value for data assimilation into weather forecast and reanalysis models or for cross-comparisons and validation of meteorological analysis systems (e.g. NAVGEM-HA).