Search

Specular meteor radars (SMRs) and partial reflection radars (PRRs) have been observing mesospheric winds for more than a solar cycle over Germany (~54 °N) and northern Norway (~69 °N). This work investigates the mesospheric mean zonal wind and the zonal mean geostrophic zonal wind from the Microwave Limb Sounder (MLS) over these two regions between 2004 and 2020. Our study focuses on the summer when strong planetary waves are absent and the stratospheric and tropospheric conditions are relatively stable. We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower thermosphere summer length (MLT-SL) using SMR and PRR winds, and (2) the mesosphere summer length (M-SL) using PRR and MLS. Under both definitions, the summer begins around April and ends around mid-September. The largest year to year variability is found in the summer beginning in both definitions, particularly at high-latitudes, possibly due to the influence of the polar vortex. At high-latitudes, the year 2004 has a longer summer length compared to the mean value for MLT-SL, as well as 2012 for both definitions. The M-SL exhibits an increasing trend over the years, while MLT-SL does not have a well-defined trend. We explore a possible influence of solar activity, as well as large-scale atmospheric influences (e.g. quasi-biennial oscillations (QBO), El Niño-southern oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at mid-latitudes using only PRR winds. In this case, the summer length shows a breakpoint, suggesting a non-uniform trend, and periods similar to those known for ENSO and QBO.

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.

In the extratropical atmosphere, Rossby waves (RWs) and internal gravity waves (GWs) propagating from the troposphere mediate a coupling with the middle atmosphere by influencing the dynamics herein. In the current generation chemistry-climate models (CCMs), RW effects are well resolved while GW effects have to be parameterized. Here, we analyze orographic GW (OGW) interaction with resolved dynamics in a comprehensive CCM on the time scale of days. For this, we apply a recently developed method of strong OGW drag event composites for the three strongest northern hemisphere OGW hotspots. We show that locally-strong OGW events considerably alter the properties of resolved wave propagation into the middle atmosphere, which subsequently influences zonal winds and RW transience. Our results demonstrate that the influence of OGWs is critically dependent on the hotspot region, which underlines the OGW-resolved dynamics interaction being a two-way process.

The thermosphere–ionosphere system shows high complexity due to its interaction with the continuously varying solar radiation flux. We investigate the temporal and spatial response of the ionosphere to solar activity using 18 years (1999–2017) of total electron content (TEC) maps provided by the international global navigation satellite systems service and 12 solar proxies (F10.7, F1.8, F3.2, F8, F15, F30, He II, Mg II index, Ly-α, Ca II K, daily sunspot area (SSA), and sunspot number (SSN)). Cross-wavelet and Lomb–Scargle periodogram (LSP) analyses are used to evaluate the different solar proxies with respect to their impact on the global mean TEC (GTEC), which is important for improved ionosphere modeling and forecasts. A 16 to 32 d periodicity in all the solar proxies and GTEC has been identified. The maximum correlation at this timescale is observed between the He II, Mg II, and F30 indices and GTEC, with an effective time delay of about 1 d. The LSP analysis shows that the most dominant period is 27 d, which is owing to the mean solar rotation, followed by a 45 d periodicity. In addition, a semi-annual and an annual variation were observed in GTEC, with the strongest correlation near the equatorial region where a time delay of about 1–2 d exists. The wavelet variance estimation method is used to find the variance of GTEC and F10.7 during the maxima of the solar cycles SC 23 and SC 24. Wavelet variance estimation suggests that the GTEC variance is highest for the seasonal timescale (32 to 64 d period) followed by the 16 to 32 d period, similar to the F10.7 index. The variance during SC 23 is larger than during SC 24. The most suitable proxy to represent solar activity at the timescales of 16 to 32 d and 32 to 64 d is He II. The Mg II index, Ly-α, and F30 may be placed second as these indices show the strongest correlation with GTEC, but there are some differences in correlation during solar maximum and minimum years, as the behavior of proxies is not always the same. The indices F1.8 and daily SSA are of limited use to represent the solar impact on GTEC. The empirical orthogonal function (EOF) analysis of the TEC data shows that the first EOF component captures more than 86 % of the variance, and the first three EOF components explain 99 % of the total variance. EOF analysis suggests that the first component is associated with the solar flux and the third EOF component captures the geomagnetic activity as well as the remaining part of EOF1. The EOF2 captures 11 % of the total variability and demonstrates the hemispheric asymmetry.

We investigate the forcing mechanisms of the terdiurnal solar tide in the middle atmosphere using a mechanistic global circulation model. In order to quantify their individual contributions, we perform several model experiments and separate each forcing mechanism by switching off the remaining sources. We find that the primary excitation is owing to the terdiurnal component of solar radiation absorption in the troposphere and stratosphere. Secondary sources are nonlinear tide–tide interactions and gravity wave–tide interactions. Thus, although the solar heating clearly dominates the terdiurnal forcing in our simulations, we find that nonlinear tidal and gravity wave interactions contribute in certain seasons and at certain altitudes. By slightly enhancing the different excitation sources, we test the sensitivity of the background circulation to these changes of the dynamics. As a result, the increase of terdiurnal gravity wave drag can strongly affect the middle and upper atmosphere dynamics, including an irregular change of the terdiurnal amplitude, a weakening of neutral winds in the thermosphere, and a significant temperature change in the thermosphere, depending on the strength of the forcing. On the contrary, the influence of nonlinear tidal interactions on the middle atmosphere background dynamics is rather small.

We analyze tidal (diurnal, semidiurnal, terdiurnal, quarterdiurnal) phases and related wind shear in the mesosphere/lower thermosphere as observed by meteor radar over Collm (51.3∘ N, 13.0∘ E). The wind shear phases are compared with those of sporadic E (Es) occurrence rates, which were derived from GPS radio occultation signal-to-noise ratio (SNR) profiles measured by the COSMIC/FORMOSAT-3 satellites. At middle latitudes Es are mainly produced by wind shear, which, in the presence of a horizontal component of the Earth's magnetic field, leads to ion convergence in the region where the wind shear is negative. Consequently, we find good correspondence between radar derived wind shear and Es phases for the semidiurnal, terdiurnal, and quarterdiurnal tidal components. The diurnal tidal wind shear, however, does not correspond to the Es diurnal signal.

The mid-latitude mesosphere and lower thermosphere (MLT) wind speeds measured by two SKiYMET meteor radars (MRs) at Collm (51°N, 13°E) and Kazan (56°N, 49°E) during 2016–2017 were analyzed to study longitudinal wind structures. The differences between monthly mean prevailing wind speeds and tidal amplitudes were compared with the corresponding differences obtained from TIMED/TIDI satellite winds and gradient wind speeds from the AURA/MLS instrument. It is shown that the MR wind difference between the two sites is statistically significant. The difference of the horizontal prevailing winds can be explained by a superposition of the background zonal flow, which is different at the two latitudes, with stationary planetary waves of different origin. Non-migrating tides contribute significantly to the difference of the semidiurnal tidal winds between the two sites.

The GPS radio occultation (RO) technique is used to study sporadic E (Es) layer plasma irregularities of the Earth's ionosphere on a global scale using GPS signal-to-noise ratio (SNR) profiles from the COSMIC/FORMOSAT-3 satellite. The maximum deviation from the mean SNR can be attributed to the height of the Es layer. Es are generally accepted to be produced by ion convergence due to vertical wind shear in the presence of a horizontal component of the Earth's magnetic field, while the wind shear is provided mainly by the solar tides. Here we present analyses of quarterdiurnal tide (QDT) signatures in Es occurrence rates. From a local comparison with mesosphere/lower thermosphere wind shear obtained with a meteor radar at Collm (51.3∘ N, 13.0∘ E), we find that the phases of the QDT in Es agree well with those of negative vertical shear of the zonal wind for all seasons except for summer, when the QDT amplitudes are small. We also compare the global QDT Es signal with numerical model results. The global distribution of the Es occurrence rates qualitatively agrees with the modeled zonal wind shears. The results indicate that zonal wind shear is indeed an important driving mechanism for the QDT seen in Es.

Objectives Children after liver transplantation show increased rates of impaired cognitive functioning. We aimed to assess the potential effects of immunosuppressive therapy on executive functioning measured by the Children's Color Trail Test and the cognitive functioning module of the PedsQL (cogPedsQL) in liver transplanted children to explore potential targets for intervention to improve executive functioning. Methods We performed a cross-sectional study in 155 children (78 girls) aged 10.4 (2-18) years at 5.0 (0.1-17) years after liver transplantation, with follow-up at 6 months in n = 114. Executive functioning was assessed by Children's Color Trail Test (ages 8-16) and by patients and parent-proxy cogPedsQL (ages 5-18/2-18, respectively). Results were correlated with clinical parameters. Stability of results over time was compared between n = 23 patients who for clinical reasons switched from twice daily calcineurin inhibitor (CNI) to once-daily slow-release tacrolimus (Tac) during the study period, and patients with unchanged CNI. Results Worse executive functioning was associated with longer stay in the intensive care unit and longer time elapsed since transplantation. No difference was found between users of cyclosporine and Tac. Children on once-daily slow-release Tac performed better than children on twice-daily Tac. In children who switched from twice-daily CNI to once-daily Tac, parent-proxy cogPedsQL improved significantly compared to stable results in the nonswitch group. Conclusions In addition to a strong impact of disease burden around transplantation, executive functioning appears to deteriorate over time. Although there is no clear-cut advantage of any CNI, once-daily Tac appears to be advantageous compared to twice-daily Tac.

Mesospheric winds from two longitudinal sectors at 53°N latitude are combined to investigate quasi‐two‐day waves (Q2DWs) and their nonlinear interactions with tides. In a summer 2019 case study, we diagnose the zonal wavenumber m of spectral peaks at expected frequencies through two dual‐station approaches, a phase differencing technique (PDT) on individual spectral peaks and a least squares procedure on family batched peaks. Consistent results from the approaches verify the occurrences of Rossby‐gravity modes (m = 3 and 4 at periods T = 2.1 and 1.7 days), and their secondary waves (SWs) generated from interactions with diurnal, semi‐diurnal, ter‐diurnal, and quatra‐diurnal migrating tides. We further extend the PDT to 2012–2019, illustrating that Q2DWs exhibit significant interannual variability. Composite analysis reveals seasonal and altitude variations of the Rossby‐gravity modes and their SWs. The Rossby‐gravity modes maximize in local summer, whereas their 16‐ and 9.6‐h SWs appear more in winter., Plain Language Summary: The quasi‐two‐day wave is the strongest and most widely studied planetary wave occurring in the mesosphere. Existing observational analyses are based on either single‐satellite or ‐station approaches, which suffer from temporal and spatial aliasing, respectively. The current study implements and develops dual‐station approaches to investigate the mesospheric quasi‐two‐day wave at 53°N latitude, in a case and a statistical study. Our approaches allow diagnosing both the frequency and zonal wavenumber. In the case study, we diagnosed two Rossby‐gravity modes and the secondary waves (SWs) of the nonlinear interactions between the Rossby‐gravity modes and the migrating tides at periods of 24, 12, 8, and 6 h. While the interactions with the 24‐ and 12‐h tides are expected, those with the 8‐ and 6‐h tides are reported for the first time. In the statistical study, we report the seasonality and altitude variation of the Rossby‐gravity modes and their most dominant SWs., Key Points: Multi‐station approaches are developed and applied to diagnose zonal wavenumber m of near‐2‐day, ‐16‐h, ‐9.6‐h, and ‐6.9‐h spectral peaks. Diagnosed are Rossby‐gravity modes with m = 3 and 4, and their secondary waves from nonlinear interactions with 24‐, 12‐, 8‐, and 6‐h migrating tides. Seasonally, the most dominant near‐2‐day, ‐16‐h, ‐9.6‐h waves occur in summer, winter, and winter, respectively.

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.

Mesospheric winds collected by multiple meteor radars at mid-latitudes in the northern hemispheric are combined to investigate wave activities in June—October 2019. Dual-station approaches are developed and implemented to diagnose zonal wavenumber $m$ of spectral peaks. In September—October, diagnosed are quasi‐10‐ and 6‐day planetary waves (Q10DW and Q6DW, $m=$1), solar semi-diurnal tides with $m=$1, 2, 3 (SW1, SW2, and SW3), lunar semi-diurnal tide, and the upper and lower sidebands (USB and LSB, $m=$ 1 and 3) of Q10DW‐SW2 nonlinear interactions. During June— September, diagnosed are Rossby-gravity modes ($m=$3 and 4 at periods $T=$ 2.1d and 1.7d), and their USBs and LSBs generated from interactions with diurnal, semi-diurnal, ter-diurnal, and quatra-diurnal migrating tides. These results demonstrate that the planetary wave-tide nonlinear interactions significantly increase the variety of waves in the mesosphere and lower thermosphere region (MLT).

The ionospheric total electron content (TEC) provided by the International GNSS Service (IGS) and the TEC simulated by the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model have been used to investigate the delayed ionospheric response against solar flux and its trend during the years 2011 to 2013. The analysis of the distinct low-latitude and midlatitude TEC response over 15∘ E shows a better correlation of observed TEC and the solar radio flux index F10.7 in the Southern Hemisphere compared to the Northern Hemisphere. Thus, a significant hemispheric asymmetry is observed. The ionospheric delay estimated using model-simulated TEC is in good agreement with the delay estimated for observed TEC against the flux measured by the Solar Dynamics Observatory (SDO) extreme ultraviolet (EUV) Variability Experiment (EVE). The average delay for the observed (modeled) TEC is 17(16) h. The average delay calculated for observed and modeled TEC is 1 and 2 h longer in the Southern Hemisphere compared to the Northern Hemisphere. Furthermore, the observed TEC is compared with the modeled TEC simulated using the SOLAR2000 and EUVAC flux models within CTIPe over northern and southern hemispheric grid points. The analysis suggests that TEC simulated using the SOLAR2000 flux model overestimates the observed TEC, which is not the case when using the EUVAC flux model.

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).

A new method is presented to derive spectrally resolved global and local annual changes in the Earth Energy Imbalance (ΔEEI(λ, Δλ)) from measurements of Total and Spectral Solar Irradiance (TSI and SSI) and Total Outgoing Radiation (TOR) and the Spectral Outgoing Radiation (SOR) of the Earth. Since TSI space radiometers provide data with a long-term absolute accuracy -2, the Sun should be used as a TSI referenced radiation source to obtain SSI data using the method of the Solar Auto-Calibrating XUV-IR Spectrometer (SOLACER). By repeatedly calibrating the solar and Earth observation instruments, the degradation should be compensated to accurately determine the outgoing flux Φ(λ, Δλ) entering the instrument. If the instruments on a pointing device are moved within the Angular Range of Sensitivity (ARS) in two angular dimensions through the solar disk, the instruments are also regularly calibrated with regard to their dependence of the angular sensitivity. ARS is independent of the environmental conditions. To improve the accuracy of SOR data, a normalization factor Ωa / ARS is used to extend the annual averaged outgoing flux data Φ(λ, Δλ)a to the SOR(λ, Δλ)a. The strength of the method is demonstrated by describing space-evaluated instruments to be adapted for solar and/or Earth observation from a small satellite. In the spectral range from 120 nm to 3000 nm, spectrometers and highly sensitive photometers with signal-to-noise ratios >1:107 are described to generate data records with high statistical accuracy. Given the compactness of the instruments, more than 20 different data sets should be compiled to complement, verify each other and improve accuracy.

The quasi-two-day wave (Q2DW) is the strongest and most widely-studied planetary wave occurring in the mesosphere. Existing observational analyses are based on either single-satellite or -station approaches, which suffer from temporal and spatial aliasing, respectively. The current work implements and develops dual-station approaches to investigate the mesospheric Q2DWs and their nonlinear interactions with tides using winds from two longitudinal sectors at 53°N latitude. An 8-year composite analysis reveals seasonal and altitude variations of Q2DWs and their secondary waves (SWs) from nonlinear interactions with tides. The Q2DWs maximize in local summer, whereas their 16hr and 9.6hr SWs appear more in winter.

The loss of Arctic sea ice as a consequence of global warming is changing the forcing of the atmospheric large-scale circulation. Areas not covered with sea ice anymore may act as an additional heat source. Associated changes in Rossby wave propagation can initiate tropospheric and stratospheric pathways of Arctic - Mid-latitude linkages. These pathways have the potential to impact on the large-scale energy transport into the Arctic. On the other hand, studies show that the large-scale circulation contributes to Arctic warming by poleward transport of moist static energy. This presentation shows results from research within the Transregional Collaborative Research Center “ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)3” funded by the Deutsche Forschungsgemeinschaft. Using the ERA interim and ERA5 reanalyses the meridional moist static energy transport during high ice and low ice periods is compared. The investigation discriminates between contributions from planetary and synoptic scale. Special emphasis is put on the seasonality of the modulations of the large-scale energy transport.

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.

Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension simulations are used to investigate the solar migrating semidiurnal tide (SW2) around September equinox at middle to high latitudes in the Northern Hemisphere. A pronounced minimum in SW2 occurs around September equinox, and is characterized by a ∼50% reduction in tidal amplitudes for 20–30 days. Analysis of the simulation results indicates that the SW2 minimum occurs due to the seasonal transition of the zonal mean zonal winds, which alter the generation and propagation of different symmetric and antisymmetric modes of SW2. In particular, the antisymmetric modes notably decrease due to the more hemispherically symmetric zonal winds around equinox. It is further demonstrated that interannual variability in the timing of the SW2 minimum is related to the timing of the seasonal transition of the zonal mean zonal winds in the middle atmosphere. This leads to an earlier occurrence of the SW2 minimum during years when the seasonal transition occurs earlier, such as the recent 2019 September equinox which saw an earlier transition of the Southern Hemisphere zonal mean zonal winds following the occurrence of a sudden stratosphere warming. The connection between the timing of the SW2 minimum in the Northern Hemisphere and the timing of the seasonal transition in the middle atmosphere winds is confirmed by seasonal variability of 12‐h tides deduced from specular meteor radar observations at middle to high latitudes in the Northern Hemisphere.

We analyze sporadic E (ES) occurrence rates (OR) obtained from ionospheric radio occultation measurements by the FORMOSAT-3/COSMIC constellation. Maximum OR are seen at 95 - 105 km altitude at lower midlatitudes. Midlatitude ES are mainly due to wind shear in the presence of tides, and the strongest signals are a migrating diurnal and semidiurnal com- ponent. Especially at high latitudes of the southern hemisphere, nonmigrating components such as a diurnal westward wave 2 and a semidiurnal westward wave 1 are also visible. Near the equator, a strong diurnal eastward wavenumber 3 component and a semidiurnal eastward wavenumber 2 component is found in summer and autumn. Terdiurnal and quarterdiurnal components are weaker than the diurnal and semidiurnal ones

The ionospheric Total Electron Content (TEC) provided by the International GNSS Service (IGS), and the Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model simulated TEC have been used to investigate the delayed ionospheric response against solar flux and its trend during the years 2011 to 2013. The analysis of the distinct low and mid-latitudes TEC response over 15° E shows a better correlation of observed TEC and the solar radio flux index F10.7 in the Southern Hemisphere compared to the Northern Hemisphere. Thus, a significant hemispheric asymmetry is observed. The ionospheric delay estimated using model simulated TEC is in good agreement with the delay estimated for observed TEC against Solar Dynamics Observatory (SDO) EUV Variability Experiment (EVE) measured flux. The average delay for the observed (modeled) TEC is 17(16) h. The average delay calculated for observed and modeled TEC is 1 and 2 h longer in the Southern Hemisphere compared to the Northern Hemisphere. Furthermore, the observed TEC is compared with the modeled TEC simulated using the SOLAR2000 and EUVAC flux models within CTIPe over Northern and Southern Hemispheric grid points. The analysis suggests that TEC simulated using the SOLAR2000 flux model overestimates the observed TEC, which is not the case when using the EUVAC flux model.

Implementing a nonlinear gravity wave (GW) parameterization into a mechanistic middle and upper atmosphere model, which extends to the lower thermosphere (160 km), we study the response of the atmosphere in terms of the circulation patterns, temperature distribution, and migrating terdiurnal solar tide activity to the upward propagating small-scale internal GWs originating in the lower atmosphere. We perform three test simulations for the Northern Hemisphere winter conditions in order to assess the effects of variations in the initial GW spectrum on the climatology and tidal patterns of the mesosphere and lower thermosphere. We find that the overall strength of the source level momentum flux has a relatively small impact on the zonal mean climatology. The tails of the GW source level spectrum, however, are crucial for the lower thermosphere climatology. With respect to the terdiurnal tide, we find a strong dependence of tidal amplitude on the induced GW drag, generally being larger when GW drag is increased.

In recent years, multistatic specular meteor radars (SMRs) have been introduced to study the Mesosphere and Lower Thermosphere (MLT) dynamics. In this paper, the statistics of mesoscale MLT power spectra are explored through observations from a campaign using the SIMONe (Spread-spectrum Interferometric Multistatic meteor radar Observing Network) approach conducted in northern Germany in 2018 (hereafter SIMONe 2018). The seven-day SIMONe 2018 comprised of fourteen multistatic SMR links and allows to build a substantial database of specular meteor trail events, collecting more than one hundred thousand detections per day within a geographic area of ~ 500 km x 500 km. The two methods we propose to obtain the power spectra in frequency range are (1) Wind field Correlation Function Inversion (WCFI), which utilizes two-point correlations of specular meteor observations, and (2) Mean Wind Estimation (MWE), which determines the MLT winds and gradients from specular meteor observations. Monte Carlo simulations of a gravity wave spectral model were implemented to validate and compare both methods. The simulation analyses suggest that the WCFI is the viable option among them to study the second-order statistics of the MLT winds that helps to capture the energy of small-scale wind fluctuations. Characterization of the spectral slope at different MLT altitudes has been conducted on the SIMONe 2018, and it provides evidence that gravity waves with periods smaller than seven hours and greater than two hours are dominated by waves with horizontal wavelength significantly larger than 500 km, which might be associated to secondary gravity waves. We believe that the presented methods can help us bridge the observational gap between large and small-scale mesospheric wind fluctuations and also improve the capabilities of SMRs.

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.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.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.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.This work received funding from the Bavarian State Ministry of the Environment and Consumer Protection.

In the atmosphere, internal gravity waves (GWs) are a naturally occurring and ubiquitous, though intermittent phenomenon. In addition, GWs (especially orographic; OGWs) are asymmetrically distributed around the globe. In current generation global climate models (GCMs), GWs are usually smaller than the model grid resolution and the majority of their spectrum therefore must be parameterized. To some extent, the intermittency and asymmetry of a spatial distribution of the resulting OGW drag (OGWD) is present also in GCMs. As the GW parameterization schemes in GCMs are usually tuned to get the zonal mean climatology of particular features right, an important question emerges: what kind of influence do GW parameterizations have on the individual models atmosphere locally? Here we focus on answering this question regarding the impact of spatiotemporally intermittent OGW forcing in the extra-tropical lower stratosphere region (LS). The LS region is characterized by a strong interplay of chemical, physical and dynamical processes. To date, the representation of this dynamically active region in models frequently mismatches observations. Although we can find a climatological maximum of oGWD in the LS, the role of OGW forcing for the transport and composition in this region is poorly understood. We combine observational evidence, idealized modeling and statistical analysis of GCM outputs to study both the short-term and long-term model response to the OGW forcing. The results presented will question the relationship between the advective part of the Brewer- Dobson circulation and the zonally asymmetric GW forcing, and a so-far neglected link between oGWD and large-scale quasi-isentropic stirring will be discussed.

A strong mountain wave, observed over Central Europe on 12 January 2016, is simulated in 2D under two fixed background wind conditions representing opposite tidal phases. The aim of the simulation is to investigate the breaking of the mountain wave and subsequent generation of nonprimary waves in the upper atmosphere. The model results show that the mountain wave first breaks as it approaches a mesospheric critical level creating turbulence on horizontal scales of 8–30 km. These turbulence scales couple directly to horizontal secondary waves scales, but those scales are prevented from reaching the hermosphere by the tidal winds, which act like a filter. Initial secondary waves that can reach the thermosphere range from 60 to 120 km in horizontal scale and are influenced by the scales of the horizontal and vertical forcing associated with wave breaking at mountain wave zonal phase width, and horizontal wavelength scales. Large-scale nonprimary waves dominate over the whole duration of the simulation with horizontal scales of 107–300 km and periods of 11–22 minutes. The thermosphere winds heavily influence the time-averaged spatial distribution of wave forcing in the thermosphere, which peaks at 150 km altitude and occurs both westward and eastward of the source in the 2 UT background simulation and primarily eastward of the source in the 7 UT background simulation. The forcing amplitude is ∼2× that of the primary mountain wave breaking and dissipation. This suggests that nonprimary waves play a significant role in gravity waves dynamics and improved understanding of the thermospheric winds is crucial to understanding their forcing distribution.

The Radar data for each location is identified with its Name and U,V by zonal and meridional winds. In each file, the first column is height and second column onwards wind data from 2017-12-01 to 2018-03-31.

Implementing a nonlinear whole atmosphere gravity wave (GW) parameterization into the Middle and Upper Atmosphere Model extending to the lower thermosphere (160 km), we study the response of the atmosphere in terms of the circulation patterns, temperature distribution, and migrating terdiurnal solar tide activity to the upward propagating small-scale internal GWs originating in the lower atmosphere. We perform three test simulations for the Northern Hemisphere winter conditions in order to assess the effects of variations in the initial GW spectrum on the dynamics of the mesosphere and lower thermosphere. We find that the overall strength of the source level momentum flux has a comparatively small impact on zonal mean dynamics. The tails of the GW source level spectrum, however, are crucial for the lower thermosphere dynamics. With respect to the terdiurnal tide, we find a strong dependence of tidal amplitude on the induced GW drag, generally being larger when GW drag is increased.

This study correlates different ionospheric parameters with the integrated solar extreme ultraviolet radiation (EUV) radiation to analyze the delayed ionospheric response, testing and improving upon previous studies on the ionospheric delay. Several time series of correlation coefficients and delays are presented to characterize the trend of the ionospheric delay from January 2011 to December 2013. The impact of the diurnal variations of ionospheric parameters in the analysis at an hourly resolution for fixed locations are discussed and specified with calculations in different timescales and with comparison to solar and geomagnetic activity. An average delay for the total electron content (TEC) of ≈18.7 h and for foF2 of ≈18.6 h is calculated at four European stations. The difference between the Northern and Southern hemispheres is analyzed by comparisons with the Australian region. A seasonal variation of the delay between the Northern and Southern hemispheres is calculated for TEC with ≈5±0.7 h and foF2 with ≈8±0.8 h. The latitudinal and longitudinal variability of the delay is analyzed for the European region, and found to be characterized by a decrease in the delay from ≈21.5 h at 30∘ N to ≈19.0 h at 70∘ N for summer months. For winter months, a roughly constant delay of ≈19.5 h is calculated. The results based on solar and ionospheric data at an hourly resolution and the analysis of the delayed ionospheric response to solar EUV show seasonal and latitudinal variations. Results also indicate a relationship of the ionospheric delay with geomagnetic activity and a possible correlation with the 11-year solar cycle in the analyzed time period.

Using a nonlinear mechanistic global circulation model we analyze the migrating terdiurnal tide in the middle atmosphere with respect to its possible forcing mechanisms, i.e. the absorption of solar radiation in the water vapor and ozone band, nonlinear tidal interactions, and gravity wave-tide interactions. In comparison to the forcing mechanisms of diurnal and semidiurnal tides, these terdiurnal forcings are less well understood and there are contradictory opinions about their respective relevance. In our simulations we remove the wavenumber 3 pattern for each forcing individually and analyze the remaining tidal wind and temperature fields. We find that the direct solar forcing is dominant and explains most of the migrating terdiurnal tide's amplitude. Nonlinear interactions due to other tides or gravity waves are most important during local winter. Further analyses show that the nonlinear forcings are locally counteracting the solar forcing due to destructive interferences. Therefore, tidal amplitudes can become even larger for simulations with removed nonlinear forcings.

The seasonal and interannual variability of the quarterdiurnal tide is analysed using meteor radar wind observations at the two midlatitude sites Collm and Obninsk. Generally tidal amplitudes increase with height. Maximum tidal amplitudes are found in winter. Meridional amplitudes are smaller than zonal ones on an average. Phases mainly differ between summer and winter. Zonal and meridional phases differ by slightly less than 90°. The vertical wavelengths are very long in winter, but shorter and on the order of 20 km in summer. Collm and Obninsk amplitudes and phases agree well, indicating that the migrating quarterdiurnal tide may be responsible for a major part of the observed waves. Observations since 1980 show that the tidal amplitudes have increased on a whole, although the increase is not linear but mainly happening during the late 1990s and the early 2000s.

Mesosphere/lower thermosphere winds obtained by meteor radar over Cachoeira Paulista (22.7° S, 45.0° W), Brazil, have been used to investigate the interannual variability of the diurnal tidal (DT) wind amplitude. The monthly DT displays year to year variations and their amplitudes are strongest during the westerly phase of the quasi-biennial oscillation (QBO) at the 30 hPa level. This can be observed in all seasons in the meridional component, whilst in the zonal component the signal is clearer during austral autumn, when the diurnal tide is strongest in this latitude. The spectrum obtained from the deseasonalized amplitudes shows a peak near 26 months in the meridional component, which can be associated to the stratospheric QBO. The QBO modulation of the DT amplitude shows a quasi-decadal variation, and it is stronger during the maximum of the solar cycle.

We used a nonlinear mechanistic global circulation model to analyze the migrating quarterdiurnal tide (QDT) in the middle atmosphere with focus on its possible forcing mechanisms. These are absorption of solar radiation by ozone and water vapor, nonlinear tidal interactions, and gravity wave-tide interactions. We show a climatology of the QDT amplitudes, and we examined the contribution of the different forcing mechanisms on the QDT amplitude. To this end, we first extracted the QDT in the model tendency terms. Then, we separately removed the QDT contribution in different tendency terms. We find that the solar forcing mechanism is the most important one for the QDT, but also the nonlinear and gravity wave forcing mechanism play a role in certain seasons, latitudes and altitudes. Furthermore, destructive interference between the individual forcing mechanisms are observed. Therefore, tidal amplitudes partly become even larger in simulations with removed nonlinear or gravity wave forcing mechanism.

In order to investigate the impact of a locally confined gravity wave (GW) hotspot, a sensitivity study based on simulations of the middle atmosphere circulation during northern winter was performed with a nonlinear, mechanistic, general circulation model. To this end, we selected a fixed longitude range in the East Asian region (120–170∘ E) and a latitude range from 22.5 to 52.5∘ N between 18 and 30 km for the hotspot region, which was then shifted northward in steps of 5∘. For the southernmost hotspots, we observe a decreased stationary planetary wave (SPW) with wave number 1 (SPW 1) activity in the upper stratosphere and lower mesosphere, i.e., fewer SPWs 1 are propagating upwards. These GW hotspots lead to a negative refractive index, inhibiting SPW propagation at midlatitudes. The decreased SPW 1 activity is connected to an increased zonal mean zonal wind at lower latitudes. This, in turn, decreases the meridional potential vorticity gradient (qy) from midlatitudes towards the polar region. A reversed qy indicates local baroclinic instability, which generates SPWs with wave number 1 in the polar region, where we observe a strong positive Eliassen–Palm (EP) divergence. As a result, the EP flux increases towards the polar stratosphere (corresponding to enhanced SPW 1 amplitudes), where the SPWs with wave number 1 break, and the zonal mean zonal wind decreases. Thus, the local GW forcing leads to a displacement of the polar vortex towards lower latitudes. The effect of the local baroclinic instability indicated by the reversed qy also produces SPWs with wave number 1 in the lower mesosphere. The effect on the dynamics in the middle atmosphere due to GW hotspots that are located northward of 50∘ N is negligible, as the refractive index of the atmosphere is strongly negative in the polar region. Thus, any changes in the SPW activity due to the local GW forcing are quite ineffective.

We investigate the forcing mechanisms of the terdiurnal solar tide in the middle atmosphere using a mechanistic global circulation model. In order to quantify their individual contributions, we perform several model experiments and separate each forcing mechanism by switching off the remaining sources. We find that the primary excitation is owing to the terdiurnal component of solar radiation absorption in the troposphere and stratosphere. Secondary sources are nonlinear tide-tide interactions and gravity wave-tide interactions. Thus, although the solar heating clearly dominates the terdiurnal forcing in our simulations, we find that nonlinear tidal and gravity wave interactions contribute in certain seasons and altitudes. By slightly enhancing the different excitation sources, we test the sensitivity of the background circulation on these changes of the dynamics. As a result, the increase of terdiurnal gravity wave drag can strongly affect the middle and upper atmosphere dynamics, including an irregular change of the terdiurnal amplitude, a weakening of neutral winds in the thermosphere, and a significant temperature change in the thermosphere, depending on the strength of the forcing. On the contrary, the influence of nonlinear tidal interactions on the middle atmosphere background dynamics is rather small.

Arctic amplification causes the meridional temperature gradient between middle and high latitudes to decrease. Through this decrease the large-scale circulation in the midlatitudes may change and therefore the meridional transport of heat and moisture increases. This in turn may increase Arctic warming even further. To investigate patterns of Arctic temperature, horizontal transports and their changes in time, we analysed ERA-Interim daily winter data of vertically integrated horizontal moist static energy transport using self-organizing maps (SOMs). Three general transport pathways have been identified: the North Atlantic pathway with transport mainly over the northern Atlantic, the North Pacific pathway with transport from the Pacific region, and the Siberian pathway with transport towards the Arctic over the eastern Siberian region. Transports that originate from the North Pacific are connected to negative temperature anomalies over the central Arctic. These North Pacific pathways have been becoming less frequent during the last decades. Patterns with origin of transport in Siberia are found to have no trend and show cold temperature anomalies north of Svalbard. It was found that transport patterns that favour transport through the North Atlantic into the central Arctic are connected to positive temperature anomalies over large regions of the Arctic. These temperature anomalies resemble the warm Arctic–cold continents pattern. Further, it could be shown that transport through the North Atlantic has been becoming more frequent during the last decades.

The thermosphere-ionosphere system shows high complexity due to interaction with the continuously varying solar radiation flux. We investigate the ionospheric response to the temporal and spatial dynamics of the solar activity using 18 years (1999–2017) of total electron content (TEC) maps provided by the international GNSS service (IGS) and twelve solar proxies (F10.7, F1.8, F3.2, F8, F15, F30, He-II, MG-II index, Ly-α, Ca K, DSA and SSN). Cross-wavelet and Lomb Scargle periodogram (LSP) analysis are used to evaluate the different solar proxies in respect to their impact on the global mean TEC (GTEC), which is important for improved ionosphere modelling and forecasts. A 16–32 days period in all the solar proxies and GTEC has been identified. The maximum correlation at this time scale is observed between the He-II, Mg-II, and F30 with respect to GTEC, with an effective time delay of about one day. LSP analysis shows that the most dominant period is 27 days, which is based on mean solar rotation, followed by a 44-day periodicity. In addition, a semi-annual and an annual variation has been observed in GTEC, with the strongest correlation near the equator region where a time delay about 1–2 days exists. The wavelet variance estimation method is used to find the variance in the maximum of the solar cycles (SC) 23 (2000–2002) and 24 (2012–2014), for GTEC and F10.7 index, respectively. Wavelet variance estimation suggests that GTEC variance is highest for the seasonal timescale followed by the 16–32 days period, similar to the F10.7 index highest variance for the 16–32 days period. Variance during SC 23 is larger than during SC 24. The most suitable proxy to represent the solar activity at the time scales of 16–32 days and 32–64 days is He-II. The MG-II index, Ly-α, and F30 may be placed at the second as these indices show the strongest correlation with GTEC, but there are some differences between solar maximum and minimum. The F1.8 and DSA are of limited use to represent the solar impact on GTEC. Empirical orthogonal function (EOF) analysis of the TEC data shows that the first EOF components capture more than 86 % of the variance, and the first three EOF components explain 99 % of the total variance. EOF analysis suggests that the first component is associated with the solar flux.

Solar tides such as the diurnal and semidiurnal tide, are forced in the lower and middle atmosphere through the diurnal cycle of solar radiation absorption. This is also the case with higher harmonics like the quarterdiurnal tide (QDT), but for these also non-linear interaction of tides such as the self-interaction of the semidiurnal tide, or the interaction of terdiurnal and diurnal tides, are discussed as possible forcing mechanism. To shed more light on the sources of the QDT, 12 years of meteor radar data at Collm (51.3∘ N, 13∘ E) have been analyzed with respect to the seasonal variability of the QDT at 82–97 km altitude, and bispectral analysis has been applied. The results indicate that non-linear interaction, in particular self-interaction of the semidiurnal tide probably plays an important role in winter, but to a lesser degree in summer. Numerical modelling of 6 h amplitudes qualitatively reproduces the gross seasonal structure of the observed 6 h wave at Collm. Model experiments with removed tidal forcing mechanisms lead to the conclusion that, although non-linear tidal interaction is one source of the QDT, the major forcing mechanism is direct solar forcing of the 6 h tidal components.

Atmospheric tides dominate the mesosphere and lower thermosphere (MLT) dynamics and show seasonal, interannual as well as long term variability. In the present study, MLT wind data obtained between 1999 and 2019 from meteor radar observations at Cachoeira Paulista (22.7∘S, 45.0∘W), Brazil, have been used to investigate similarities and differences in the diurnal, semidiurnal and terdiurnal tide amplitudes during the declining phases of solar cycles 23 and 24. The equinoctial peaks of the terdiurnal and semidiurnal tide amplitudes decay from solar maximum to solar minimum, while diurnal tide amplitudes present high variability, but no obvious long-term trend or decadal variability. The deseasonalized MLT terdiurnal and semidiurnal tidal amplitudes show higher positive correlation with solar activity during solar cycle 24 ( ρ = 0.97 and 0.90) than during solar cycle 23 ( ρ = 0.85 and 0.67) and an insignificant correlation with the quasi-biennial oscillation (QBO), during both solar cycles, in disagreement to some results reported in the literature. The deseasonalized MLT diurnal tidal amplitudes show QBO signals whose amplitudes decrease with solar activity during the declining phase of solar cycle 23, suggesting a modulation of the QBO amplitude of the MLT diurnal tide by solar activity. However, in contrast to solar cycle 23, quasi-biennial amplitudes of the diurnal tide increase during the declining phase of the very weak solar cycle 24.

A rare noctilucent cloud (NLC) event was observed at 48.8° N, 13.7° E above GERES station in southern Germany on 18/19 July 2016 using the CORAL Rayleigh lidar. Strong southward winds due to a quasi 2-day planetary wave allowed for the influx of mesospheric polar air to mid-latitudes on this day. The NLC observed by lidar was preceded by strong NLC displays in CIPS satellite images above the North Sea and by strong MSE radar echoes 800 km north of the lidar site, and was also observed visually in central Europe. The NLC occurred at low altitude and was bright and thin with strong oscillations in altitude and brightness. Darkness allowed for high-resolution temperature measurements at NLC altitudes. The ice particles were embedded in the upper part of a cold region with temperatures below 150 K. Significantly higher temperatures were found directly above the cloud with large vertical temperature gradients of 25 K/km at the top boundary. Spectral analysis reveals that NLC particles existed within cold phases of gravity waves within a region of high static stability. In order to study the evolution of NLC brightness in this environment we drive the microphysical model CARMA with lidar temperature soundings. We find that NLC particles can survive and grow in the conditions defining this mid-latitude event. We conclude that the ice particles did not nucleate at the site of observation, but were meridionally transported and vertically confined to a thin layer due to a large vertical temperature gradient, wind reversal, and low levels of mesospheric turbulence.

The 2015/16 Northern Hemisphere winter season was marked by peculiarities in the circulation pattern in the high-latitude mesopause region. Wind measurements from the Andenes (69° N, 13° E) meteor radar show westward winds below 84 km and eastward winds above. This wind pattern in the zonal wind was observable between the end of December 2015 and the end of January 2016, i.e., conditions that are typical for the summer were found during winter. Additional meteor radar measurements at midlatitude stations did not show such a zonal wind reversal but indicate, together with the polar latitude stations, a reversal of the horizontal temperature gradient. This is confirmed by global satellite measurements. Therefore, it is plausible that the polar latitude summer-like zonal wind reversal in December–January is in accordance with the reversed horizontal temperature gradient assuming a thermal wind balance between mid- and polar latitudes. The reversed horizontal temperature gradient itself is induced by stationary planetary waves at lower and midlatitudes in the mesosphere, leading to a weakening of the residual circulation above the European sector.

We investigate the ionospheric response to solar Extreme Ultraviolet (EUV) variations using different proxies, based on solar EUV spectra observed from the Solar Extreme Ultraviolet Experiment (SEE) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite, the F10.7 index (solar irradiance at 10.7 cm), and the Bremen composite Mg-II index during January 2003 to December 2016. The daily mean solar proxies are compared with global mean Total Electron Content (GTEC) values calculated from global IGS TEC maps. The preliminary analysis shows a significant correlation between GTEC and both the integrated flux from SEE and the Mg II index, while F10.7 correlates less strongly with GTEC. The correlations of EUV proxies and GTEC at different time periods are presented. An ionospheric delay in GTEC is observed at the 27 days solar rotation period with the time scale of about ∼1–2 days. An experiment with the physics based global 3-D Coupled Thermosphere/Ionosphere Plasmasphere electrodynamics (CTIPe) numerical model was performed to reproduce the ionospheric delay. Model simulations were performed for different values of the F10.7 index while keeping all the other model inputs constant. Preliminary results qualitatively reproduce the observed ∼1–2 days delay in GTEC, which is might be due to vertical transport processes.