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  • 1
    Electronic Resource
    Electronic Resource
    The influence of time-dependent wind on gravity-wave propagation in the middle atmosphere (1995)
    Springer
    Annales geophysicae 13 (1995), S. 375-394 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Ray-tracing techniques are used to computationally investigate the propagation of gravity waves through the middle atmosphere, as characterized by the vertically varying CIRA-86 wind and temperature models, plus a tidal wind model that varies temporally as well as vertically. For the wave parameters studied here, the background wind variation has a much stronger influence on the ray path and changes in wave characteristics than does the temperature variation. The temporal variation of the tidal component of the wind changes the observed frequency, sometimes substantially, while leaving the intrinsic frequency unaltered. It also renders temporary any critical levels that occur in the tidal region. Different starting times for the rays relative to the tidal phase provide different propagation environments, so that the temporary critical levels appear at different heights. The lateral component of the tidal wind is shown to advect propagating wave packets; the maximum lateral displacement of a packet varies inversely with its vertical group velocity. Time-dependent effects are more pronounced in local winter than in summer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-995-0375-6
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  • 2
    Electronic Resource
    Electronic Resource
    Climatological monthly characteristics of middle atmosphere gravity waves (10 min-10 h) during 1979–1993 at Saskatoon (1995)
    Springer
    Annales geophysicae 13 (1995), S. 285-295 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Saskatoon (52° N, 107°W) medium frequency (MF) radar data from 1979 to 1993 have been analyzed to investigate the climatology of irregular wind components in the height region 60–100 km. This component is usually treated in terms of internal gravity waves (IGW). Three different band-pass filters have been used to separate the intensities of IGWs having periods 0.2-2.5; 1.5-6 and 2–10 h, respectively. Height, seasonal and inter-annual variations of IGW intensities, anisotropy and predominant directions of propagation are investigated. Mean over 14 years’ seasonal variation of the intensity of long-period IGWs shows a dominant annual component with winter maximum and summer minimum. Seasonal variations of the intensity of short-period waves have a strong semi-annual component as well, which forms a secondary maximum in summer. Predominant azimuths of long-period IGWs are generally zonal, though they vary with season. For short-period IGWs, the predominant azimuth is closer to the meridional direction. Anisotropy of IGW intensity is larger in summer, winter and at lower altitudes. The IGW intensity shows apparent correlation with both solar and geomagnetic activity. In most cases, this correlation appears to be negative. The variations versus solar activity is larger for longer-period IGW. Possible reasons and consequences of the observed climatological variations of IGW intensity are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-995-0285-7
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  • 3
    Electronic Resource
    Electronic Resource
    Simultaneous observations of the 2-day wave at London (43°N, 81°W) and Saskatoon (52°N, 107°W) near 91 km altitude during the two years of 1993 and 1994 (1997)
    Springer
    Annales geophysicae 15 (1997), S. 1324-1339 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Simultaneous observations are valuable in providing further insights into the character of the quasi 2-day wave. In this study we investigate the period and amplitude for the quasi 2-day wave near 91 km using MF radars at London and Saskatoon, Canada, and in addition look at possible values of the zonal wave number. The results of the present study bring out certain new aspects of the quasi 2-day wave at mid-latitudes in the Northern Hemisphere. In particular we show that the period of the quasi 2-day wave determined from our study (specially at large amplitudes) is smaller (46-47 h) than the 51-52 h period often suggested by other Northern Hemisphere results, and that the periods also showed variability as a function of time. We also draw attention to the annual variability, and especially highlight the occurrence of the wave in non-summer months. Our observations show significant wave correlation between the London and Saskatoon sites during time intervals of strong 2-day wave activity. These results suggest that the 2-day waves of 1993/4 are westward propagating waves of zonal wave number 3, although sometimes the zonal wave number 5 is also indicated (specially at large amplitudes). Our study also contributes additional mid-latitude geographical data which should aid in developing a better picture of the quasi 2-day wave.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-997-1324-3
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  • 4
    Electronic Resource
    Electronic Resource
    An intercomparison between the GSWM, UARS, and ground based radar observations: a case-study in January 1993 (1997)
    Springer
    Annales geophysicae 15 (1997), S. 1123-1141 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The Global-Scale Wave Model (GSWM) is a steady-state two-dimensional linearized model capable of simulating the solar tides and planetary waves. In an effort to understand the capabilities and limitations of the GSWM throughout the upper mesosphere and thermosphere a comparative analysis with observational data is presented. A majority of the observational data used in this study was collected during the World Day campaign which ran from 20 January to 30 January 1993. During this campaign data from 18 ground-based observational sites across the globe and two instruments located on the UARS spacecraft were analyzed. Comparisons of these data with the simulations from the GSWM indicate that the GSWM results are in reasonable agreement with the observations. However, there are a number of cases where the agreement is not particularly good. One such instance is for the semidiurnal tide in the northern hemisphere, where the GSWM estimates may exceed observations by 50%. Through a number of numerical simulations, it appears that this discrepancy may be due to the eddy diffusivity profiles used by the GSWM. Other differences relating to the diurnal tide and the quasi-two-day wave are presented and discussed. Additionally, a discussion on the biases and aliasing difficulties which may arise in the observational data is alos presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-997-1123-x
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  • 5
    Electronic Resource
    Electronic Resource
    Spectral characteristics of spring arctic mesosphere dynamics (1998)
    Springer
    Annales geophysicae 16 (1998), S. 1607-1618 
    Details
    ISSN: 0992-7689
    Keywords: Meteorology and atmospheric dynamics (middle atmosphere dynamics; turbulence; waves and tides)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The spring of 1997 has represented a stable period of operation for the joint University of Tromsø/University of Saskatchewan MF radar, being between refurbishment and upgrades. We examine the horizontal winds from the February to June inclusive and also include estimates of energy dissipation rates derived from signal fading times and presented as upper limits on the turbulent energy dissipation rate, ɛ. Here we address the periodicity in the dynamics of the upper mesosphere for time scales from hours to one month. Thus, we are able to examine the changes in the spectral signature of the mesospheric dynamics during the transition from winter to summer states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-998-1607-3
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  • 6
    Electronic Resource
    Electronic Resource
    PMSE observations at three different frequencies in northern Europe during summer 1994 (1997)
    Springer
    Annales geophysicae 14 (1997), S. 1317-1327 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Simultaneous observations of polar mesosphere summer echoes (PMSE) have been carried out during summer 1994 in northern Norway using three radars on different frequencies: the ALOMAR SOUSY radar at Andenes on 53.5 MHz, the EISCAT VHP radar at Tromsø on 224 MHz and the MF radar at Tromsø on 2.78 MHz. During the common measuring period in July/August 1994, PMSE could be detected at 224 and 53.5 MHz, and there are strong hints that PMSE also occur at 2.78 MHz. Reliable correlations between hourly backscattered power values indicate that the PMSE structures have zonal extensions of more than 130 km and can be detected at very different scales (half wavelength) between 0.67 (EISCAT VHP radar) and 54 m (MF radar). Using the wind values derived by the MF radar it can be shown that the mesospheric wind field influences the structure of PMSE. The diurnal variation of PMSE is strongly connected with tidal-wind components, whereas spatial differences of PMSE can partly be explained by the mean wind field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-996-1317-7
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  • 7
    Electronic Resource
    Electronic Resource
    Gravity wave exclusion circles in background flows modulated by the semidiurnal tide (1996)
    Springer
    Annales geophysicae 14 (1996), S. 557-565 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract In this short paper the exclusion circles and vertical phase locities for gravity waves launched from the ground into a time-varying wind are studied using a ray-tracing technique. It is shown that waves with initial observed phase speeds that should place them within the local temporally varying exclusion circle, are often Doppler shifted outside of the circle. This, and the finite lifetime of some critical levels, allow waves to survive the critical layer and reach higher altitudes. Also, for slower phase-speed waves, the temporally varying wind can shift the observed frequency to negative values, so that the observed phase motions will be opposite (i.e. horizontally reversed and vertically upward), even though the energy still propagates upward. This effect can also cause the phase velocity to move inside the local exclusion circle. Due to the directional filtering of wave sources by the stratospheric wind, the percentage of such reverse-propagating waves will change systematically with local time and height in our simplified but realistic model. These results are related to ground-based systems, optical and radar, which sample the wind field and gravity waves in the middle atmosphere.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-996-0557-x
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  • 8
    Electronic Resource
    Electronic Resource
    Extension of MF radar tidal measurements to E-region heights (95-125 km): Saskatoon (52○N, 107○W), Canada (1994)
    Springer
    Annales geophysicae 12 (1994), S. 333-341 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Efforts have been made to extend the MF radar tidal profiles to E-region heights. The totally reflected MF radar echoes from E-region heights during daytime are known to be group-retarded and the corresponding wind and tidal data will have associated height discrepancies. The estimation of the E-region real heights (Namboothiri et al., 1993), and the elimination of the data for which the group retardation is significant, are selected as the basic criteria to extend the tidal profiles to 100–125 km. The analysis of the quiet (Ap〈19) days of the winter and summer seasons of 1988/89 shows that the tidal propagation continues to higher altitudes with some changes in their pattern, e.g. longer wavelengths, compared to that in the lower altitudes. Comparison with the model profiles shows some resemblance and some disagreements. The reliability of the MF radar tidal measurements of E-region heights and the propagation of tides in this region have been discussed in the light of existing theories and other experimental observations. It is concluded that, based on the initial studies with UHF and MF systems and within the limits of the available theories, the information on tides presented here for the 100–125 km region using the MF radar observations is useful. Suggestions for future work in this direction are also given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-994-0333-8
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  • 9
    Electronic Resource
    Electronic Resource
    N2 and M2 lunar tides: atmospheric resonance revisited (1996)
    Springer
    Annales geophysicae 14 (1996), S. 826-836 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A numerical model has been used to calculate the atmospheric response to forcing at periods in the region of 12-13.5 h. The results show that the response is enhanced in the neighbourhood of 13 h. These results have been compared with lunar tidal analyses of mesospheric wind data and geomagnetic variations at a number of stations. It is found that the N2 lunar tidal component (period 12.66 h) is significantly enhanced relative to the main lunar tidal component M2 (period 12.42 h) in both types of data, compared with what would be expected from the gravitational tidal potential. This supports the predictions of the numerical model. An appreciable phase shift is also found in the experimental data between the N2 and M2 tides, agreeing in sense with what would be expected for a resonance at a period around 13 h.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-996-0826-8
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  • 10
    Electronic Resource
    Electronic Resource
    Zonal mean and tidal dynamics from space: an empirical examination of aliasing and sampling issues (1997)
    Springer
    Annales geophysicae 15 (1997), S. 1158-1164 
    Details
    ISSN: 0992-7689
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Interpretations of space-based measurements of atmospheric parameters in the mesosphere and thermosphere are complicated by large local-time variations at these altitudes. For this reason, satellite orbits are often preferred which precess through all local times one or more times per season. However, the local-time structure of the atmosphere is inherently non-stationary, which can lead to sampling and aliasing difficulties when attempting to deconvolve the measurements into zonal mean and tidal components. In the present study, hourly radar measurements of mesopause-region winds are used to form a mock data base which can be used to gain insight into implications of the aforementioned problems; the use of actual measurements introduces a realistic element of geophysical temporal variability. Assuming zonal symmetry (i.e., migrating tides superimposed on a zonal mean circulation), the radar measurements are sampled from the satellite perspective for orbital inclinations of 57° and 70°, and compared to the ground or true perspective. These comparisons provide realistic estimates of the errors to be expected when attempting to derive mean and tidal components from space-based measurements. For both diurnal and semidiurnal components, and the quoted satellite inclinations, acceptable errors (3–4 m/s rms) are obtained for data covering 24 h local time (i.e., ascending plus descending nodes); the corresponding errors for singlenode data (12 h local-time coverage) are of order 8–11 m/s, and therefore may not represent reliable estimates of the actual tidal components. There exist certain caveats in connection with the latter conclusion which are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s00585-997-1158-z
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