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In the course of the GW-LCYCLE II campaign, conducted in Jan/Feb 2016 from Kiruna, Sweden, coherent Doppler wind lidar (2 µm DWL) measurements were performed from the DLR Falcon aircraft to investigate gravity waves induced by flow across the Scandinavian Alps. During a mountain wave event on 28 January 2016, a novel momentum flux (MF) scan pattern with fore and aft propagating laser beams was applied to the 2 µm DWL. This allows us to measure the vertical wind and the horizontal wind along the flight track simultaneously with a high horizontal resolution of ≈800 m and hence enables us to derive the horizontal MF profile for a broad wavelength spectrum from a few hundred meters to several hundred kilometers. The functionality of this method and the corresponding retrieval algorithm is validated using a comparison against in situ wind data measured by the High Altitude and Long Range (HALO) aircraft which was also deployed in Kiruna for the POLSTRACC (Polar Stratosphere in a Changing Climate) campaign. Based on that, the systematic and random error of the wind speeds retrieved from the 2 µm DWL observations are determined. Further, the measurements performed on that day are used to reveal significant changes in the horizontal wavelengths of the vertical wind speed and of the leg-averaged momentum fluxes in the tropopause inversion layer (TIL) region, which are likely to be induced by interfacial waves as recently presented by Gisinger et al. (2020).

The Balloon Lidar Experiment (BOLIDE), which was part of the Polar Mesospheric Cloud Turbulence (PMC Turbo) Balloon Mission has captured vertical profiles of PMCs during a 6 d flight along the Arctic circle in July 2018. The high-resolution soundings (20 m vertical and 10 s temporal resolution) reveal highly structured layers with large gradients in the volume backscatter coefficient. We systematically screen the BOLIDE dataset for small-scale variability by assessing these gradients at high resolution. We find longer tails of the probability density distributions of these gradients compared to a normal distribution, indicating intermittent behaviour. The high occurrence rate of large gradients is assessed in relation to the 15 min averaged layer brightness and the spectral power of short-period (5–62 min) gravity waves based on PMC layer altitude variations. We find that variability on small scales occurs during weak, moderate, and strong gravity wave activity. Layers with below-average brightness are less likely to show small-scale variability in conditions of strong gravity wave activity. We present and discuss the signatures of this small-scale variability, and possibly related dynamical processes, and identify potential cases for future case studies and modelling efforts.

The Balloon Lidar Experiment (BOLIDE) observed polar mesospheric clouds (PMCs) along the Arctic circle between Sweden and Canada during the balloon flight of PMC Turbo in July 2018. The purpose of the mission was to study small-scale dynamical processes induced by the breaking of atmospheric gravity waves by high-resolution imaging and profiling of the PMC layer. The primary parameter of the lidar soundings is the time- and range-resolved volume backscatter coefficient β. These data are available at high resolutions of 20 m and 10 s (Kaifler, 2021, https://doi.org/10.5281/zenodo.5722385). This document describes how we calculate β from the BOLIDE photon count data and balloon floating altitude. We compile information relevant for the scientific exploration of this dataset, including statistics, mean values, and temporal evolution of parameters like PMC brightness, altitude, and occurrence rate. Special emphasis is given to the stability of the gondola pointing and the effect of resolution on the signal-to-noise ratio and thus the detection threshold of PMC. PMC layers were detected during 49.7 h in total, accounting for 36.8 % of the 5.7 d flight duration and a total of 178 924 PMC profiles at 10 s resolution. Up to the present, published results from subsets of this dataset include the evolution of small-scale vortex rings, distinct Kelvin–Helmholtz instabilities, and mesospheric bores. The lidar soundings reveal a wide range of responses of the PMC layer to larger-scale gravity waves and breaking gravity waves, including the accompanying instabilities, that await scientific analysis.

The mesosphere and lower thermosphere (MLT) is a dynamic layer of the earth's atmosphere. This region marks the interface at which neutral atmosphere dynamics begin to influence the upper atmosphere and ionosphere. However, our understanding of this region and our ability to accurately simulate it in global circulation models (GCMs) is limited by a lack of observations, especially in remote locations. To this end, a meteor radar was deployed from 2016 to 2020 on the remote mountainous island of South Georgia (54∘ S, 36∘ W) in the Southern Ocean. In this study we use these new measurements to characterise the fundamental dynamics of the MLT above South Georgia including large-scale winds, solar tides, planetary waves (PWs), and mesoscale gravity waves (GWs). We first present an improved method for time–height localisation of radar wind measurements and characterise the large-scale MLT winds. We then determine the amplitudes and phases of the diurnal (24 h), semidiurnal (12 h), terdiurnal (8 h), and quardiurnal (6 h) solar tides at this latitude. We find very large amplitudes up to 30 m s−1 for the quasi 2 d PW in summer and, combining our measurements with the meteor SAAMER radar in Argentina, show that the dominant modes of the quasi 5, 10, and 16 d PWs are westward 1 and 2. We investigate and compare wind variance due to both large-scale “resolved” GWs and small-scale “sub-volume” GWs in the MLT and characterise their seasonal cycles. Last, we use our radar observations and satellite temperature observations from the Microwave Limb Sounder to test a climatological simulation of the Whole Atmosphere Community Climate Model (WACCM). We find that WACCM exhibits a summertime mesopause near 80 km altitude that is around 10 K warmer and 10 km lower in altitude than observed. Above 95 km altitude, summertime meridional winds in WACCM reverse to poleward, but this not observed in radar observations in this altitude range. More significantly, we find that wintertime zonal winds between 85 to 105 km altitude are eastward up to 40 m s−1 in radar observations, but in WACCM they are westward up to 20 m s−1. We propose that this large discrepancy may be linked to the impacts of secondary GWs (2GWs) on the residual circulation, which are not included in most global models, including WACCM. These radar measurements can therefore provide vital constraints that can guide the development of GCMs as they extend upwards into this important region of the atmosphere.

The main goals of this work are to characterize and investigate the potential wave sources of four mesospheric fronts identified in the hydroxyl near-infrared (OH-NIR) airglow images, obtained with an all-sky airglow imager installed at Comandante Ferraz Antarctic Station (EACF, as per its Portuguese acronym) located on King George Island in the Antarctic Peninsula. We identified and analyzed four mesospheric fronts in 2011 over King George Island. In addition, we investigate the atmospheric background environment between 80 and 100 km altitude and discuss the ducts and propagation conditions for these waves. For that, we used wind data obtained from a meteor radar operated at EACF and temperature data obtained from the TIMED/SABER satellite. The vertical wavenumber squared, m2, was calculated for each of the four waves. Even though no clearly defined duct (indicated by positive values of m2 sandwiched between layers above and below with m2

A study of the quasi-5-day wave (5DW) was performed using meteor radars at conjugate latitudes in the Northern and Southern hemispheres. These radars are located at Esrange, Sweden (68° N) and Juliusruh, Germany (55° N) in the Northern Hemisphere, and at Tierra del Fuego, Argentina (54° S) and Rothera Station, Antarctica (68° S) in the Southern Hemisphere. The analysis was performed using data collected during simultaneous measurements by the four radars from June 2010 to December 2012 at altitudes from 84 to 96 km. The 5DW was found to exhibit significant short-term, seasonal, and interannual variability at all sites. Typical events had planetary wave periods that ranged between 4 and 7 days, durations of only a few cycles, and infrequent strongly peaked variances and covariances. Winds exhibited rotary structures that varied strongly among sites and between events, and maximum amplitudes up to ~ 20 m s−1. Mean horizontal velocity covariances tended to be largely negative at all sites throughout the interval studied.

We employ a modified composite day extension of the Hocking (2005) analysis method to study gravity wave (GW) activity in the mesosphere and lower thermosphere using 4 meteor radars spanning latitudes from 7° S to 53.6° S. Diurnal and semidiurnal modulations were observed in GW variances over all sites. Semidiurnal modulation with downward phase propagation was observed at lower latitudes mainly near the equinoxes. Diurnal modulations occur mainly near solstice and, except for the zonal component at Cariri (7° S), do not exhibit downward phase propagation. At a higher latitude (SAAMER, 53.6° S) these modulations are only observed in the meridional component where we can observe diurnal variation from March to May, and semidiurnal, during January, February, October (above 88 km) and November. Some of these modulations with downward phase progression correlate well with wind shear. When the wind shear is well correlated with the maximum of the variances the diurnal tide has its largest amplitudes, i.e., near equinox. Correlations exhibiting variations with tidal phases suggest significant GW-tidal interactions that have different characters depending on the tidal components and possible mean wind shears. Modulations that do not exhibit phase variations could be indicative of diurnal variations in GW sources.

The advantages of using a composite day analysis for all-sky interferometric meteor radars when measuring mean winds and tides are widely known. On the other hand, problems arise if this technique is applied to Hocking's (2005) gravity wave analysis for all-sky meteor radars. In this paper we describe how a simple change in the procedure makes it possible to use a composite day in Hocking's analysis. Also, we explain how a modified composite day can be constructed to test its ability to measure gravity wave momentum fluxes. Test results for specified mean, tidal, and gravity wave fields, including tidal amplitudes and gravity wave momentum fluxes varying strongly with altitude and/or time, suggest that the modified composite day allows characterization of monthly mean profiles of the gravity wave momentum fluxes, with good accuracy at least at the altitudes where the meteor counts are large (from 89 to 92.5 km). In the present work we also show that the variances measured with Hocking's method are often contaminated by the tidal fields and suggest a method of empirical correction derived from a simple simulation model. The results presented here greatly increase our confidence because they show that our technique is able to remove the tide-induced false variances from Hocking's analysis.

In this paper, we develop an estimator for the vertical flux of horizontal momentum with arbitrary beam pointing, applicable to the case of arbitrary but fixed beam pointing with systems such as the Poker Flat Incoherent Scatter Radar (PFISR). This method uses information from all available beams to resolve the variances of the wind field in addition to the vertical flux of both meridional and zonal momentum, targeted for high-frequency wave motions. The estimator utilises the full covariance of the distributed measurements, which provides a significant reduction in errors over the direct extension of previously developed techniques and allows for the calculation of an error covariance matrix of the estimated quantities. We find that for the PFISR experiment, we can construct an unbiased and robust estimator of the momentum flux if sufficient and proper beam orientations are chosen, which can in the future be optimized for the expected frequency distribution of momentum-containing scales. However, there is a potential trade-off between biases and standard errors introduced with the new approach, which must be taken into account when assessing the momentum fluxes. We apply the estimator to PFISR measurements on 23 April 2008 and 21 December 2007, from 60–85 km altitude, and show expected results as compared to mean winds and in relation to the measured vertical velocity variances.

The data from ground based experiments conducted during the 2005 SpreadFEx campaign in Brazil are used, with the help of theoretical model calculations, to investigate the precursor conditions, and especially, the role of gravity waves, in the instability initiation leading to equatorial spread F development. Data from a digisonde and a 30 MHz coherent back-scatter radar operated at an equatorial site, Sao Luis (dip angle: 2.7°) and from a digisonde operated at another equatorial site (dip angle: −11.5°) are analyzed during selected days representative of differing precursor conditions of the evening prereversal vertical drift, F layer bottom-side density gradients and density perturbations due to gravity waves. It is found that radar irregularity plumes indicative of topside bubbles, can be generated for precursor vertical drift velocities exceeding 30 m/s even when the precursor GW induced density oscillations are marginally detectable by the digisonde. For drift velocities ≤20 m/s the presence of precursor gravity waves of detectable intensity is found to be a necessary condition for spread F instability initiation. Theoretical model calculations show that the zonal polarization electric field in an instability development, even as judged from its linear growth phase, can be significantly enhanced under the action of perturbation winds from gravity waves. Comparison of the observational results with the theoretical model calculations provides evidence for gravity wave seeding of equatorial spread F.

From September to November 2005, the NASA Living with a Star program supported the Spread-F Experiment campaign (SpreadFEx) in Brazil to study the effects of convectively generated gravity waves on the ionosphere and their role in seeding Rayleigh-Taylor instabilities, and associated equatorial plasma bubbles. Several US and Brazilian institutes deployed a broad range of instruments (all-sky imagers, digisondes, photometers, meteor/VHF radars, GPS receivers) covering a large area of Brazil. The campaign was divided in two observational phases centered on the September and October new moon periods. During these periods, an Utah State University (USU) all-sky CCD imager operated at São João d'Aliança (14.8° S, 47.6° W), near Brasilia, and a Brazilian all-sky CCD imager located at Cariri (7.4° S, 36° W), observed simultaneously the evolution of the ionospheric bubbles in the OI (630 nm) emission and the mesospheric gravity wave field. The two sites had approximately the same magnetic latitude (9–10° S) but were separated in longitude by ~1500 km. Plasma bubbles were observed on every clear night (17 from Brasilia and 19 from Cariri, with 8 coincident nights). These joint datasets provided important information for characterizing the ionospheric depletions during the campaign and to perform a novel longitudinal investigation of their variability. Measurements of the drift velocities at both sites are in good agreement with previous studies, however, the overlapping fields of view revealed significant differences in the occurrence and structure of the plasma bubbles, providing new evidence for localized generation. This paper summarizes the observed bubble characteristics important for related investigations of their seeding mechanisms associated with gravity wave activity.

We provide here an overview of, and a summary of results arising from, an extensive experimental campaign (the Spread F Experiment, or SpreadFEx) performed from September to November 2005, with primary measurements in Brazil. The motivation was to define the potential role of neutral atmosphere dynamics, specifically gravity wave motions propagating upward from the lower atmosphere, in seeding Rayleigh-Taylor instability (RTI) and plasma bubbles extending to higher altitudes. Campaign measurements focused on the Brazilian sector and included ground-based optical, radar, digisonde, and GPS measurements at a number of fixed and temporary sites. Related data on convection and plasma bubble structures were also collected by GOES 12, and the GUVI instrument aboard the TIMED satellite. Initial results of our SpreadFEx analyses are described separately by Fritts et al. (2009). Further analyses of these data provide additional evidence of 1) gravity wave (GW) activity near the mesopause apparently linked to deep convection predominantly to the west of our measurement sites, 2) small-scale GWs largely confined to lower altitudes, 3) larger-scale GWs apparently penetrating to much higher altitudes, 4) substantial GW amplitudes implied by digisonde electron densities, and 5) apparent influences of these perturbations in the lower F-region on the formation of equatorial spread F, RTI, and plasma bubbles extending to much higher altitudes. Other efforts with SpreadFEx data have also yielded 6) the occurrence, locations, and scales of deep convection, 7) the spatial and temporal evolutions of plasma bubbles, 8) 2-D (height-resolved) structures in electron density fluctuations and equatorial spread F at lower altitudes and plasma bubbles above, and 9) the occurrence of substantial tidal perturbations to the large-scale wind and temperature fields extending to bottomside F-layer and higher altitudes. Collectively, our various SpreadFEx analyses suggest direct links between deep tropical convection and large GW perturbations at large spatial scales at the bottomside F-layer and their likely contributions to the excitation of RTI and plasma bubbles extending to much higher altitudes.

During the Spread F Experiment campaign, under NASA Living with a Star (LWS) program, carried out in the South American Magnetic Equator region from 22 September to 8 November 2005, two airglow CCD imagers, located at Cariri (7.4° S, 36.5° W, geomag. 11° S) and near Brasilia (14.8° S, 47.6° W, geomag. 10° S) were operated simultaneously and measured the equatorial ionospheric bubbles and their time evolution by monitoring the airglow OI 6300 intensity depletions. Simultaneous observation of the mesospheric OH wave structures made it possible to investigate the relationship between the bubble formation in the ionosphere and the gravity wave activity at around 90 km. On the evening of 30 September 2005, comb-like OI 6300 depletions with a distance of ~130 km between the adjacent ones were observed. During the same period, a mesospheric gravity wave with a horizontal wavelength of ~130 km was observed. From the 17 nights of observation during the campaign period, there was a good correlation between the OI 6300 depletion distances and the gravity wave horizontal wavelengths in the mesosphere with a statistically significant level, suggesting a direct contribution of the mesospheric gravity wave to plasma bubble seeding in the equatorial ionosphere.

The nonlinear evolution of equatorial F-region plasma bubbles under varying ambient ionospheric conditions and gravity wave seeding perturbations in the bottomside F-layer is studied. To do so, the gravity wave propagation from the convective source region in the lower atmosphere to the thermosphere is simulated using a model of gravity wave propagation in a compressible atmosphere. The wind perturbation associated with this gravity wave is taken as a seeding perturbation in the bottomside F-region to excite collisional-interchange instability. A nonlinear model of collisional-interchange instability (CII) is implemented to study the influences of gravity wave seeding on plasma bubble formation and development. Based on observations during the SpreadFEx campaign, two events are selected for detailed studies. Results of these simulations suggest that gravity waves can play a key role in plasma bubble seeding, but that they are also neither necessary nor certain to do so. Large gravity wave perturbations can result in deep plasma bubbles when ionospheric conditions are not conducive by themselves; conversely weaker gravity wave perturbations can trigger significant bubble events when ionospheric conditions are more favorable. But weak gravity wave perturbations in less favorable environments cannot, by themselves, lead to strong plasma bubble responses.

We developed a technique to identify and estimate the size, intensity, and Tropopause overshoot of thunderstorm convective cores expected to be significant sources of gravity waves. The work was based on GOES IR images of South America on the night of 30 September to 1 October and 25–26 October 2005, as part of the Spread F Experiment (SpreadFEx) in Brazil in 2005. We also characterized, for the first time, the convective activity of three small TLE producing thunderstorms that yielded 11 TLEs on 25–26 October 2005. The campaign occurred during the dry to wet season transition in central Brazil, marked by the presence of extra-tropical cyclogenesis over the Atlantic Ocean, and cold fronts penetrating inland. The Tropopause temperature was typically −76°C with a corresponding altitude of ~15 200 m. Vigorous convective cores capable of generating strong gravity waves were located in convective regions having areas with cloud top temperatures ≤−76°C. They had typical cloud-top temperature deficits of ΔT−2.0°C to −8.0°C from the average surroundings, implying overshoot heights of 200 to 3100 m, which are within the typical range. Fast vertical development and high horizontal growth rates were associated with a large number of simultaneously active vigorous convective cores, indicating that their dynamics may have determined the spatial-temporal development of the thunderstorms analyzed. Moderate convective cores were also present in areas with cloud top −76°C≤T≤−70°C. They had ΔT of −1.9°C to −5.3°C producing overshoots between 80–300 m. All convective cores had typical diameters of 5–20 km and their size tended to increase with ΔT, there was a 57% correlation between the two parameters. Analysis of the relationship of cloud top T with positive and negative cloud-to-ground lightning (+/−CG) occurrence rate and with peak current showed that lighting activity may provide an independent way to identify convective cores and measure their intensity, since they were characterized by a high incidence of low peak current −CGs that forms the bulk of the −CG population.

As part of the SpreadFEx campaign, coordinated optical and radio measurements were made from Brazil to investigate the occurrence and properties of equatorial Spread F, and to characterize the regional mesospheric gravity wave field. All-sky image measurements were made from two sites: Brasilia and Cariri located ~10° S of the magnetic equator and separated by ~1500 km. In particular, the observations from Brasilia provided key data in relatively close proximity to expected convective sources of the gravity waves. High-quality image measurements of the mesospheric OH emission and the thermospheric OI (630 nm) emission were made during two consecutive new moon periods (22 September to 9 November 2005) providing extensive data on the occurrence and properties of F-region depletions and regional measurements of the dominant gravity wave characteristics at each site. A total of 120 wave displays were observed, comprising 94 short-period events and 26 medium-scale gravity waves. The characteristics of the small-scale waves agreed well with previous gravity wave studies from Brazil and other sites. However, significant differences in the wave propagation headings indicate dissimilar source regions for the Brasilia and Cariri datasets. The observed medium-scale gravity wave events constitute an important new dataset to study their mesospheric properties at equatorial latitudes. These data exhibited similar propagation headings to the short-period events, suggesting they originated from the same source regions. Medium-scale waves are generally less susceptible to wind filtering effects and modeling studies utilizing these data have successfully identified localized regions of strong convection, mainly to the west of Brasilia, as their most likely sources (Vadas et al., 2009).

We perform a series of numerical simulations employing a model describing nonlinear incompressible dynamics to explore bore generation and morphology for several ducting geometries. These include idealized thermal ducts in close proximity to, and more remote from, reflecting boundaries, an idealized Doppler duct remote from boundaries, and a combination of thermal and Doppler ducts at nearby altitudes. Based on observed bore environments, we assume initial long-wave perturbations having sinusoidal form and large amplitudes. Results indicate that close proximity of a duct to reflecting boundaries enhances bore development, that both thermal and Doppler ducts support bore development for long-wave perturbations of sufficiently large amplitudes, and that bore structures and evolutions exhibit both features anticipated by idealized bore theory and departures that remain to be explored in detail.

We implement gravity wave (GW) phases into our convective plume and anelastic ray trace models. This allows us to successfully reconstruct the GW velocity, temperature, and density perturbation amplitudes and phases in the Mesosphere-Lower-Thermosphere (MLT) via ray tracing (in real space) those GWs that are excited from a deep convective plume. We find that the ray trace solutions agree very well with the exact, isothermal, zero-wind, Fourier-Laplace solutions in the Boussinesq limit. This comparison also allows us to determine the normalization factor which converts the GW spectral amplitudes to real-space amplitudes in the ray trace model. This normalization factor can then be used for ray tracing GWs through varying temperature and wind profiles. We show that by adding GW reflection off the Earth's surface, the resulting GW spectrum has more power at larger vertical and horizontal wavelengths. We determine the form of the momentum flux and velocity spectra which allows for easy calculation of GW amplitudes in the MLT and thermosphere. Finally, we find that the reconstructed (ray traced) solution for a deep, convective plume with a duration much shorter than the buoyancy period does not equal the Fourier-Laplace Boussinesq solution; this is likely due to errors in the Boussinesq dispersion relation for very high frequency GWs.

We previously considered various aspects of gravity wave penetration and effects at mesospheric and thermospheric altitudes, including propagation, viscous effects on wave structure, characteristics, and damping, local body forcing, responses to solar cycle temperature variations, and filtering by mean winds. Several of these efforts focused on gravity waves arising from deep convection or in situ body forcing accompanying wave dissipation. Here we generalize these results to a broad range of gravity wave phase speeds, spatial scales, and intrinsic frequencies in order to address all of the major gravity wave sources in the lower atmosphere potentially impacting the thermosphere. We show how penetration altitudes depend on gravity wave phase speed, horizontal and vertical wavelengths, and observed frequencies for a range of thermospheric temperatures spanning realistic solar conditions and winds spanning reasonable mean and tidal amplitudes. Our results emphasize that independent of gravity wave source, thermospheric temperature, and filtering conditions, those gravity waves that penetrate to the highest altitudes have increasing vertical wavelengths and decreasing intrinsic frequencies with increasing altitude. The spatial scales at the highest altitudes at which gravity wave perturbations are observed are inevitably horizontal wavelengths of ~150 to 1000 km and vertical wavelengths of ~150 to 500 km or more, with the larger horizontal scales only becoming important for the stronger Doppler-shifting conditions. Observed and intrinsic periods are typically ~10 to 60 min and ~10 to 30 min, respectively, with the intrinsic periods shorter at the highest altitudes because of preferential penetration of GWs that are up-shifted in frequency by thermospheric winds.

The Spread F Experiment, or SpreadFEx, was performed from September to November 2005 to define the potential role of neutral atmosphere dynamics, primarily gravity waves propagating upward from the lower atmosphere, in seeding equatorial spread F (ESF) and plasma bubbles extending to higher altitudes. A description of the SpreadFEx campaign motivations, goals, instrumentation, and structure, and an overview of the results presented in this special issue, are provided by Fritts et al. (2008a). The various analyses of neutral atmosphere and ionosphere dynamics and structure described in this special issue provide enticing evidence of gravity waves arising from deep convection in plasma bubble seeding at the bottomside F layer. Our purpose here is to employ these results to estimate gravity wave characteristics at the bottomside F layer, and to assess their possible contributions to optimal seeding conditions for ESF and plasma instability growth rates. We also assess expected tidal influences on the environment in which plasma bubble seeding occurs, given their apparent large wind and temperature amplitudes at these altitudes. We conclude 1) that gravity waves can achieve large amplitudes at the bottomside F layer, 2) that tidal winds likely control the orientations of the gravity waves that attain the highest altitudes and have the greatest effects, 3) that the favored gravity wave orientations enhance most or all of the parameters influencing plasma instability growth rates, and 4) that gravity wave and tidal structures acting together have an even greater potential impact on plasma instability growth rates and plasma bubble seeding.

Following earlier comparisons using the Canadian Middle Atmosphere Model (CMAM, without interactive chemistry), the dynamical characteristics of the model are assessed with interactive chemistry activated. Time-sequences of temperatures and winds at Tromsø (70° N) show that the model has more frequent and earlier stratospheric winter warmings than typically observed. Wavelets at mesospheric heights (76, 85 km) and from equator to polar regions show that CMAM tides are generally larger, but planetary waves (PW) smaller, than medium frequency (MF) radar-derived values. Tides modelled for eight geographic locations during the four seasons are not strikingly different from the earlier CMAM experiment; although monthly data now allow these detailed seasonal variations (local combinations of migrating and non-migrating components) within the mesosphere (circa 50–80 km) to be demonstrated for the first time. The dominant semi-diurnal tide of middle latitudes is, as in the earlier papers, quite well realized in CMAM. Regarding the diurnal tide, it is shown here and in an earlier study by one of the authors, that the main characteristics of the diurnal tide at low latitudes (where the S (1,1) mode dominates) are well captured by the model. However, in this experiment there are some other unobserved features for the diurnal tide, which are quite similar to those noted in the earlier CMAM experiment: low latitude amplitudes are larger than observed at 82 km, and middle latitudes feature an unobserved low altitude (73 km) summer maximum. Phases, especially at low and middle (circa 42° N) latitudes, do not match observations well. Mesospheric seasonal tidal variations available from the CUJO (Canada U.S.\ Japan Opportunity) radar (MFR) network (sites 40–45° N) reveal interesting longitudinal differences between the CMAM and the MFR observations. In addition, model and observations differ in the character of the vertical phase variations at each network-location. Finally, the seasonal variations of planetary wave (PW) activity available from CMAM and the MFR show quite good agreement, apart from the amplitude differences (smaller in CMAM above 70 km). A major difference for the 16-d PW is that CMAM shows large amplitudes before the winter solstice; and for the 2-d PW, while both CMAM and MFR show summer and winter activity, the observed summer mesopause and winter mesospheric wave activities are stronger and more extended in height. Models such as CMAM, operated without data-assimilation, are now able to provide increasingly realistic climatologies of middle atmosphere tides and PW activity. Differences do exist however, and so discussion of the various factors affecting tidal and PW characteristics in atmospheres, modelled and observed, is provided. Other diagnostics of model-characteristics and of future desirable model experiments are suggested.

The winter MaCWAVE (Mountain and convective waves ascending vertically) rocket campaign took place in January 2003 at Esrange, Sweden and the ALOMAR observatory in Andenes, Norway. The campaign combined balloon, lidar, radar, and rocket measurements to produce full temperature and wind profiles from the ground to 105 km. This paper will investigate gravity wave propagation in the mesosphere and lower thermosphere using data from the Weber sodium lidar on 28–29 January 2003. A very large semidiurnal tide was present in the zonal wind above 80 km that grew to a 90 m/s amplitude at 100 km. The superposition of smaller-scale gravity waves and the tide caused small regions of possible convective or shear instabilities to form along the downward progressing phase fronts of the tide. The gravity waves had periods ranging from the Nyquist period of 30 min up to 4 h, vertical wavelengths ranging from 7 km to more than 20 km, and the frequency spectra had the expected –5/3 slope. The dominant gravity waves had long vertical wavelengths and experienced rapid downward phase progression. The gravity wave variance grew exponentially with height up from 86 to 94 km, consistent with the measured scale height, suggesting that the waves were not dissipated strongly by the tidal gradients and resulting unstable regions in this altitude range.

As part of the MaCWAVE (Mountain and Convective Waves Ascending Vertically) winter campaign an all-sky monochromatic CCD imager has been used to investigate the properties of short-period mesospheric gravity waves at high northern latitudes. Sequential measurements of several nightglow emissions were made from Esrange, Sweden, during a limited period from 27–31 January 2003. Coincident wind measurements over the altitude range (~80–100 km) using two meteor radar systems located at Esrange and Andenes have been used to perform a novel investigation of the intrinsic properties of five distinct wave events observed during this period. Additional lidar and MSIS model temperature data have been used to investigate their nature (i.e. freely propagating or ducted). Four of these extensive wave events were found to be freely propagating with potential source regions to the north of Scandinavia. No evidence was found for strong orographic forcing by short-period waves in the airglow emission layers. The fifth event was most unusual exhibiting an extensive, but much smaller and variable wavelength pattern that appeared to be embedded in the background wind field. Coincident wind measurements indicated the presence of a strong shear suggesting this event was probably due to a large-scale Kelvin-Helmholtz instability.

MaCWAVE (Mountain and Convective Waves Ascending VErtically) was a highly coordinated rocket, ground-based, and satellite program designed to address gravity wave forcing of the mesosphere and lower thermosphere (MLT). The MaCWAVE program was conducted at the Norwegian Andøya Rocket Range (ARR, 69.3° N) in July 2002, and continued at the Swedish Rocket Range (Esrange, 67.9° N) during January 2003. Correlative instrumentation included the ALOMAR MF and MST radars and RMR and Na lidars, Esrange MST and meteor radars and RMR lidar, radiosondes, and TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) satellite measurements of thermal structures. The data have been used to define both the mean fields and the wave field structures and turbulence generation leading to forcing of the large-scale flow. In summer, launch sequences coupled with ground-based measurements at ARR addressed the forcing of the summer mesopause environment by anticipated convective and shear generated gravity waves. These motions were measured with two 12-h rocket sequences, each involving one Terrier-Orion payload accompanied by a mix of MET rockets, all at ARR in Norway. The MET rockets were used to define the temperature and wind structure of the stratosphere and mesosphere. The Terrier-Orions were designed to measure small-scale plasma fluctuations and turbulence that might be induced by wave breaking in the mesosphere. For the summer series, three European MIDAS (Middle Atmosphere Dynamics and Structure) rockets were also launched from ARR in coordination with the MaCWAVE payloads. These were designed to measure plasma and neutral turbulence within the MLT. The summer program exhibited a number of indications of significant departures of the mean wind and temperature structures from ``normal" polar summer conditions, including an unusually warm mesopause and a slowing of the formation of polar mesospheric summer echoes (PMSE) and noctilucent clouds (NLC). This was suggested to be due to enhanced planetary wave activity in the Southern Hemisphere and a surprising degree of inter-hemispheric coupling. The winter program was designed to study the upward propagation and penetration of mountain waves from northern Scandinavia into the MLT at a site favored for such penetration. As the major response was expected to be downstream (east) of Norway, these motions were measured with similar rocket sequences to those used in the summer campaign, but this time at Esrange. However, a major polar stratospheric warming just prior to the rocket launch window induced small or reversed stratospheric zonal winds, which prevented mountain wave penetration into the mesosphere. Instead, mountain waves encountered critical levels at lower altitudes and the observed wave structure in the mesosphere originated from other sources. For example, a large-amplitude semidiurnal tide was observed in the mesosphere on 28 and 29 January, and appears to have contributed to significant instability and small-scale structures at higher altitudes. The resulting energy deposition was found to be competitive with summertime values. Hence, our MaCWAVE measurements as a whole are the first to characterize influences in the MLT region of planetary wave activity and related stratospheric warmings during both winter and summer.

We present new sensitivity experiments that link observed anomalies of the mesosphere and lower thermosphere at high latitudes during the MaCWAVE/MIDAS summer program 2002 to enhanced planetary Rossby-wave activity in the austral winter troposphere. We employ the same general concept of a GCM having simplified representations of radiative and latent heating as in a previous study by Becker et al. (2004). In the present version, however, the model includes no gravity wave (GW) parameterization. Instead we employ a high vertical and a moderate horizontal resolution in order to describe GW effects explicitly. This is supported by advanced, nonlinear momentum diffusion schemes that allow for a self-consistent generation of inertia and mid-frequency GWs in the lower atmosphere, their vertical propagation into the mesosphere and lower thermosphere, and their subsequent dissipation which is induced by prescribed horizontal and vertical mixing lengths as functions of height. The main anomalies in northern summer 2002 consist of higher temperatures than usual above 82 km, an anomalous eastward mean zonal wind between 70 and 90 km, an altered meridional flow, enhanced turbulent dissipation below 80 km, and enhanced temperature variations associated with GWs. These signals are all reasonably described by differences between two long-integration perpetual model runs, one with normal July conditions, and another run with modified latent heating in the tropics and Southern Hemisphere to mimic conditions that correspond to the unusual austral winter 2002. The model response to the enhanced winter hemisphere Rossby-wave activity has resulted in both an interhemispheric coupling through a downward shift of the GW-driven branch of the residual circulation and an increased GW activity at high summer latitudes. Thus a quantitative explanation of the dynamical state of the northern mesosphere and lower thermosphere during June-August 2002 requires an enhanced Lorenz energy cycle and correspondingly enhanced GW sources in the troposphere, which in the model show up in both hemispheres.

Falling sphere and balloon wind and temperature data from the MaCWAVE winter campaign, which was conducted in northern Scandinavia during January 2003, are analyzed to investigate gravity wave characteristics in the stratosphere and mesosphere. There were two stratospheric warming events occurring during the campaign, one having a maximum temperature perturbation at ~45 km during 17–19 January, and the other having a maximum perturbation at ~30 km during 24–27 January. The former was a major event, whereas the latter was a minor one. Both warmings were accompanied by upper mesospheric coolings, and during the second warming, the upper mesospheric cooling propagated downward. Falling sphere data from the two salvos on 24–25 January and 28 January were analyzed for gravity wave characteristics. Gravity wave perturbations maximized at ~45–50 km, with a secondary maximum at ~60 km during Salvo 1; for Salvo 2, wave activity was most pronounced at ~60 km and above. Gravity wave horizontal propagation directions are estimated using the conventional hodographic analysis combined with the S-transform (a Gaussian wavelet analysis method). The results are compared with those from a Stokes analysis. They agree in general, though the former appears to provide better estimates for some cases, likely due to the capability of the S-transform to obtain robust estimates of wave amplitudes and phase differences between different fields. For Salvo 1 at ~60 km and above, gravity waves propagated towards the southeast, whereas for Salvo 2 at similar altitudes, waves propagated predominantly towards the northwest or west. These waves were found not to be topographic waves. Gravity wave motions at ~45–50 km in Salvo 1 were more complicated, but they generally had large amplitudes, short vertical scales, and their hodographs revealed a northwest-southeast orientation. In addition, the ratios between wave amplitudes and intrinsic phase speeds generally displayed a marked peak at ~45–50 km and decreased sharply at ~50 km, where the background winds were very weak. These results suggest that these wave motions were most likely topographic waves approaching their critical levels. Waves were more nearly isotropic in the lower stratosphere.

Images of mesospheric airglow and radar-wind measurements have been combined to estimate the difference in the vertical flux of horizontal momentum carried by high-frequency gravity waves over two dissimilar Antarctic stations. Rothera (67° S, 68° W) is situated in the mountains of the Peninsula near the edge of the wintertime polar vortex. In contrast, Halley (76° S, 27° W), some 1658 km to the southeast, is located on an ice sheet at the edge of the Antarctic Plateau and deep within the polar vortex during winter. The cross-correlation coefficients between the vertical and horizontal wind perturbations were calculated from sodium (Na) airglow imager data collected during the austral winter seasons of 2002 and 2003 at Rothera for comparison with the 2000 and 2001 results from Halley reported previously (Espy et al., 2004). These cross-correlation coefficients were combined with wind-velocity variances from coincident radar measurements to estimate the daily averaged upper-limit of the vertical flux of horizontal momentum due to gravity waves near the peak emission altitude of the Na nightglow layer, 90km. The resulting momentum flux at both stations displayed a large day-to-day variability and showed a marked seasonal rotation from the northwest to the southwest throughout the winter. However, the magnitude of the flux at Rothera was about 4 times larger than that at Halley, suggesting that the differences in the gravity-wave source functions and filtering by the underlying winds at the two stations create significant regional differences in wave forcing on the scale of the station separation.

A medium frequency spaced-antenna radar has been operating at Rothera station, Antarctica (67° S, 68° W) for two periods, between 1997-1998 and since 2002, measuring winds in the mesosphere and lower thermosphere. In this paper monthly mean winds are derived and presented along with three years of radiosonde balloon data for comparison with the HWM-93 model atmosphere and other high latitude southern hemisphere sites. The observed meridional winds are slightly more northwards than those predicted by the model above 80 km in the winter months and below 80 km in summer. In addition, the altitude of the summer time zero crossing of the zonal winds above the westward jet is overestimated by the model by up to 8 km. These data are then merged with the wind climatology obtained from falling sphere measurements made during the PORTA campaign at Rothera in early 1998 and the HWM-93 model atmosphere to generate a complete zonal wind climatology between 0 and 100 km as a benchmark for future studies at Rothera. A westwards (eastwards) maximum of 44 ms-1 at 67 km altitude occurs in mid December (62 ms-1 at 37 km in mid July). The 0 ms-1 wind contour reaches a maximum altitude of 90 km in mid November and a minimum altitude of 18 km in January extending into mid March at 75 km and early October at 76 km.

There have been reports in the literature that spaced-antenna MF radars may provide a source of data on Polar Mesospheric Summer Echoes (PMSE). Even though the expected scatter from PMSE at MF frequencies is very much weaker than at VHF, the wide distribution of sites and long duration of data sets for MF radar systems could provide valuable information about the occurrence of PMSE. This paper tests whether there is any evidence of PMSE in the profiles derived using the MF radar at Rothera, Antarctica, one of the few such radars at high southern latitudes. Over a year of data during 1997/1998 has been analysed for the occurrence of persistent features around midday in the altitude range 60-95km. Criteria were chosen to test the likelihood that some of the narrow peaks in the power profiles were manifestations of electron density structures associated with PMSE. Although a small number of persistent features were seen at altitudes of 80-85km that are typically associated with PMSE, there was no seasonality in their occurrence. A detailed analysis of specific days showed that two peaks were often seen with altitude separations consistent with the vertical wavelength of the diurnal tide. Persistent features were also detected at altitudes of 70km and 90km during the winter months, thus showing a quite different seasonality to that of PMSE. An estimate of the turbulence caused by the breaking of gravity waves that have propagated up from the lower atmosphere shows that at Rothera significant energy is deposited near 80km during summer, and near 70 and 90km during winter. This seasonal variability is driven by the screening effect of stratospheric winds, and it appears that breaking gravity wave dynamics, rather than PMSE phenomena, can explain many of the localised altitude features in the MF radar data.

The mesospheric and lower thermospheric (MLT) winds (60–100 km) obtained by multiple MF radars, located from the arctic to equator at Tromsø (70° N, 19° E), Saskatoon (52° N, 107° W), London (43° N, 81° W), Hawaii (21° N, 157° W) and Christmas Island (2° N, 157° W), respectively, are used to study the planetary-scale 16-day waves. Based on the simultaneous observations (1993/1994), the variabilities of the wave amplitudes, periods and phases are derived. At mid- and high-latitude locations the 16-day waves are usually pervasive in the winter-centred seasons (October through March), with the amplitude gradually decreasing with height. From the subtropical location to the equator, the summer wave activities become strong at some particular altitude where the inter-hemisphere wave ducts possibly allow for the leakage of the wave from the other hemispheric winter. The observational results are in good agreement with the theoretical conclusion that, for slowly westward-traveling waves, such as the 16-day wave, vertical propagation is permitted only in an eastward background flow of moderate speed which is present in the winter hemisphere. The wave period also varies with height and time in a range of about 12–24 days. The wave latitudinal differences and the vertical structures are compared with the Global Scale Wave Model (GSWM) for the winter situation. Although their amplitude variations and profiles have a similar tendency, the discrepancies are considerable. For example, the maximum zonal amplitude occurs around 40° N for radar but 30° N for the model. The phase differences between sites due to the latitudinal effect are basically consistent with the model prediction of equatorward phase-propagation. The global 16-day waves at 95 km from the HRDI wind measurements during 1992 through 1995 are also displayed. Again, the wave is a winter dominant phenomenon with strong amplitude around the 40–60° latitude-band on both hemispheres.Key words. Meteorology and atmospheric dynamics – waves and tides – middle atmosphere dynamics – thermospheric dynamics

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