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Mbatha, N., V. Sivakumar, S. B. Malinga, H. Bencherif i S. R. Pillay. "Study on the impact of sudden stratosphere warming in the upper mesosphere-lower thermosphere regions using satellite and HF radar measurements". Atmospheric Chemistry and Physics Discussions 9, nr 6 (2.11.2009): 23051–72. http://dx.doi.org/10.5194/acpd-9-23051-2009.
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Abstract. The occurrence of sudden stratospheric warming (SSW) excites disturbances in the mesosphere-lower thermospheric (MLT) wind and temperature. Here, we have examined the high frequency (HF) radar wind data from the South African National Antarctic Expedition, SANAE (72° S, 3° W), a radar which is part of the Super Dual Auroral Radar Network (SuperDARN). Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on board the Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics (TIMED) satellite temperature data and National Centre for Environmental Prediction (NCEP) temperature and wind data were use to investigate the dynamical effects of the unprecedented September 2002 SSW in the Antarctica stratosphere and MLT. The mean zonal wind (from SANAE HF radar) at the MLT shows reversal in approximately 7 days before the reversal at 10 hPa (from NCEP). This indicates that there was a downwards propagation of circulation disturbance. Westerly zonal winds dominate the winter MLT, but during the 2002 winter there were many periods of westward winds observed compared to other years. The dynamic spectrums of both meridional and zonal winds show presence of planetary waves (of ~14-day period) before the occurrence of the SSW. The SABER vertical temperature profiles indicated the cooling of the MLT region before the SSW event.
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Mbatha, N., V. Sivakumar, S. B. Malinga, H. Bencherif i S. R. Pillay. "Study on the impact of sudden stratosphere warming in the upper mesosphere-lower thermosphere regions using satellite and HF radar measurements". Atmospheric Chemistry and Physics 10, nr 7 (12.04.2010): 3397–404. http://dx.doi.org/10.5194/acp-10-3397-2010.
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Abstract. The occurrence of a sudden stratospheric warming (SSW) excites disturbances in the mesosphere-lower thermospheric (MLT) wind and temperature. Here, we have examined the high frequency (HF) radar wind data from the South African National Antarctic Expedition, SANAE (72° S, 3° W), a radar which is part of the Super Dual Auroral Radar Network (SuperDARN). Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on board the Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics (TIMED) satellite temperature data and National Centre for Environmental Prediction (NCEP) temperature and wind data are used to investigate the dynamical effects of the unprecedented September 2002 SSW in the Antarctica stratosphere and MLT. The mean zonal wind (from SANAE HF radar) at the MLT shows reversal approximately 7 days before the reversal at 10 hPa (from NCEP). This indicates that there was a downwards propagation of circulation disturbance. Westerly zonal winds dominate the winter MLT, but during the 2002 winter there are many periods of westward winds observed compared to other years. The normalised power spectrums of both meridional and zonal winds show presence of planetary waves (of ~14-day period) before the occurrence of the SSW. The SABER vertical temperature profiles indicated the cooling of the MLT region before the SSW event.
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Portnyagin, Y. I., T. V. Solovjova, N. A. Makarov, E. G. Merzlyakov, A. H. Manson, C. E. Meek, W. Hocking i in. "Monthly mean climatology of the prevailing winds and tides in the Arctic mesosphere/lower thermosphere". Annales Geophysicae 22, nr 10 (3.11.2004): 3395–410. http://dx.doi.org/10.5194/angeo-22-3395-2004.
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Abstract. The Arctic MLT wind regime parameters measured at the ground-based network of MF and meteor radar stations (Andenes 69° N, Tromsø 70° N, Esrange 68° N, Dixon 73.5° N, Poker Flat 65° N and Resolute Bay 75° N) are discussed and compared with those observed in the mid-latitudes. The network of the ground-based MF and meteor radars for measuring winds in the Arctic upper mesosphere and lower thermosphere provides an excellent opportunity for study of the main global dynamical structures in this height region and their dependence from longitude. Preliminary estimates of the differences between the measured winds and tides from the different radar types, situated 125-273km apart (Tromsø, Andenes and Esrange), are provided. Despite some differences arising from using different types of radars it is possible to study the dynamical wind structures. It is revealed that most of the observed dynamical structures are persistent from year to year, thus permitting the analysis of the Arctic MLT dynamics in a climatological sense. The seasonal behaviour of the zonally averaged wind parameters is, to some extent, similar to that observed at the moderate latitudes. However, the strength of the winds (except the prevailing meridional wind and the diurnal tide amplitudes) in the Arctic MLT region is, in general, less than that detected at the moderate latitudes, decreasing toward the pole. There are also some features in the vertical structure and seasonal variations of the Arctic MLT winds which are different from the expectations of the well-known empirical wind models CIRA-86 and HWM-93. The tidal phases show a very definite longitudinal dependence that permits the determination of the corresponding zonal wave numbers. It is shown that the migrating tides play an important role in the dynamics of the Arctic MLT region. However, there are clear indications with the presence in some months of non-migrating tidal modes of significant appreciable amplitude.
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Griffith, Matthew J., Shaun M. Dempsey, David R. Jackson, Tracy Moffat-Griffin i Nicholas J. Mitchell. "Winds and tides of the Extended Unified Model in the mesosphere and lower thermosphere validated with meteor radar observations". Annales Geophysicae 39, nr 3 (10.06.2021): 487–514. http://dx.doi.org/10.5194/angeo-39-487-2021.
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Abstract. The mesosphere and lower thermosphere (MLT) is a critical region that must be accurately reproduced in general circulation models (GCMs) that aim to include the coupling between the lower and middle atmosphere and the thermosphere. An accurate representation of the MLT is thus important for improved climate modelling and the development of a whole atmosphere model. This is because the atmospheric waves at these heights are particularly large, and so the energy and momentum they carry is an important driver of climatological phenomena through the whole atmosphere, affecting terrestrial and space weather. The Extended Unified Model (ExUM) is the recently developed version of the Met Office's Unified Model which has been extended to model the MLT. The capability of the ExUM to model atmospheric winds and tides in the MLT is currently unknown. Here, we present the first study of winds and tides from the ExUM. We make a comparison against meteor radar observations of winds and tides from 2006 between 80 and 100 km over two radar stations – Rothera (68∘ S, 68∘ W) and Ascension Island (8∘ S, 14∘ W). These locations are chosen to study tides in two very different tidal regimes – the equatorial regime, where the diurnal (24 h) tide dominates, and the polar regime, where the semi-diurnal (12 h) tide dominates. The results of this study illustrate that the ExUM is capable of reproducing atmospheric winds and tides that capture many of the key characteristics seen in meteor radar observations, such as zonal and meridional wind maxima and minima, the increase in tidal amplitude with increasing height, and the decrease in tidal phase with increasing height. In particular, in the equatorial regime some essential characteristics of the background winds, tidal amplitudes and tidal phases are well captured but with significant differences in detail. In the polar regime, the difference is more pronounced. The ExUM zonal background winds in austral winter are primarily westward rather than eastward, and in austral summer they are larger than observed above 90 km. The ExUM tidal amplitudes here are in general consistent with observed values, but they are also larger than observed values above 90 km in austral summer. The tidal phases are generally well replicated in this regime. We propose that the bias in background winds in the polar regime is a consequence of the lack of in situ gravity wave generation to generate eastward fluxes in the MLT. The results of this study indicate that the ExUM has a good natural capability for modelling atmospheric winds and tides in the MLT but that there is room for improvement in the model physics in this region. This highlights the need for modifications to the physical parameterization schemes used in the model in this region – such as the non-orographic spectral gravity wave scheme – to improve aspects such as polar circulation. To this end, we make specific recommendations of changes that can be implemented to improve the accuracy of the ExUM in the MLT.
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Rokade, M. V., R. Kondala Rao, S. S. Nikte, R. N. Ghodpage, P. T. Patil, A. K. Sharma i S. Gurubaran. "Intraseasonal oscillation (ISO) in the MLT zonal wind over Kolhapur (16.8° N) and Tirunelveli (8.7° N)". Annales Geophysicae 30, nr 12 (5.12.2012): 1623–31. http://dx.doi.org/10.5194/angeo-30-1623-2012.
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Abstract. Simultaneous observations of the mean zonal winds at 88 km obtained by the medium-frequency (MF) radars at Kolhapur (16.8° N, 74.2° E) and Tirunelveli (8.7° N, 77.8° E) have been used to study the intraseasonal oscillation (ISO) in the MLT region. The influences of the intraseasonal variations in the lower tropospheric convective activity associated with the Madden-Julian oscillations on the latitudinal behavior of intraseasonal oscillations (ISO) of the zonal winds in the equatorial mesosphere and lower thermosphere (MLT) have been studied. The ISO activity in the lower tropospheric convective activity is examined by employing outgoing long wave radiation (OLR) as a proxy for deep convective activity occurring in the tropical lower atmosphere. The ISO activity in the zonal wind over TIR is more correlated with that in the convective activity compared to the ISO over KOL. The latitudinal and temporal variabilities of the ISO in MLT zonal winds are explained in terms of the intraseasonal variabilities in the convective activity.
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Hasebe, F., T. Tsuda, T. Nakamura i M. D. Burrage. "Validation of HRDI MLT winds with meteor radars". Annales Geophysicae 15, nr 9 (30.09.1997): 1142–57. http://dx.doi.org/10.1007/s00585-997-1142-7.
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Abstract. A validation study of the mesospheric and lower-thermospheric (MLT) wind velocities measured by the High-Resolution Doppler Imager (HRDI) on board the Upper-Atmosphere Research Satellite (UARS) has been carried out, comparing with observations by meteor radars located at Shigaraki, Japan and Jakarta, Indonesia. The accuracy of the HRDI winds relative to the meteor radars is obtained by a series of simultaneous wind measurements at the time of UARS overpasses. Statistical tests on the difference in the wind vectors observed by HRDI and the meteor radars are applied to determine whether the wind speed has been overestimated by HRDI (or underestimated by the MF radars) as previously noticed in HRDI vs. MF radar comparisons. The techniques employed are the conventional t-test applied to the mean values of the paired wind vector components as well as wind speeds, and two nonparametric tests suitable for testing the paired wind speed. The square-root transformation has been applied before the t-tests of the wind speed in order to fit the wind-speed distribution function to the normal distribution. The overall results show little evidence of overestimation by HRDI (underestimation by meteor radars) of wind velocities in the MLT region. Some exceptions are noticed, however, at the altitudes around 88 km, where statistical differences occasionally reach a level of significance of 0.01. The validation is extended to estimate the precision of the wind velocities by both HRDI and meteor radars. In the procedure, the structure function defined by the mean square difference of the observed anomalies is applied in the vertical direction for the profile data. This method assumes the isotropy and the homogeneity of variance for the physical quantity and the homogeneity of variance for the observational errors. The estimated precision is about 6ms–1 for the Shigaraki meteor radar, 15ms–1 for the Jakarta meteor radar, and 20ms–1 for HRDI at 90-km altitude. These values can be used to confirm the statistical significance of the wind field obtained by averaging the observed winds.
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Jacobi, Christoph, Tatiana Ermakova, Daniel Mewes i Alexander I. Pogoreltsev. "El Niño influence on the mesosphere/lower thermosphere circulation at midlatitudes as seen by a VHF meteor radar at Collm (51.3 ° N, 13 ° E)". Advances in Radio Science 15 (21.09.2017): 199–206. http://dx.doi.org/10.5194/ars-15-199-2017.
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Abstract. Mesosphere/lower thermosphere (MLT) zonal winds continuously measured by a VHF meteor radar at Collm, Germany (51.3° N, 13.0° E) in the height range 82 – 97 km from 2004 to date are analyzed with respect to the signature of El Niño. The comparison of Niño3 equatorial SST index and MLT wind time series shows that in January and especially in February zonal winds are positively correlated with the Niño3 index. We note a delay of about one month of the MLT zonal wind effect with respect to equatorial sea surface temperature variability. The signal is strong for the upper altitudes (above 90 km) accessible to the radar observations, but weakens with decreasing height. This reflects the fact that during El Niño years the westerly winter middle atmosphere wind jet is weaker, and this is also the case with the easterly lower thermospheric jet. Owing to the reversal of the absolute El Niño signal from negative to positive with altitude, at the height of the maximum meteor flux, which is around 90 km, the El Niño signal is weak. The experimental results can be qualitatively reproduced by numerical experiments using a mechanistic global circulation model with prescribed tropospheric temperatures and latent heat release for El Niño and La Niña conditions.
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Naniwadekar, G. P., S. Gurubaran, A. P. Jadhav, R. N. Ghodpage, P. T. Patil i D. S. Burud. "Studies on the variability of mean winds in the mesosphere and lower thermosphere region (MLT) over Kolhapur (16.8oN, 74.2oE)". Journal of Geomatics 17, nr 1 (28.04.2023): 93–100. http://dx.doi.org/10.58825/jog.2023.17.1.78.
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We present the study of mesospheric winds in the 78–98 km height range using observations by a partial reflection radar station (MF–radar) situated at Kolhapur (16.8° N, 74.2° E), India. The sequential wind profiles over the period of 2014–2019 obtained from this radar operated at 1.98 MHz are used for this study. To delineate the behaviour of the winds in the mesosphere and lower thermosphere (MLT) region, we use wind data providing horizontal wind velocities averaged for an hour. Details of the seasonal, annual, and inter-annual variations and also the climatology of mean motion in zonal (East-West) and meridional (North-South) components in the MLT region over the aforementioned period are presented. The zonal wind below 90 km has been observed with eastward flow for the period of solstices and westward flow at equinoxes, showing strong semi-annual oscillations (SAO). While above 90 km, annual oscillations (AO) are seen to be dominant. Annual oscillations (AO) are observed in the mean meridional wind, with poleward motion during winter and equatorward motion during the remaining seasons. At higher altitudes (above 92 km), the poleward motion weakens and the equatorward wind flow becomes strong.
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Middleton, H. R., N. J. Mitchell i H. G. Muller. "Mean winds of the mesosphere and lower thermosphere at 52° N in the period 1988–2000". Annales Geophysicae 20, nr 1 (31.01.2002): 81–91. http://dx.doi.org/10.5194/angeo-20-81-2002.
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Abstract. A meteor radar in the UK (near 52° N) has been used to measure the mean winds of the mesosphere/lower-thermosphere (MLT) region over the period 1988–2000. The seasonal course and interannual variability is characterised and comparisons are made with a number of models. Annual mean wind trends were found to be + 0.37 ms-1 yr-1 for the zonal component and + 0.157 ms-1 yr-1 for the meridional component. Seasonal means revealed significant trends in the case of meridional winds in spring ( + 0.38 ms-1 yr-1) and autumn ( + 0.29 ms-1 yr-1), and zonal winds in summer ( + 0.48 ms-1 yr-1) and autumn ( + 0.38 ms-1 yr-1). Significant correlation coefficients, R, between the sunspot number and seasonal mean wind are found in four instances. In the case of the summer zonal winds, R = + 0.732; for the winter meridional winds, R = - 0.677; for the winter zonal winds, R = - 0.472; and for the autumn zonal winds R = + 0.508.Key words. Meteorology and atmospheric dynamics (climatology; general circulation; middle atmospheric dynamics)
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Stober, Gunter, Alan Liu, Alexander Kozlovsky, Zishun Qiao, Witali Krochin, Guochun Shi, Johan Kero i in. "Identifying gravity waves launched by the Hunga Tonga–Hunga Ha′apai volcanic eruption in mesosphere/lower-thermosphere winds derived from CONDOR and the Nordic Meteor Radar Cluster". Annales Geophysicae 41, nr 1 (18.04.2023): 197–208. http://dx.doi.org/10.5194/angeo-41-197-2023.
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Abstract. The Hunga Tonga–Hunga Ha′apai volcano eruption was a unique event that caused many atmospheric phenomena around the globe. In this study, we investigate the atmospheric gravity waves in the mesosphere/lower-thermosphere (MLT) launched by the volcanic explosion in the Pacific, leveraging multistatic meteor radar observations from the Chilean Observation Network De Meteor Radars (CONDOR) and the Nordic Meteor Radar Cluster in Fennoscandia. MLT winds are computed using a recently developed 3DVAR+DIV algorithm. We found eastward- and westward-traveling gravity waves in the CONDOR zonal and meridional wind measurements, which arrived 12 and 48 h after the eruption, and we found one in the Nordic Meteor Radar Cluster that arrived 27.5 h after the volcanic detonation. We obtained observed phase speeds for the eastward great circle path at both locations of about 250 m s−1, and they were 170–150 m s−1 for the opposite propagation direction. The intrinsic phase speed was estimated to be 200–212 m s−1. Furthermore, we identified a potential lamb wave signature in the MLT winds using 5 min resolved 3DVAR+DIV retrievals.
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Jaen, Juliana, Toralf Renkwitz, Jorge L. Chau, Maosheng He, Peter Hoffmann, Yosuke Yamazaki, Christoph Jacobi, Masaki Tsutsumi, Vivien Matthias i Chris Hall. "Long-term studies of mesosphere and lower-thermosphere summer length definitions based on mean zonal wind features observed for more than one solar cycle at middle and high latitudes in the Northern Hemisphere". Annales Geophysicae 40, nr 1 (20.01.2022): 23–35. http://dx.doi.org/10.5194/angeo-40-23-2022.
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Abstract. 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 the PRR and MLS. Under both definitions, the summer begins around April and ends around middle 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 oscillation (QBO), El Niño–Southern Oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at middle 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.
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Day, K. A., i N. J. Mitchell. "Mean winds in the MLT, the SQBO and MSAO over Ascension Island (8° S, 14° W)". Atmospheric Chemistry and Physics 13, nr 18 (27.09.2013): 9515–23. http://dx.doi.org/10.5194/acp-13-9515-2013.
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Abstract. Mean winds in the mesosphere and lower thermosphere (MLT) over Ascension Island (8° S, 14° W) have been measured at heights of approximately 80–100 km by a meteor radar. The results presented in this study are from the interval October 2001 to December 2011. In all years, the monthly-mean meridional winds display a clear annual oscillation. Typically, these winds are found to be southward during April–October, when they reach velocities of up to about −23 m s−1, and northward throughout the rest of the year, when they reach velocities up to about 16 m s−1. The monthly-mean zonal winds are generally westward throughout most of the year and reach velocities of up to about −46 m s−1. However, eastward winds are observed in May–August and again in December at the lower heights observed. These eastward winds reach a maximum at heights of about 86 km with velocities of up to about 36 m s−1, but decay quickly at heights above and below that level. The mesospheric semi-annual oscillation (MSAO) is clearly apparent in the observed monthly-mean zonal winds. The winds in first westward phase of the MSAO are observed to be much stronger than in the second phase. The westward phase of the MSAO is found to maximise at heights of about 84 km with typical first-phase wind velocities reaching about −35 m s−1. These meteor-radar observations have been compared to the HWM-07 empirical model. The observed meridional winds are found to be generally more southward than those of the model during May–August, when at the lower heights observed the model suggests there will be only weakly southward, or even northward, winds. The zonal monthly-mean winds are in generally good agreement, although in the model they are somewhat less westward than those observed. Throughout the observations there were eight occasions in which the first westward phase of the MSAO was observed. Strikingly, in 2002 there was an event in which the westward winds during the first phase of the MSAO were much stronger than normal and reached velocities of about −75 m s−1. This event is explained in terms of a previously proposed mechanism in which the relative phasing of the stratospheric quasi-biennial oscillation (SQBO) and the MSAO allows an unusually large flux of gravity waves of large westward phase speed to reach the mesosphere. It is the dissipation of these gravity waves that then drives the MLT winds to the large westward velocities observed. It is demonstrated that the necessary SQBO–MSAO phase relationship did indeed exist during 2002, but not during the other years observed here. This demonstration provides strong support for the suggestion that extreme zonal-wind events during the MSAO result from the modulation of gravity-wave fluxes.
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Kishore Kumar, G., i W. K. Hocking. "Climatology of northern polar latitude MLT dynamics: mean winds and tides". Annales Geophysicae 28, nr 10 (7.10.2010): 1859–76. http://dx.doi.org/10.5194/angeo-28-1859-2010.
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Abstract. Mean winds and tides in the northern polar Mesosphere and Lower Thermosphere (MLT) have been studied using meteor radars located at Resolute Bay (75° N, 95° W) and Yellowknife (62.5° N, 114.3° W). The measurements for Resolute Bay span almost 12 years from July 1997 to February 2009 and the Yellowknife data cover 7 years from June 2002 to October 2008. The analysis reveals similar wind flow over both sites with a difference in magnitude. The summer zonal flow is westward at lower heights, eastward at upper heights and the winter zonal flow is eastward at all heights. The winter meridional flow is poleward and sometimes weakly equatorward, while non winter months show equatorward flow, with a strong equatorward jet during mid-summer months. The zonal and meridional winds show strong interannual variation with a dominant annual variation as well as significant latitudinal variation. Year to year variability in both zonal and meridional winds exists, with a possible solar cycle dependence. The diurnal, semidiurnal and terdiurnal tides also show large interannual variability and latitudinal variation. The diurnal amplitudes are dominated by an annual variation. The climatological monthly mean winds are compared with CIRA 86, GEWM and HWM07 and the climatological monthly mean amplitudes and phases of diurnal and semidiurnal tides are compared with GSWM00 predictions. The GEWM shows better agreement with observations than the CIRA 86 and HWM07. The GSWM00 model predictions need to be modified above 90 km. The agreements and disagreements between observations and models are discussed.
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Altadill, D., E. M. Apostolov, Ch Jacobi i N. J. Mitchell. "Six-day westward propagating wave in the maximum electron density of the ionosphere". Annales Geophysicae 21, nr 7 (31.07.2003): 1577–88. http://dx.doi.org/10.5194/angeo-21-1577-2003.
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Abstract. Analyses of time-spatial variations of critical plasma frequency foF2 during the summer of 1998 reveal the existence of an oscillation activity with attributes of a 6-day westward propagating wave. This event manifests itself as a global scale wave in the foF2 of the Northern Hemisphere, having a zonal wave number 2. This event coincides with a 6-day oscillation activity in the meridional neutral winds of the mesosphere/lower thermosphere (MLT). The oscillation in neutral winds seems to be linked to the 6–7-day global scale unstable mode westward propagating wave number 1 in the MLT. The forcing mechanisms of the 6-day wave event in the ionosphere from the wave activity in the MLT are discussed.Key words. Ionosphere (Ionosphere-Atmosphere interactions; Mid-latitude Ionosphere) – Meterology and atmospheric dynamics (waves and tides)
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Singer, W., J. Bremer, P. Hoffmann, A. H. Manson, C. E. Meek, R. Schminder, D. Kürschner i in. "Geomagnetic influences upon tides—winds from MLT radars". Journal of Atmospheric and Terrestrial Physics 56, nr 10 (sierpień 1994): 1301–11. http://dx.doi.org/10.1016/0021-9169(94)90068-x.
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Liu, X., J. Xu, H. L. Liu, J. Yue i W. Yuan. "Simulations of large winds and wind shears induced by gravity wave breaking in the mesosphere and lower thermosphere (MLT) region". Annales Geophysicae 32, nr 5 (23.05.2014): 543–52. http://dx.doi.org/10.5194/angeo-32-543-2014.
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Abstract. Using a fully nonlinear two-dimensional (2-D) numerical model, we simulated gravity waves (GWs) breaking and their contributions to the formation of large winds and wind shears in the mesosphere and lower thermosphere (MLT). An eddy diffusion coefficient is used in the 2-D numerical model to parameterize realistic turbulent mixing. Our study shows that the momentum deposited by breaking GWs accelerates the mean wind. The resultant large background wind increases the GW's apparent horizontal phase velocity and decreases the GW's intrinsic frequency and vertical wavelength. Both the accelerated mean wind and the decreased GW vertical wavelength contribute to the enhancement of wind shears. This, in turn, creates a background condition that favors the occurrence of GW instability, breaking, and momentum deposition, as well as mean wind acceleration, which further enhances the wind shears. We find that GWs with longer vertical wavelengths and faster horizontal phase velocity can induce larger winds, but they may not necessarily induce larger wind shears. In addition, the background temperature can affect the time and height of GW breaking, thus causing accelerated mean winds and wind shears.
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Jacobi, C. "Low-frequency lower E-region wind and reflection height measurements as sensor for climate variability". Advances in Radio Science 6 (21.07.2008): 331–35. http://dx.doi.org/10.5194/ars-6-331-2008.
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Abstract. Measurements of reflection heights of low-frequency (LF) radio waves at oblique incidence and estimates of mesosphere/lower thermosphere (MLT) region horizontal winds applying the D1 spaced receiver method on LF field strength registrations are analyzed with respect to possible long-term trends and interdecadal variability in the time interval from ~1980 to date. While no clear signal of mesospheric height trend is registered during the last two decades, significant trends of MLT horizontal winds are found. These trends are non-linear, in particular a change of trends around 1990 is found, which is probably connected with changes in tropospheric and stratospheric conditions at that time.
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Hindley, Neil P., Nicholas J. Mitchell, Neil Cobbett, Anne K. Smith, Dave C. Fritts, Diego Janches, Corwin J. Wright i Tracy Moffat-Griffin. "Radar observations of winds, waves and tides in the mesosphere and lower thermosphere over South Georgia island (54° S, 36° W) and comparison with WACCM simulations". Atmospheric Chemistry and Physics 22, nr 14 (22.07.2022): 9435–59. http://dx.doi.org/10.5194/acp-22-9435-2022.
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Abstract. 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.
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Jacobi, Christoph, Friederike Lilienthal, Dmitry Korotyshkin, Evgeny Merzlyakov i Gunter Stober. "Influence of geomagnetic disturbances on mean winds and tides in the mesosphere/lower thermosphere at midlatitudes". Advances in Radio Science 19 (17.12.2021): 185–93. http://dx.doi.org/10.5194/ars-19-185-2021.
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Abstract. Observations of upper mesosphere/lower thermosphere (MLT) wind have been performed at Collm (51.3∘ N, 13.0∘ 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 analyse 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 for geomagnetic effects on mean winds over Collm and Kazan qualitatively agree during most of the year. For the diurnal tide, amplitudes in summer are smaller in the mesosphere and 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.
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Chen, X., X. Hu i C. Xiao. "Variability of MLT winds and waves over mid-latitude during the 2000/2001 and 2009/2010 winter stratospheric sudden warming". Annales Geophysicae 30, nr 6 (28.06.2012): 991–1001. http://dx.doi.org/10.5194/angeo-30-991-2012.
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Abstract. The mesosphere and lower thermosphere (MLT) wind structure over Wuhan (30° N, 114° E) in 2000/2001 winter and over Langfang (39.4° N, 116.6° E) in 2009/2010 winter are examined to reveal the effects of stratospheric sudden warming (SSW) in mid-low-latitude MLT region. The result shows that the MLT daily zonal wind over these two sites reversed from eastward wind to westward wind for several days during the SSW events. The reversals were almost coincident with the polar stratospheric temperature reaching its maximum at 10 hPa, 90° N and were about ten days prior to the reversal of high latitude stratospheric zonal wind at 10 hPa, 60° N. The temporal variations of tides, gravity waves and 2-day planetary waves in the mid-latitude MLT showed different behavior during the two SSW events. During the 2001 SSW event, MLT diurnal tide reached its maximum when the MLT zonal wind decreased rapidly and SSW event began in polar stratosphere; the activity of 2-day waves decreased after the onset of the 2001 SSW, while the gravity wave increased when the 2001 SSW developed into a major warming. However, in the 2009/2010 winter, the semidiurnal tide and 2-day wave in MLT over Langfang reached a peak about two days earlier than zonal wind reversal at 10 hPa, 60° N; no significant features were found in diurnal tides, terdiurnal tides and gravity waves related to the 2010 SSW event.
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Yi, Wen, Xianghui Xue, Jie Zeng, Jianyuan Wang, Baozhu Zhou, Hailun Ye, Tingdi Chen i Xiankang Dou. "Observation of MLT region winds and tides by the USTC Mengcheng meteor radar". JUSTC 53, nr 5 (2023): 0501. http://dx.doi.org/10.52396/justc-2022-0158.
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The atmospheric winds and waves in the mesosphere and lower thermosphere (MLT) region are essential for studying the dynamics and climate in the middle and upper atmosphere. The University of Science and Technology of China (USTC) meteor radar located at Mengcheng (33.36°N, 116.49°E) has been operating continuously since April 2014. More than 8 years of observation of mesospheric horizontal winds and tides are presented in this study. In addition, we present an intercomparison among the meteor radar observations and the Navy Global Environmental Model-High Altitude (NAVGEM-HA) analysis results. The meteor number at northern lower midlatitudes suffers from diurnal variations in meteor occurrence, with a high count rate in the local morning and a low rate during local afternoon-to-midnight. The meteor count rates show a clear annual variation, with a maximum in September–October and a minimum in February. The horizontal wind in the MLT region has dominant annual variations at lower midlatitudes, with the eastward wind during summer and the westward wind during winter above 84 km, and the eastward wind during winter and the westward wind during spring below 84 km. The meridional wind is northward during winter and southward during summer. The diurnal amplitude is dominant, followed by the semidiurnal tides at lower midlatitudes. The zonal and meridional diurnal tides show enhancements during spring (March) with amplitudes that can reach up to 40 m/s and 30 m/s and during autumn (September) with amplitudes that can reach up to 30 m/s and 25 m/s, respectively. The seasonal variations in diurnal tidal amplitude basically show characteristics that are strong during the equinox and weak during the solstice. The zonal and meridional semidiurnal tides are maximized during spring (April) and autumn (September) above 90 km.
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Zhu, Xun, Jeng-Hwa Yee, E. R. Talaat, M. Mlynczak i J. M. Russell. "Diagnostic Analysis of Tidal Winds and the Eliassen–Palm Flux Divergence in the Mesosphere and Lower Thermosphere from TIMED/SABER Temperatures". Journal of the Atmospheric Sciences 65, nr 12 (1.12.2008): 3840–59. http://dx.doi.org/10.1175/2008jas2801.1.
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Abstract For migrating tides or fast-moving planetary waves, polarization relations derived from the linear wave equations are required to accurately derive the wind components from the temperature field. A common problem in diagnosing winds from the measured temperature is the error amplification associated with apparent singularities in the wave polarization relations. The authors have developed a spectral module that accurately derives tidal winds from the measured tidal temperature field and effectively eliminates the error amplification near the apparent singularities. The algorithm is used to perform a diagnostic analysis of tidal winds and the Eliassen–Palm (EP) flux divergence in the mesosphere and lower thermosphere (MLT) based on the zonal mean and tidal temperature fields derived from 6 yr of temperature measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) satellite. The derived zonal mean wind and diurnal tidal amplitude reveal new insights into the mesospheric biennial oscillation (MBO) that exists in the MLT at both equatorial and midlatitude regions. The equatorial MBO in the zonal mean wind is present in the entire mesosphere from 50 to 90 km. The equatorial MBO in the temperature amplitude of the diurnal tide occurs near the mesopause region between 80 and 90 km and is largely coincident with the downward phase propagation of the equatorial MBO in the zonal mean wind, indicating a possible mechanism of wave–mean flow interaction between the two. On the other hand, the newly discovered midlatitude MBOs in zonal mean wind and the meridional wind in diurnal tide occur at different altitudes, suggesting possibly a remote forcing–response relationship. The acceleration or deceleration of the zonal mean wind due to EP flux divergence that is contributed by the migrating tides peaks at midlatitudes with a typical value of 10–20 m s−1 day−1 around 95 km.
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Day, K. A., i N. J. Mitchell. "Mean winds, SAO and QBO in the stratosphere, mesosphere and lower thermosphere over Ascension Island (8° S 14° W)". Atmospheric Chemistry and Physics Discussions 13, nr 3 (13.03.2013): 6779–805. http://dx.doi.org/10.5194/acpd-13-6779-2013.
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Abstract. Mean winds in the mesosphere and lower thermosphere (MLT) over Ascension Island (8° S and 14° W) have been investigated using meteor radar wind observations. The results presented in this study are from the interval October 2001 to December 2011. There is a clear annual oscillation in the monthly-mean meridional winds. The monthly-mean meridional winds observed over Ascension Island at meteor heights are found to be southward during April–October, reaching velocities up to about −23 m s−1 and northward the rest of the year, reaching velocities up to about 16 m s−1. The monthly-mean zonal winds are generally westward through most of the year, reaching velocities up to about −46 m s−1. However, there are eastward winds in May–August and again in December in the lower heights that the radar observes. These winds maximises at heights of about 86 km reaching velocities up to about 36 m s−1 and decays quickly above and below. The Mesospheric Semi-Annual Oscillation (MSAO) is clearly observed in the monthly-mean zonal winds. The first westward phase of the winds is much stronger than the second. The first westward phase of the MSAO was found to maximise at heights of about 84 km and to in general reach amplitudes of about −35 m s−1. We have compared the HWM-07 model to our observations. Our observed meridional winds are generally more southward than those of the model at meteor heights in the southern hemispheric winter, whereas HWM-07 suggests that in this season only weakly southward, or even northward flows occur at the lower heights. The zonal monthly-mean winds are in general agreement but somewhat less westward than observed by the radar. In one of the eight events in which the first westward phase of the MSAO was observed, the strongest westward winds reached about −75 m s−1, compared to the mean of about −35 m s−1 for other events. We explain this observation in terms of a mechanism which has been previously proposed by others. In this the relative phasing of the Stratospheric Quasi-Biennial Oscillation (SQBO) and the MSAO allow an unusually large flux of gravity waves with westward phase speed to reach the mesosphere. The dissipation of these waves then drives the MLT winds to large westward velocities. We demonstrate that the necessary phase relationship existed during the event we observed in 2002 and not during other times. This provides strong support for the suggestion that those extremes in zonal flow are a~result of modulated gravity-wave fluxes.
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Davis, R. N., Y. W. Chen, S. Miyahara i N. J. Mitchell. "The climatology, propagation and excitation of ultra-fast Kelvin waves as observed by meteor radar, Aura MLS, TRMM and in the Kyushu-GCM". Atmospheric Chemistry and Physics Discussions 11, nr 10 (31.10.2011): 29479–525. http://dx.doi.org/10.5194/acpd-11-29479-2011.
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Abstract. Wind measurements from a meteor radar on Ascension Island (8° S, 14° W) and simultaneous temperature measurements from the Aura MLS instrument are used to characterise ultra-fast Kelvin waves (UFKW) of zonal wavenumber 1 (E1) in the mesosphere and lower thermosphere (MLT) in the years 2005–2010. These observations are compared with some predictions of the Kyushu-general circulation model. Good agreement is found between observations of the UFKW in the winds and temperatures, and also with the properties of the waves in the Kyushu-GCM. UFKW are found at periods between 2.5–4.5 days with amplitudes of up to 40 m s−1 in the zonal winds and 6 K in the temperatures. The average vertical wavelength is found to be 44 km. Amplitudes vary with latitude in a Gaussian manner with the profiles centred over the equator. Dissipation of the waves results in monthly-mean eastward accelerations of 0.2–0.9 m s−1 day−1 at heights around 95 km, with 5-day mean peak values of 4 m s−1 day−1. Largest wave amplitudes and variances are observed over Indonesia and central Africa and may be a result of very strong moist convective heating over those regions. Rainfall data from TRMM are used as a proxy for latent-heat release in an investigation of the excitation of these waves. No strong correlation is found between the occurrence of large-amplitude mesospheric UFKW events and either the magnitude of the equatorial rainfall or the amplitudes of E1 signatures in the rainfall time series, indicating that either other sources or the propagation environment are more important in determining the amplitude of UFKW in the MLT. A strong semiannual variation in wave amplitudes is observed. Intraseasonal oscillations (ISOs) with periods 25–60 days are evident in the zonal background winds, zonal-mean temperature, UFKW amplitudes, UFKW accelerations and the rainfall rate. This suggests that UFKW play a role in carrying the signature of tropospheric ISOs to the MLT region.
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Davis, R. N., Y. W. Chen, S. Miyahara i N. J. Mitchell. "The climatology, propagation and excitation of ultra-fast Kelvin waves as observed by meteor radar, Aura MLS, TRMM and in the Kyushu-GCM". Atmospheric Chemistry and Physics 12, nr 4 (17.02.2012): 1865–79. http://dx.doi.org/10.5194/acp-12-1865-2012.
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Abstract. Wind measurements from a meteor radar on Ascension Island (8° S, 14° W) and simultaneous temperature measurements from the Aura MLS instrument are used to characterise ultra-fast Kelvin waves (UFKW) of zonal wavenumber 1 (E1) in the mesosphere and lower thermosphere (MLT) in the years 2005–2010. These observations are compared with some predictions of the Kyushu-general circulation model. Good agreement is found between observations of the UFKW in the winds and temperatures, and also with the properties of the waves in the Kyushu-GCM. UFKW are found at periods between 2.5–4.5 days with amplitudes of up to 40 m s−1 in the zonal winds and 6 K in the temperatures. The average vertical wavelength is found to be 44 km. Amplitudes vary with latitude in a Gaussian manner with the maxima centred over the equator. Dissipation of the waves results in monthly-mean eastward accelerations of 0.2–0.9 m s−1 day−1 at heights around 95 km, with 5-day mean peak values of 4 m s−1 day−1. Largest wave amplitudes and variances are observed over Indonesia and central Africa and may be a result of very strong moist convective heating over those regions. Rainfall data from TRMM are used as a proxy for latent-heat release in an investigation of the excitation of these waves. No strong correlation is found between the occurrence of large-amplitude mesospheric UFKW events and either the magnitude of the equatorial rainfall or the amplitudes of E1 signatures in the rainfall time series, indicating that either other sources or the propagation environment are more important in determining the amplitude of UFKW in the MLT. A strong semiannual variation in wave amplitudes is observed. Intraseasonal oscillations (ISOs) with periods 25–60 days are evident in the zonal background winds, zonal-mean temperature, UFKW amplitudes, UFKW accelerations and the rainfall rate. This suggests that UFKW play a role in carrying the signature of tropospheric ISOs to the MLT region.
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Eckermann, Stephen D., Jun Ma, Karl W. Hoppel, David D. Kuhl, Douglas R. Allen, James A. Doyle, Kevin C. Viner i in. "High-Altitude (0–100 km) Global Atmospheric Reanalysis System: Description and Application to the 2014 Austral Winter of the Deep Propagating Gravity Wave Experiment (DEEPWAVE)". Monthly Weather Review 146, nr 8 (1.08.2018): 2639–66. http://dx.doi.org/10.1175/mwr-d-17-0386.1.
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AbstractA data assimilation system (DAS) is described for global atmospheric reanalysis from 0- to 100-km altitude. We apply it to the 2014 austral winter of the Deep Propagating Gravity Wave Experiment (DEEPWAVE), an international field campaign focused on gravity wave dynamics from 0 to 100 km, where an absence of reanalysis above 60 km inhibits research. Four experiments were performed from April to September 2014 and assessed for reanalysis skill above 50 km. A four-dimensional variational (4DVAR) run specified initial background error covariances statically. A hybrid-4DVAR (HYBRID) run formed background error covariances from an 80-member forecast ensemble blended with a static estimate. Each configuration was run at low and high horizontal resolution. In addition to operational observations below 50 km, each experiment assimilated 105 observations of the mesosphere and lower thermosphere (MLT) every 6 h. While all MLT reanalyses show skill relative to independent wind and temperature measurements, HYBRID outperforms 4DVAR. MLT fields at 1-h resolution (6-h analysis and 1–5-h forecasts) outperform 6-h analysis alone due to a migrating semidiurnal (SW2) tide that dominates MLT dynamics and is temporally aliased in 6-h time series. MLT reanalyses reproduce observed SW2 winds and temperatures, including phase structures and 10–15-day amplitude vacillations. The 0–100-km reanalyses reveal quasi-stationary planetary waves splitting the stratopause jet in July over New Zealand, decaying from 50 to 80 km then reintensifying above 80 km, most likely via MLT forcing due to zonal asymmetries in stratospheric gravity wave filtering.
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Day, K. A., R. E. Hibbins i N. J. Mitchell. "Aura MLS observations of the westward-propagating <i>s</i>=1, 16-day planetary wave in the middle atmosphere: climatology and cross-equatorial propagation". Atmospheric Chemistry and Physics Discussions 10, nr 10 (7.10.2010): 23197–227. http://dx.doi.org/10.5194/acpd-10-23197-2010.
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Abstract. The Microwave Limb Sounder (MLS) on the Aura satellite has been used to measure temperatures in the stratosphere, mesosphere and lower thermosphere (MLT). The data used here are from August 2004 to June 2010 and latitudes 75° S to 75° N. The temperature data reveal the persistent presence of a westward propagating 16-day planetary wave with zonal wavenumber 1. The wave amplitude maximises in winter in the stratosphere and MLT at middle to high latitudes, where monthly-mean amplitudes can be as large as ~8 K. Significant wave amplitudes are observed in the summer-time MLT and at lower stratospheric heights of up to ~20 km at middle to high latitudes. Wave amplitudes in the Northern Hemisphere approach values twice as large as those in the Southern Hemisphere. Wave amplitudes are also closely related to climatological zonal winds and are largest in regions of strongest eastward flow. There is a~reduction in wave amplitudes at the stratopause. No significant wave amplitude is observed near the equator or in the strongly westward background winds of the atmosphere in summer. This behaviour is interpreted as a consequence of wave/mean-flow interactions. It has been suggested that the summer-time 16-day wave in the MLT is ducted across the equator from the winter hemisphere and that this ducting is modulated by the equatorial Quasi-Biennial Oscillation (QBO) in the westerly phase. Here we observe that the QBO modulates the 16-day wave in the polar summer-time MLT in the Northern Hemisphere as previously observed, but this modulation is not seen in the Southern Hemisphere.
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Tian, Caixia, Xiong Hu, Yurong Liu, Xuan Cheng, Zhaoai Yan i Bing Cai. "Seasonal Variations of High-Frequency Gravity Wave Momentum Fluxes and Their Forcing toward Zonal Winds in the Mesosphere and Lower Thermosphere over Langfang, China (39.4° N, 116.7° E)". Atmosphere 11, nr 11 (20.11.2020): 1253. http://dx.doi.org/10.3390/atmos11111253.
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Meteor radar data collected over Langfang, China (39.4° N, 116.7° E) were used to estimate the momentum flux of short-period (less than 2 h) gravity waves (GWs) in the mesosphere and lower thermosphere (MLT), using the Hocking (2005) analysis technique. Seasonal variations in GW momentum flux exhibited annual oscillation (AO), semiannual oscillation (SAO), and quasi-4-month oscillation. Quantitative estimations of GW forcing toward the mean zonal flow were provided using the determined GW momentum flux. The mean flow acceleration estimated from the divergence of this flux was compared with the observed acceleration of zonal winds displaying SAO and quasi-4-month oscillations. These comparisons were used to analyze the contribution of zonal momentum fluxes of SAO and quasi-4-month oscillations to zonal winds. The estimated acceleration from high-frequency GWs was in the same direction as the observed acceleration of zonal winds for quasi-4-month oscillation winds, with GWs contributing more than 69%. The estimated acceleration due to Coriolis forces to the zonal wind was studied; the findings were opposite to the estimated acceleration of high-frequency GWs for quasi-4-month oscillation winds. The significance of this study lies in estimating and quantifying the contribution of the GW momentum fluxes to zonal winds with quasi-4-month periods over mid-latitude regions for the first time.
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Jacobi, Ch, i D. Kürschner. "Long-term trends of MLT region winds over Central Europe". Physics and Chemistry of the Earth, Parts A/B/C 31, nr 1-3 (styczeń 2006): 16–21. http://dx.doi.org/10.1016/j.pce.2005.01.004.
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Wilhelm, Sven, Gunter Stober, Vivien Matthias, Christoph Jacobi i Damian J. Murphy. "Connection between the length of day and wind measurements in the mesosphere and lower thermosphere at mid- and high latitudes". Annales Geophysicae 37, nr 1 (11.01.2019): 1–14. http://dx.doi.org/10.5194/angeo-37-1-2019.
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Abstract. This work presents a connection between the density variation within the mesosphere and lower thermosphere (MLT) and changes in the intensity of solar radiation. On a seasonal timescale, these changes take place due to the revolution of the Earth around the Sun. While the Earth, during the northern-hemispheric (NH) winter, is closer to the Sun, the upper mesosphere expands due to an increased radiation intensity, which results in changes in density at these heights. These density variations, i.e., a vertical redistribution of atmospheric mass, have an effect on the rotation rate of Earth's upper atmosphere owing to angular momentum conservation. In order to test this effect, we applied a theoretical model, which shows a decrease in the atmospheric rotation speed of about ∼4 m s−1 at a latitude of 45∘ in the case of a density change of 1 % between 70 and 100 km. To support this statement, we compare the wind variability obtained from meteor radar (MR) and Microwave Limb Sounder (MLS) satellite observations with fluctuations in the length of a day (LOD). Changes in the LOD on timescales of a year and less are primarily driven by tropospheric large-scale geophysical processes and their impact on the Earth's rotation. A global increase in lower-atmospheric eastward-directed winds leads, due to friction with the Earth's surface, to an acceleration of the Earth's rotation by up to a few milliseconds per rotation. The LOD shows an increase during northern winter and decreases during summer, which corresponds to changes in the MLT density due to the Earth–Sun movement. Within the MLT the mean zonal wind shows similar fluctuations to the LOD on annual scales as well as longer time series, which are connected to the seasonal wind regime as well as to density changes excited by variations in the solar radiation. A direct correlation between the local measured winds and the LOD on shorter timescales cannot clearly be identified, due to stronger influences of other natural oscillations on the wind. Further, we show that, even after removing the seasonal and 11-year solar cycle variations, the mean zonal wind and the LOD are connected by analyzing long-term tendencies for the years 2005–2016.
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Wilhelm, Sven, Gunter Stober i Jorge L. Chau. "A comparison of 11-year mesospheric and lower thermospheric winds determined by meteor and MF radar at 69 ° N". Annales Geophysicae 35, nr 4 (31.07.2017): 893–906. http://dx.doi.org/10.5194/angeo-35-893-2017.
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Abstract. The Andenes Meteor Radar (MR) and the Saura Medium Frequency (MF) Radar are located in northern Norway (69° N, 16° E) and operate continuously to provide wind measurements of the mesosphere and lower thermosphere (MLT) region. We compare the two systems to find potential biases between the radars and combine the data from both systems to enhance altitudinal coverage between 60 and 110 km. The systems have altitudinal overlap between 78 and 100 km at which we compare winds and tides on the basis of hourly winds with 2 km altitude bins. Our results indicate reasonable agreement for the zonal and meridional wind components between 78 and 92 km. An exception to this is the altitude range below 84 km during the summer, at which the correlation decreases. We also compare semidiurnal and diurnal tides according to their amplitudes and phases with good agreement below 90 km for the diurnal and below 96 km for the semidiurnal tides. Based on these findings we have taken the MR data as a reference. By comparing the MF and MR winds within the overlapping region, we have empirically estimated correction factors to be applied to the MF winds. Existing gaps in that data set will be filled with weighted MF data. This weighting is done due to underestimated wind values of the MF compared to the MR, and the resulting correction factors fit to a polynomial function of second degree within the overlapping area. We are therefore able to construct a consistent and homogenous wind from approximately 60 to 110 km.
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Stober, Gunter, Kathrin Baumgarten, John P. McCormack, Peter Brown i Jerry Czarnecki. "Comparative study between ground-based observations and NAVGEM-HA analysis data in the mesosphere and lower thermosphere region". Atmospheric Chemistry and Physics 20, nr 20 (26.10.2020): 11979–2010. http://dx.doi.org/10.5194/acp-20-11979-2020.
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Abstract. Recent studies have shown that day-to-day variability of the migrating semidiurnal solar (SW2) tide within the mesosphere and lower thermosphere (MLT) is a key driver of anomalies in the thermosphere–ionosphere system. Here, we study the variability in both the amplitude and phase of SW2 using meteor radar wind and lidar temperature observations at altitudes of 75–110 km as well as wind and temperature output from the Navy Global Environmental Model – High Altitude (NAVGEM-HA), a high-altitude meteorological analysis system. Application of a new adaptive spectral filter technique to both local radar wind observations and global NAVGEM-HA analyses offers an important cross-validation of both data sets and makes it possible to distinguish between migrating and non-migrating tidal components, which is difficult using local measurements alone. Comparisons of NAVGEM-HA, meteor radar and lidar observations over a 12-month period show that the meteorological analyses consistently reproduce the seasonal as well as day-to-day variability in mean winds, mean temperatures and SW2 features from the ground-based observations. This study also examines in detail the day-to-day variability in SW2 during two sudden stratospheric warming, events that have been implicated in producing ionospheric anomalies. During this period, both meteor radar and NAVGEM-HA winds show a significant phase shift and amplitude modulation, but no signs of coupling to the lunar tide as previous studies have suggested. Overall, these findings demonstrate the benefit of combining global high-altitude meteorological analyses with ground-based observations of the MLT region to better understand the tidal variability in the atmosphere.
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Namboothiri, S. P., P. Kishore i K. Igarashi. "Climatological studies of the quasi 16-day oscillations in the mesosphere and lower thermosphere at Yamagawa (31.2° N, 130.6° E), Japan". Annales Geophysicae 20, nr 8 (31.08.2002): 1239–46. http://dx.doi.org/10.5194/angeo-20-1239-2002.
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Abstract. The 16-day wave climatology at Yamagawa (31.2° N, 130.6° E) is investigated by analyzing the mesosphere and lower thermosphere (MLT) wind data collected with the recently installed MF radar. We have utilized the continuous data gathered during the five-year period 1996–2000. The wave climatology clearly shows some seasonal variations. The period of late autumn-spring is marked with larger wave activity, with the strongest waves being observed in the winter months. The maximum amplitude observed at Yamagawa is about 20 m/s, which is comparatively larger than the amplitudes observed at mid-latitude stations. The height dependence of the 16-day wave suggests that the maximum amplitude is observed at altitudes below 80 km. The summer months are characterized with much weaker wave activity. The vertical wavelength appears to be larger in the winter months and shorter in the summer months. The present analysis again confirms that the 16-day wave is highly sensitive to the background mean winds. Eastward motion of the background winds is a more favourable condition for the 16-day wave penetration to the MLT heights. The wave features show some signs of interannual variability. Overall, the observed features of the 16-day wave at Yamagawa, which is located at the edge of the subtropical latitudes, show some correspondence with the results reported for mid-latitude stations.Key words. Meteorology and atmospheric dynamics (climatology; thermospheric dynamics)
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Yasui, Ryosuke, Kaoru Sato i Yasunobu Miyoshi. "The Momentum Budget in the Stratosphere, Mesosphere, and Lower Thermosphere. Part II: The In Situ Generation of Gravity Waves". Journal of the Atmospheric Sciences 75, nr 10 (październik 2018): 3635–51. http://dx.doi.org/10.1175/jas-d-17-0337.1.
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The contributions of gravity waves to the momentum budget in the mesosphere and lower thermosphere (MLT) is examined using simulation data from the Ground-to-Topside Model of Atmosphere and Ionosphere for Aeronomy (GAIA) whole-atmosphere model. Regardless of the relatively coarse model resolution, gravity waves appear in the MLT region. The resolved gravity waves largely contribute to the MLT momentum budget. A pair of positive and negative Eliassen–Palm flux divergences of the resolved gravity waves are observed in the summer MLT region, suggesting that the resolved gravity waves are likely in situ generated in the MLT region. In the summer MLT region, the mean zonal winds have a strong vertical shear that is likely formed by parameterized gravity wave forcing. The Richardson number sometimes becomes less than a quarter in the strong-shear region, suggesting that the resolved gravity waves are generated by shear instability. In addition, shear instability occurs in the low (middle) latitudes of the summer (winter) MLT region and is associated with diurnal (semidiurnal) migrating tides. Resolved gravity waves are also radiated from these regions. In Part I of this paper, it was shown that Rossby waves in the MLT region are also radiated by the barotropic and/or baroclinic instability formed by parameterized gravity wave forcing. These results strongly suggest that the forcing by gravity waves originating from the lower atmosphere causes the barotropic/baroclinic and shear instabilities in the mesosphere that, respectively, generate Rossby and gravity waves and suggest that the in situ generation and dissipation of these waves play important roles in the momentum budget of the MLT region.
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John, S. R., K. V. Subrahmanyam, G. Manju i Q. Wu. "Meteor radar measurements of MLT winds near the equatorial electro jet region over Thumba (8.5° N, 77° E): comparison with TIDI observations". Annales Geophysicae 29, nr 7 (1.07.2011): 1209–14. http://dx.doi.org/10.5194/angeo-29-1209-2011.
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Abstract. The All-Sky interferometric meteor (SKYiMET) radar (MR) derived winds in the vicinity of the equatorial electrojet (EEJ) are discussed. As Thumba (8.5° N, 77° E; dip lat. 0.5° N) is under the EEJ belt, there has been some debate on the reliability of the meteor radar derived winds near the EEJ height region. In this regard, the composite diurnal variations of zonal wind profiles in the mesosphere-lower thermosphere (MLT) region derived from TIMED Doppler Interferometer (TIDI) and ground based meteor radar at Thumba are compared. In this study, emphasis is given to verify the meteor radar observations at 98 km height region, especially during the EEJ peaking time (11:00 to 14:00 LT). The composite diurnal cycles of zonal winds over Thumba are constructed during four seasons of the year 2006 using TIDI and meteor radar observations, which showed good agreement especially during the peak EEJ hours, thus assuring the reliability of meteor radar measurements of neutral winds close to the EEJ height region. It is evident from the present study that on seasonal scales, the radar measurements are not biased by the EEJ. The day-time variations of HF radar measured E-region drifts at the EEJ region are also compared with MR measurements to show there are large differences between ionospheric drifts and MR measurements. The significance of the present study lies in validating the meteor radar technique over Thumba located at magnetic equator by comparing with other than the radio technique for the first time.
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Day, K. A., R. E. Hibbins i N. J. Mitchell. "Aura MLS observations of the westward-propagating <i>s</i>=1, 16-day planetary wave in the stratosphere, mesosphere and lower thermosphere". Atmospheric Chemistry and Physics 11, nr 9 (5.05.2011): 4149–61. http://dx.doi.org/10.5194/acp-11-4149-2011.
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Abstract. The Microwave Limb Sounder (MLS) on the Aura satellite has been used to measure temperatures in the stratosphere, mesosphere and lower thermosphere. The data used here are from August 2004 to December 2010 and latitudes 75° N to 75° S. The temperature data reveal the regular presence of a westward-propagating 16-day planetary wave with zonal wavenumber 1. The wave amplitudes maximise in winter at middle to high latitudes, where monthly-mean amplitudes can be as large as ~8 K. Significant wave amplitudes are also observed in the summer-time mesosphere and lower thermosphere (MLT) and at lower stratospheric heights of up to ~20 km at middle to high latitudes. Wave amplitudes in the Northern Hemisphere approach values twice as large as those in the Southern Hemisphere. Wave amplitudes are also closely related to mean zonal winds and are largest in regions of strongest eastward flow. There is a reduction in wave amplitudes at the stratopause. No significant wave amplitudes are observed near the equator or in the strongly westward background winds of the atmosphere in summer. This behaviour is interpreted as a consequence of wave/mean-flow interactions. Perturbations in wave amplitude summer MLT are compared to those simultaneously observed in the winter stratosphere of the opposite hemisphere and found to have a correlation coefficient of +0.22, suggesting a small degrees of inter-hemispheric coupling. We interpret this to mean that some of the summer-time MLT wave may originate in the winter stratosphere of the opposite hemisphere and have been ducted across the equator. We do not observe a significant QBO modulation of the 16-day wave amplitude in the polar summer-time MLT. Wave amplitudes were also observed to be suppressed during the major sudden stratospheric warming events of the Northern Hemisphere winters of 2006 and 2009.
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Chen, Zhou, Yi Liu, Zhitao Du, Zhiqiang Fan, Haiyang Sun i Chen Zhou. "Validation of MIGHTI/ICON Atmospheric Wind Observations over China Region Based on Meteor Radar and Horizontal Wind Model (HWM14)". Atmosphere 13, nr 7 (7.07.2022): 1078. http://dx.doi.org/10.3390/atmos13071078.
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The Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) on board the ICON satellite provides effective measurement of horizontal winds in the mesosphere and lower thermosphere (MLT) region. In order to verify the measurement accuracy of the horizontal wind, this study uses the measurements of the meteor radar in Wuhan and the simulation results of a horizontal wind field model (HWM14) to compare and analyze the measurement results of MIGHTI/ICON in the whole year of 2020. The comparative analysis indicated that two datasets from MIGHTI/ICON and meteor radar are strongly correlated (r = 0.65,0.76) with an RMS difference of 39.21 m/s (30.31 m/s). The consistency for meridional wind from MIGHTI/ICON, meteor radar, and HWM14 is worse than that of zonal wind. The accuracy of horizontal wind observations is influenced by altitude, diurnal, and seasonal patterns.
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Svoboda, Aaron A., Jeffrey M. Forbes i Saburo Miyahara. "A space-based climatology of diurnal MLT tidal winds, temperatures and densities from UARS wind measurements". Journal of Atmospheric and Solar-Terrestrial Physics 67, nr 16 (listopad 2005): 1533–43. http://dx.doi.org/10.1016/j.jastp.2005.08.018.
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Manson, A. H., Y. Luo i C. Meek. "Global distributions of diurnal and semi-diurnal tides: observations from HRDI-UARS of the MLT region". Annales Geophysicae 20, nr 11 (30.11.2002): 1877–90. http://dx.doi.org/10.5194/angeo-20-1877-2002.
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Abstract. HRDI (High Resolution Doppler Interferometer-UARS) winds data have been analyzed in 4° latitude by 10° longitude cells at 96 km to obtain global contour maps of solar-tidal amplitudes and phases, and also mean winds. The solstices June–July (1993), December–January (1993–1994), and one equinox September–October (1994) are shown. The 24-h diurnal tide that maximizes near the 20–25° latitude has significant seasonal changes with equinoctial maxima, and very clear longitudinal variability. Maxima are very clear over the oceans. In contrast, the 12-h semi-diurnal tides that maximize near the 40–55° latitude have very strong seasonal changes with winter maxima, and more modest longitudinal changes. The similarities with MLT (mesosphere-lower thermosphere) radar observations (90 km) and the GSWM (Global Scale Wave Model) are very satisfactory. The mean winds are consistent with expectations and show clear poleward flow from summer to winter hemispheres in the solstices.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides) Radio science (remote sensing)
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Sridharan, S., i S. Sathishkumar. "Seasonal and interannual variations of gravity wave activity in the low-latitude mesosphere and lower thermosphere over Tirunelveli (8.7° N, 77.8° E)". Annales Geophysicae 26, nr 11 (21.10.2008): 3215–23. http://dx.doi.org/10.5194/angeo-26-3215-2008.
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Abstract. The Mesosphere and Lower Thermosphere (MLT) winds acquired by medium frequency (MF) radar at Tirunelveli (8.7° N, 77.8° E) for the years 1993–2007 are used to study seasonal and interannual variabilities of gravity wave (GW) variances in the altitude region 84–94 km. The GW variances in zonal and meridional winds show semiannual oscillation with maximum variance during March–April and August–September and minimum during June–July and November–December months. The wind variances, in general, are observed to be enhanced during and after the year 1998 and they undergo large interannual variability, in particular, during spring equinox months. An enhancement of GW variances is observed during spring equinox months of the years 2000, 2004 and 2006. These larger GW enhancements, most of the times, coincide with eastward phase of zonally averaged stratospheric QBO at 30 hPa over equator and sudden stratospheric warming occurred at high latitudes. From the zonal and meridional variances, the perturbation ellipses are calculated and they show that the predominant direction of propagation of gravity waves is in SE-NW plane.
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Manson, A. H., C. E. Meek, C. M. Hall, S. Nozawa, N. J. Mitchell, D. Pancheva, W. Singer i P. Hoffmann. "Mesopause dynamics from the scandinavian triangle of radars within the PSMOS-DATAR Project". Annales Geophysicae 22, nr 2 (1.01.2004): 367–86. http://dx.doi.org/10.5194/angeo-22-367-2004.
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Abstract. The "Scandinavian Triangle" is a unique trio of radars within the DATAR Project (Dynamics and Temperatures from the Arctic MLT (60–97km) region): Andenes MF radar (69°N, 16°E); Tromsø MF radar (70°N, 19°E) and Esrange "Meteor" radar (68°N, 21°E). The radar-spacings range from 125-270km, making it unique for studies of wind variability associated with small-scale waves, comparisons of large-scale waves measured over small spacings, and for comparisons of winds from different radar systems. As such it complements results from arrays having spacings of 25km and 500km that have been located near Saskatoon. Correlation analysis is used to demonstrate a speed bias (MF smaller than the Meteor) between the radar types, which varies with season and altitude. Annual climatologies for the year 2000 of mean winds, solar tides, planetary and gravity waves are presented, and show indications of significant spatial variability across the Triangle and of differences in wave characteristics from middle latitudes. Key words: Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides: instrument and techniques)
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Stray, N. H., Y. J. Orsolini, P. J. Espy, V. Limpasuvan i R. E. Hibbins. "Observations of PW activity in the MLT during SSW events using a chain of SuperDARN radars and SD-WACCM". Atmospheric Chemistry and Physics Discussions 15, nr 1 (7.01.2015): 393–413. http://dx.doi.org/10.5194/acpd-15-393-2015.
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Abstract. This study investigates the effect of Stratospheric Sudden Warmings (SSWs) on Planetary Wave (PW) activity in the Mesosphere-Lower Thermosphere (MLT). PW activity near 95 km is derived from meteor wind data using a chain of 8 SuperDARN radars at high northern latitudes that span longitudes from 150° W to 25° E and latitudes from 51 to 66° N. Zonal wave number 1 and 2 components were extracted from the meridional wind for the years 2000–2008. The observed wintertime PW activity shows common features associated with the stratospheric wind reversals and the accompanying stratospheric warming events. Onset dates for seven SSW events accompanied by an elevated stratopause (ES) were identified during this time period using the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM). For the seven events, a significant enhancement in wave number 1 and 2 PW amplitudes near 95 km was found to occur after the wind reversed at 50 km, with amplitudes maximizing approximately 5 days after the onset of the wind reversal. This PW enhancement in the MLT after the event was confirmed using SD-WACCM. When all cases of polar cap wind reversals at 50 km were considered, a significant, albeit moderate, correlation of 0.4 was found between PW amplitudes near 95 km and westward polar-cap stratospheric winds at 50 km, with the maximum correlation occurring ~3 days after the maximum westward wind. These results indicate that the enhancement of PW amplitudes near 95 km are a common feature of SSWs irrespective of the strength of the wind reversal.
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Stray, N. H., Y. J. Orsolini, P. J. Espy, V. Limpasuvan i R. E. Hibbins. "Observations of planetary waves in the mesosphere-lower thermosphere during stratospheric warming events". Atmospheric Chemistry and Physics 15, nr 9 (4.05.2015): 4997–5005. http://dx.doi.org/10.5194/acp-15-4997-2015.
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Abstract. This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW) activity in the mesosphere–lower thermosphere (MLT). PW activity near 95 km is derived from meteor wind data using a chain of eight SuperDARN radars at high northern latitudes that span longitudes from 150° W to 25° E and latitudes from 51 to 66° N. Zonal wave number 1 and 2 components were extracted from the meridional wind for the years 2000–2008. The observed wintertime PW activity shows common features associated with the stratospheric wind reversals and the accompanying stratospheric warming events. Onset dates for seven SSW events accompanied by an elevated stratopause (ES) were identified during this time period using the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM). For the seven events, a significant enhancement in wave number 1 and 2 PW amplitudes near 95 km was found to occur after the wind reversed at 50 km, with amplitudes maximizing approximately 5 days after the onset of the wind reversal. This PW enhancement in the MLT after the event was confirmed using SD-WACCM. When all cases of polar cap wind reversals at 50 km were considered, a significant, albeit moderate, correlation of 0.4 was found between PW amplitudes near 95 km and westward polar-cap stratospheric winds at 50 km, with the maximum correlation occurring ∼ 3 days after the maximum westward wind. These results indicate that the enhancement of PW amplitudes near 95 km is a common feature of SSWs irrespective of the strength of the wind reversal.
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Manson, A. H., C. E. Meek, T. Chshyolkova, S. K. Avery, D. Thorsen, J. W. MacDougall, W. Hocking, Y. Murayama i K. Igarashi. "Wave activity (planetary, tidal) throughout the middle atmosphere (20-100km) over the CUJO network: Satellite (TOMS) and Medium Frequency (MF) radar observations". Annales Geophysicae 23, nr 2 (28.02.2005): 305–23. http://dx.doi.org/10.5194/angeo-23-305-2005.
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Abstract. Planetary and tidal wave activity in the tropopause-lower stratosphere and mesosphere-lower thermosphere (MLT) is studied using combinations of ground-based (GB) and satellite instruments (2000-2002). The relatively new MFR (medium frequency radar) at Platteville (40° N, 105° W) has provided the opportunity to create an operational network of middle-latitude MFRs, stretching from 81° W-142° E, which provides winds and tides 70-100km. CUJO (Canada U.S. Japan Opportunity) comprises systems at London (43° N, 81° W), Platteville (40° N, 105° W), Saskatoon (52° N, 107° W), Wakkanai (45° N, 142° E) and Yamagawa (31° N, 131° E). It offers a significant 7000-km longitudinal sector in the North American-Pacific region, and a useful range of latitudes (12-14°) at two longitudes. Satellite data mainly involve the daily values of the total ozone column measured by the Earth Probe (EP) TOMS (Total Ozone Mapping Spectrometer) and provide a measure of tropopause-lower stratospheric planetary wave activity, as well as ozone variability. Climatologies of ozone and winds/tides involving frequency versus time (wavelet) contour plots for periods from 2-d to 30-d and the interval from mid 2000 to 2002, show that the changes with altitude, longitude and latitude are very significant and distinctive. Geometric-mean wavelets for the region of the 40° N MFRs demonstrate occasions during the autumn, winter and spring months when there are similarities in the spectral features of the lower atmosphere and at mesopause (85km) heights. Both direct planetary wave (PW) propagation into the MLT, nonlinear PW-tide interactions, and disturbances in MLT tides associated with fluctuations in the ozone forcing are considered to be possible coupling processes. The complex horizontal wave numbers of the longer period oscillations are provided in frequency contour plots for the TOMS satellite data to demonstrate the differences between lower atmospheric and MLT wave motions and their directions of propagation.
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Jacobi, Ch, N. Samtleben i G. Stober. "Meteor radar observations of mesopause region long-period temperature oscillations". Advances in Radio Science 14 (28.09.2016): 169–74. http://dx.doi.org/10.5194/ars-14-169-2016.
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Abstract. Meteor radar observations of mesosphere/lower thermosphere (MLT) daily temperatures have been performed at Collm, Germany since August 2004. The data have been analyzed with respect to long-period oscillations at time scales of 2–30 days. The results reveal that oscillations with periods of up to 6 days are more frequently observed during summer, while those with longer periods have larger amplitudes during winter. The oscillations may be considered as the signature of planetary waves. The results are compared with analyses from radar wind measurements. Moreover, the temperature oscillations show considerable year-to-year variability. In particular, amplitudes of the quasi 5-day oscillation have increased during the last decade, and the quasi 10-day oscillations are larger if the equatorial stratospheric winds are eastward.
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van Caspel, Willem E., Patrick J. Espy, Robert E. Hibbins i John P. McCormack. "Migrating tide climatologies measured by a high-latitude array of SuperDARN HF radars". Annales Geophysicae 38, nr 6 (21.12.2020): 1257–65. http://dx.doi.org/10.5194/angeo-38-1257-2020.
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Abstract. This study uses hourly meteor wind measurements from a longitudinal array of 10 high-latitude SuperDARN high-frequency (HF) radars to isolate the migrating diurnal, semidiurnal, and terdiurnal tides at mesosphere–lower-thermosphere (MLT) altitudes. The planetary-scale array of radars covers 180∘ of longitude, with 8 out of 10 radars being in near-continuous operation since the year 2000. Time series spanning 16 years of tidal amplitudes and phases in both zonal and meridional wind are presented, along with their respective annual climatologies. The method to isolate the migrating tides from SuperDARN meteor winds is validated using 2 years of winds from a high-altitude meteorological analysis system. The validation steps demonstrate that, given the geographical spread of the radar stations, the derived tidal modes are most closely representative of the migrating tides at 60∘ N. Some of the main characteristics of the observed migrating tides are that the semidiurnal tide shows sharp phase jumps around the equinoxes and peak amplitudes during early fall and that the terdiurnal tide shows a pronounced secondary amplitude peak around day of year (DOY) 265. In addition, the diurnal tide is found to show a bi-modal circular polarization phase relation between summer and winter.
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Jacobi, Ch, P. Hoffmann i D. Kürschner. "Trends in MLT region winds and planetary waves, Collm (52° N, 15° E)". Annales Geophysicae 26, nr 5 (28.05.2008): 1221–32. http://dx.doi.org/10.5194/angeo-26-1221-2008.
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Abstract. Long-period oscillations in the period range 2–30 days, interpreted as planetary wave (PW) signatures, have been analysed using daily mesosphere/lower thermosphere wind measurements near 90 km over Collm (52° N, 15° E) in the time interval 1980–2005. Interannual and interdecadal variability of PW are found. Since the 1990s, a tendency for larger zonal amplitudes compared to meridional ones has been observed, i.e. some long-term trends are visible, which are positive in the zonal component, but negative in the meridional component. There is a tendency of the trend to be non-linear for waves with periods lower than 7 days, so that a climatic transition appears around 1990, with smaller changes before and after that time. A solar cycle effect on PW is weak, but there is a tendency for a positive correlation between solar flux and wave activity, if a time lag of PW activity with respect to the solar flux of about 2 years is taken into consideration.
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Lühr, H., S. Rentz, P. Ritter, H. Liu i K. Häusler. "Average thermospheric wind patterns over the polar regions, as observed by CHAMP". Annales Geophysicae 25, nr 5 (4.06.2007): 1093–101. http://dx.doi.org/10.5194/angeo-25-1093-2007.
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Abstract. Measurements of the CHAMP accelerometer are utilized to investigate the average thermospheric wind distribution in the polar regions at altitudes around 400 km. This study puts special emphasis on the seasonal differences in the wind patterns. For this purpose 131 days centered on the June solstice of 2003 are considered. Within that period CHAMP's orbit is precessing once through all local times. The cross-track wind estimates of all 2030 passes are used to construct mean wind vectors for 918 equal-area cells. These bin averages are presented in corrected geomagnetic coordinates. Both hemispheres are considered simultaneously providing summer and winter responses for the same prevailing geophysical conditions. The period under study is characterized by high magnetic activity (Kp=4−) but moderate solar flux level (F10.7=124). Our analysis reveals clear wind features in the summer (Northern) Hemisphere. Over the polar cap there is a fast day-to-night flow with mean speeds surpassing 600 m/s in the dawn sector. At auroral latitudes we find strong westward zonal winds on the dawn side. On the dusk side, however, an anti-cyclonic vortex is forming. The dawn/dusk asymmetry is attributed to the combined action of Coriolis and centrifugal forces. Along the auroral oval the sunward streaming plasma causes a stagnation of the day-to-night wind. This effect is particularly clear on the dusk side. On the dawn side it is evident only from midnight to 06:00 MLT. The winter (Southern) Hemisphere reveals similar wind features, but they are less well ordered. The mean day-to-night wind over the polar cap is weaker by about 35%. Otherwise, the seasonal differences are mainly confined to the dayside (06:00–18:00 MLT). In addition, the larger offset between geographic and geomagnetic pole in the south also causes hemispheric differences of the thermospheric wind distribution.
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Volz, Ryan, Jorge L. Chau, Philip J. Erickson, Juha P. Vierinen, J. Miguel Urco i Matthias Clahsen. "Four-dimensional mesospheric and lower thermospheric wind fields using Gaussian process regression on multistatic specular meteor radar observations". Atmospheric Measurement Techniques 14, nr 11 (17.11.2021): 7199–219. http://dx.doi.org/10.5194/amt-14-7199-2021.
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Abstract. Mesoscale dynamics in the mesosphere and lower thermosphere (MLT) region have been difficult to study from either ground- or satellite-based observations. For understanding of atmospheric coupling processes, important spatial scales at these altitudes range between tens and hundreds of kilometers in the horizontal plane. To date, this scale size is challenging observationally, so structures are usually parameterized in global circulation models. The advent of multistatic specular meteor radar networks allows exploration of MLT mesoscale dynamics on these scales using an increased number of detections and a diversity of viewing angles inherent to multistatic networks. In this work, we introduce a four-dimensional wind field inversion method that makes use of Gaussian process regression (GPR), which is a nonparametric and Bayesian approach. The method takes measured projected wind velocities and prior distributions of the wind velocity as a function of space and time, specified by the user or estimated from the data, and produces posterior distributions for the wind velocity. Computation of the predictive posterior distribution is performed on sampled points of interest and is not necessarily regularly sampled. The main benefits of the GPR method include this non-gridded sampling, the built-in statistical uncertainty estimates, and the ability to horizontally resolve winds on relatively small scales. The performance of the GPR implementation has been evaluated on Monte Carlo simulations with known distributions using the same spatial and temporal sampling as 1 d of real meteor measurements. Based on the simulation results we find that the GPR implementation is robust, providing wind fields that are statistically unbiased with statistical variances that depend on the geometry and are proportional to the prior velocity variances. A conservative and fast approach can be straightforwardly implemented by employing overestimated prior variances and distances, while a more robust but computationally intensive approach can be implemented by employing training and fitting of model hyperparameters. The latter GPR approach has been applied to a 24 h dataset and shown to compare well to previously used homogeneous and gradient methods. Small-scale features have reasonably low statistical uncertainties, implying geophysical wind field horizontal structures as low as 20–50 km. We suggest that this GPR approach forms a suitable method for MLT regional and weather studies.
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Takahashi, H., C. M. Wrasse, D. Pancheva, M. A. Abdu, I. S. Batista, L. M. Lima, P. P. Batista, B. R. Clemesha i K. Shiokawa. "Signatures of 3–6 day planetary waves in the equatorial mesosphere and ionosphere". Annales Geophysicae 24, nr 12 (21.12.2006): 3343–50. http://dx.doi.org/10.5194/angeo-24-3343-2006.
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Abstract. Common periodic oscillations have been observed in meteor radar measurements of the MLT winds at Cariri (7.4° S, 36.5° W) and Ascension Island (7.9° S, 14.4° W) and in the minimum ionospheric virtual height, h'F, measured at Fortaleza (3.9° S, 38.4° W) in 2004, all located in the near equatorial region. Wavelet analysis of these time series reveals that there are 3–4-day, 6–8-day and 12–16-day oscillations in the zonal winds and h'F. The 3–4 day oscillation appeared as a form of a wave packet from 7–17 August 2004. From the wave characteristics analyzed this might be a 3.5-day Ultra Fast Kelvin wave. The 6-day oscillation in the mesosphere was prominent during the period of August to November. In the ionosphere, however, it was apparent only in November. Spectral analysis suggests that this might be a 6.5-day wave previously identified. The 3.5-day and 6.5-day waves in the ionosphere could have important roles in the initiation of equatorial spread F (plasma bubble). These waves might modulate the post-sunset E×B uplifting of the base of the F-layer via the induced lower thermosphere zonal wind and/or the E-region conductivity.