[Show abstract][Hide abstract] ABSTRACT: The meteor radar measurements obtained at Cachoeira Paulista (22.7°S), Brazil, have been used to study a possible relationship between meteor echo height variations and solar flux during solar cycle 23. A good concordance between the normalized values of the annual mean of the meteor peak heights and F10.7 solar radio flux and Mg_II solar indexes have been observed during declining phase of the solar cycle 23. After eliminating the solar activity influence, the annual mean of the meteor echo peak heights showed a linear decrease of 30 m/year when Mg_II solar index is used and 38 m/year when F10.7 solar radio flux is used. When the trend is eliminated the relationship between meteor peak heights and F10.7 solar flux indicate a trend of 672 m/100 sfu (sfu-solar flux unit). The meteor amplitude signals and the decay time drops after mid-2004, which may be attributed to the decreasing of the electron density in the meteor trails. The meteor echo peak height decrease has been interpreted as being caused by a reduction in air density in the upper atmosphere.
Journal of Atmospheric and Solar-Terrestrial Physics 10/2015; 133:139. DOI:10.1016/j.jastp.2015.08.015 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using meteor radar observations of four years in the mesosphere and lower thermosphere (MLT) over a subtropical Brazilian station, Cachoeira Paulista (22.7°S, 45°W), the temporal variability characteristics of the diurnal tide have been studied. In addition to the semiannual, annual and interannual variations, the diurnal tide amplitude exhibits clear intermittent modulation at periods of planetary waves. The tidal amplitude exhibits clear seasonal pattern with largest amplitude in fall equinox. The dominant periods of modulation of the diurnal tide are found to be greater than 10 days in the MLT. The diurnal tide, as detected in the power spectra of the horizontal winds, shows a spread in period around the central period (24 h) which is an indication of nonlinear interactions between the diurnal tide and planetary waves. A bispectral analysis reveals prominent triplets (two primary waves and a secondary wave) confirming the interaction of the diurnal tide with planetary waves persistent over a broad spectral range. Also there is an indication of coupling of the diurnal tide with the intraseasonal oscillations at various times of the year.
Journal of Atmospheric and Solar-Terrestrial Physics 10/2015; 133:7-17. DOI:10.1016/j.jastp.2015.07.016 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have applied a modified composite day analysis to the Hocking (2005) technique to study gravity wave (GW) momentum fluxes in the mesosphere and lower thermosphere (MLT). Wind measurements from almost continuous meteor radar observations during June 2004–December 2008 over São João do Cariri (Cariri; 7° S, 36° W), April 1999–November 2008 over Cachoeira Paulista (CP; 23° S, 45° W), and February 2005–December 2009 over Santa Maria (SM; 30° S, 54° W) were used to estimate the GW momentum fluxes and variances in the MLT region. Our analysis can provide monthly mean altitude profiles of vertical fluxes of horizontal momentum for short-period (less than 2–3 h) GWs. The averages for each month throughout the entire data series have shown different behavior for the momentum fluxes depending on latitude and component. The meridional component has almost the same behavior at the three sites, being positive (northward), for most part of the year. On the other hand, the zonal component shows different behavior at each location: it is positive for almost half the year at Cariri and SM but predominantly negative over CP. Annual variation in the GW momentum fluxes is present at all sites in the zonal component and also in SM at 89 km in the meridional component. The seasonal analysis has also shown a 4-month oscillation at 92.5 km over SM in the zonal component and over CP at the same altitudes but for the meridional component.
[Show abstract][Hide abstract] ABSTRACT: Planetary scale waves in equatorial region transport a significant amount of energy and momentum through atmosphere. Quantifying the momentum transported by these waves and its effects toward the mean flow is rather important. Direct estimative of the momentum flux transported by these waves requires horizontal and vertical wind measurements. Ground-based meteor radars have provided continuous and reliable measurements of the horizontal wind components in the MLT region. On the other hand, vertical wind component does not present the same reliability. However, an approach presented by Babu et al. (2012), made possible to calculate tridimensional daily winds by using only near zenith echoes detected around their maximum counting rate, i.e., during early morning hours. From daily tridimensional winds, momentum fluxes due to planetary scale waves can be estimated. Following this approach, we have used measurements performed by a meteor radar installed at São João do Cariri (7.4ºS; 36.5ºW) in order to measure vertical winds and after determine the momentum flux transported by these waves. Vertical wind presents magnitudes of a few meters per second and occasionally reaches magnitudes higher than 10 m/s. Below 92 km vertical wind is upward during the whole year and above exhibits a semi-annual oscillation with downward phase during the equinoxes. Variations associated to planetary scale waves in the vertical wind are also observed and some of them appear simultaneously in the zonal and meridional wind. Vertical transport of zonal momentum fluxes of 3-4 day component is found to be maximum near autumn equinox, when its value reaches almost 20 m 2 s-2 , while minimum momentum fluxes were observed before winter solstice.
14th International Congress of the Brazilian Geophysical Society; 08/2015
[Show abstract][Hide abstract] ABSTRACT: Meteor radar observations at São João do Cariri (7.4°S; 36.5°W), Cachoeira Paulista (22.7°S; 45°W) and Santa Maria (29.7°S; 53.7°W) have permitted estimates to be made of winds in the mesosphere and lower thermosphere (MLT) over the Brazilian sector simultaneously. Using horizontal winds the semidiurnal lunar tide is determined from January 2005 to December 2008 for these three sites. The lunar tide is observed to reach amplitudes as large as 8. m/s. In general, the amplitude increases with height and the phase decreases with height, corresponding to an upwardly-propagating tide. The estimated vertical wavelengths are variable for some month, like December at Cachoeira Paulista for northward wind, April and June at Santa Maria for eastward wind, which indicates possible mode coupling and reflection. Characteristics similar to those seen in the Northern Hemisphere have been observed in June and October at São João do Cariri, in December at Cachoeira Paulista, in March at Santa Maria and in August at all observation sites, which suggest the presence of antisymmetric modes. Different behavior has been observed in the amplitudes, phases and vertical wavelengths at each station, indicating latitudinal variation even from the low to the equatorial region.
Journal of Atmospheric and Solar-Terrestrial Physics 08/2015; 133. DOI:10.1016/j.jastp.2015.08.011 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Laser radar technique has been used measure mesospheric sodium density over São José dos Campos, SP (23° 12' S, 45° 51' W). Changes with time in the sodium layer structure shows the presence of oscillations with vertical wavelenghts > 20 km and a probable period of 24 hours. Theoretical calculations have been carried out to predict the solar thermal tide in this region. It is shown that the oscillating structures are consistent with the predictions for yhe solar diurnal atmospheric tide, which is dominated by the Hw1,1 tidal mode with wavelength > 20 km. Number of Pages: 25
[Show abstract][Hide abstract] ABSTRACT: An exclusive study has been carried out with long term meteor wind data (2000-2014) to characterize the quasi-2- day wave (QTDW) in the mesosphere and lower thermosphere (MLT) and its interactions with the longer period planetary waves at Cachoeira Paulista (22.7°S, 45°W). The QTDW is observed to be dominant during late summer (January-February) all the years under consideration except 2013. All the wave parameters exhibit significant interannual variability. The maximum wave amplitude comes out to be 39 m/s, which is significantly higher than the reported northern hemispheric findings. The mean MLT period exhibits a wide range of variability (36-70 h) indicating the presence of multiple Rossby normal modes with varying zonal wave numbers. Modulations of the QTDW amplitude by the planetary waves with longer periodicities (>9 days) are evident, especially during summer. The nonlinear interactions between the 2- day wave and longer period waves are believed to give rise to a host of secondary waves with frequencies lying close to 2- day. The strong QTDW activity, as observed at this location, has potential to cause significant effect on the overlying ionosphere and hence the atmosphere-ionosphere dynamical coupling.
Journal of Atmospheric and Solar-Terrestrial Physics 05/2015; 130-131. DOI:10.1016/j.jastp.2015.05.010 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A comparative study of intraseasonal oscillations (ISO) in the period range 20–110 days is carried out in the mesosphere and lower thermosphere (MLT) zonal wind at two low latitude stations, Cariri (7.4°S, 36.5°W) and Cachoeira Paulista (22.7°S, 45°W) located far from the convective anomaly region. Considerable seasonal and interannual variability is observed. The ISO in the MLT and lower atmosphere are found to be well correlated during winter and spring indicating a coupling of the atmospheric regions through the ISO. On the other hand, relatively less correlation during summer and fall may suggest a dominance of the in situ excitation of the ISO in the MLT relative to the lower atmospheric contribution. The correlation between the MLT and lower atmosphere is found to be a little higher at Cachoeira Paulista than Cariri. The ISO in the MLT shows good correlation between the two stations, but correlation is insignificant in the case of lower atmosphere. The ISO is most prominent in the upper troposphere, upper stratosphere and MLT. The waves responsible for communicating the ISO signature from the troposphere to the middle atmosphere in the tropics are believed to refract through mid-latitudes in course of their propagation. An evident height variation of the high amplitude ISO in the upper troposphere is observed with a clear annual oscillation at Cariri. The observed behaviors of the ISO at the present sites are discussed in the light of plausible physical mechanisms.
Journal of Atmospheric and Solar-Terrestrial Physics 12/2014; 120. DOI:10.1016/j.jastp.2014.08.016 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Characteristics of the intraseasonal oscillations (ISO) in the mesosphere and lower thermosphere (MLT) are investigated using meteor radar wind observations from a South American, equatorial station, Sao Joao do Cariri (7.4 degrees S, 36.5 degrees W) during 2008. A prominent ISO signature is observed during January-May in the period band approximately 40 to 70 days in the MLT zonal wind. In the lower atmosphere, a dominant ISO is observed in the period band approximately 30 to 60 days in the outgoing longwave radiation (OLR) (a proxy for convection), total columnar water vapor (a proxy for tides), and zonal wind. Considerable high correlation of the ISO between the MLT and lower atmosphere indicates significant dynamical coupling between the lower and middle atmosphere during the observational period. The MLT-ISO shows conspicuous downward propagation of the peak amplitude indicating the role of the dissipating upward propagating waves for its generation. The amplitudes in the zonal wind of the dominant tidal components in the MLT exhibit conspicuous ISO modulation. The eastward propagating waves and tides are surmised to be responsible for communicating the ISO signature from the lower atmosphere to the MLT. The origin of the MLT-ISO is believed to take place in the lower troposphere, below 4 km. The MLT-ISO is believed to be the imprint of the lower atmospheric Madden-Julian oscillation which travels eastward from the Indian Ocean-western Pacific Ocean to the present location.
Earth Planets and Space 11/2014; 66(1):145. DOI:10.1186/s40623-014-0145-3 · 1.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mean winds and tidal signatures in the mesosphere and lower thermosphere (MLT) region are derived from meteor radar observations at three sites around 22°S acquired in 2005. The observed differences of mean winds and tides are discussed in relation to the meteorological situation in the lower atmosphere and the possible generation of non-migrating tides. The longitudinally well separated radar sites allowed the evaluation of the migrating tidal component. The seasonal variation of signatures of the diurnal tide derived from ground-based radar observations, TIDI measurements aboard TIMED satellite, and model results obtained with HAMMONIA (Hamburg Model of the Neutral and Ionized Atmosphere) are compared. The ground-based, satellite, and model results of the total diurnal tide are in good agreement. The same is true for the migrating diurnal tide obtained from the radar observations, TIDI observations and from the model studies of HAMMONIA and GSWM00 (Global Scale Wave Model).
Journal of Atmospheric and Solar-Terrestrial Physics 10/2014; 118:96–105. DOI:10.1016/j.jastp.2013.07.005 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The characteristics of long period atmospheric oscillations are studied using long-term meteor radar wind observations over a low-latitude Brazilian station, Cachoeira Paulista (22.7°S, 45°W). The semiannual oscillation (SAO) amplitude decreases gradually with altitude in the mesopause region, while the annual oscillation (AO) amplitude exhibits a minimum at mid-mesopause with significant interannual variability. The quasi-biennial oscillation (QBO) obtained by removing the SAO and AO from the zonal wind exhibits an out-of-phase relationship at lower mesopause heights (approximately 81 km) and mid-stratosphere (approximately 10 hPa). There is an evident signature of modulation of the mesospheric SAO (MSAO) and mesospheric AO (MAO) by the QBO. The diurnal tide is surmised to contribute significantly to drive the easterly phase of these oscillations, especially MSAO and MAO through depositing westward momentum to the background wind. The westerly phase of the MSAO is believed to support the growth of the quasi-2-day wave (QTDW) by wave filtering. The QTDW is found to be strong during the easterly phase of the MAO indicating a possible existence of the barotropic/baroclinic instability during summer and a possible role of the QTDW towards westward forcing of the background wind.
Earth Planets and Space 05/2014; 66(1). DOI:10.1186/1880-5981-66-45 · 1.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this study we have used wind observation data from the mesosphere and lower thermosphere (MLT) region, obtained from meteor radar measurements in São João do Cariri (7.4° S, 36.5° W) from July 2004 to December 2008 and in Cachoeira Paulista (22.7° S, 45.0° W) from January 2002 to July 2006 and from September 2007 to November 2008. From the spectral analysis it was possible to identify the presence of planetary-scale oscillations in the hourly winds for the two latitudes and to study their transient character, which allowed elaboration of a climatology of planetary oscillation signatures. Planetary waves with periods near 2-days, 6–7 days, and 16 days were focussed on in this study. The quasi-2-day waves in the meteoric winds showed a seasonal cycle, with intense amplitudes occurring after the austral summer solstice and extending until the end of the season. The vertical wavelengths of the 2-day wave over Cachoeira Paulista were larger than those at São João do Cariri. A possible modulation of the quasi-2-day wave amplitudes by the quasi-biennial oscillation (QBO) has been observed only at São João do Cariri. The 6–7 day oscillations presented more intense amplitudes during August–November but were present with lower amplitudes during March–April at both sites. The 6–7 day vertical wavelengths over São João do Cariri were larger than at Cachoeira Paulista. The 6–7 day amplitudes exhibited intra-seasonal and annual behavior, however, there was no clear evidence of QBO modulation. The 16-day oscillations showed a seasonal cycle at São João do Cariri, with amplifications from austral spring to mid-summer and weaker amplitudes from autumn until early winter, however, there was no clear seasonality over Cachoeira Paulista. The 16-day vertical wavelengths have assumed values of λz ~ 45–85 km over both sites. 16-day wave amplitudes at the two sites showed different long-term behaviors.
[Show abstract][Hide abstract] ABSTRACT: Long-term data available from Lidar systems located at three different locations namely São José dos Campos, Brazil (23.2° S, 45.8° W), Gadanki (13.5° N, 79.2° E) and Reunion (20.8° S, 55.5° E) have been used to investigate the long-term variations like Annual, Semi-annual, Quasi-biennial, El Nino Southern Oscillation and solar cycle. These oscillations are also extracted from simultaneous satellite borne measurements of HALogen Occultation Experiment (HALOE) instrument onboard UARS and SABER onboard TIMED over these stations making largest time series covering the entire middle atmosphere. A good agreement is found between the LIDAR and satellite-derived amplitudes and phases between 30 and 65 km altitude, which suggests that satellite measurements can be used to investigate the long-term trends globally. Latter measurements are extended to 80 km in order to further investigate these oscillations. Large difference in the amplitudes between the eastern pacific and western pacific is noticed in these oscillations. Changing from cooling trends in the stratosphere to warming trends in the mesosphere occurs more or less at altitude around 70 km altitude and this result agrees well with that observed by satellite measurements reported in the literature. The peak in the cooling trend does not occur at a fixed altitude in the stratosphere however maximum warming trend is observed around 75 km at all the stations. The observed long-term trends including various oscillations are compared with that reported with various techniques.
[Show abstract][Hide abstract] ABSTRACT: The effects of a major stratospheric sudden warming (SSW) at extratropical latitudes have been investigated with wind and temperature observations over a Brazilian station, Cachoeira Paulista (22.7°S, 45°W) during September–October 2002. In response to the warming at polar latitudes a corresponding cooling at tropical and extratropical latitudes is prominent in the stratosphere. A conspicuous signature of latitudinal propagation of a planetary wave of zonal wavenumbers 1 and 2 from polar to low latitude has been observed during the warming period. The polar vortex which split into two parts of different size is found to travel considerably low latitude. Significant air mass mixing between low and high latitudes is caused by planetary wave breaking. The meridional wind exhibits oscillations of period 2–4 days during the warming period in the stratosphere. No wave feature is evident in the mesosphere during the warming period, although a 12–14 day periodicity is observed after 2 weeks of the warming event, indicating close resemblance to the results of other simultaneous investigations carried out from high latitude Antarctic stations. Convective activity over the present extratropical station diminishes remarkably during the warming period. This behavior is possibly due to destabilization and shift of equatorial convective active regions towards the opposite hemisphere in response to changes in the mean meridional circulation in concert with the SSW.
Advances in Space Research 12/2013; 53(2):257-265. DOI:10.1016/j.asr.2013.11.002 · 1.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We employ a modified composite day extension
of the Hocking (2005) analysis method to study gravity
wave (GW) activity in the mesosphere and lower thermosphere
using 4 meteor radars spanning latitudes from 7� S to
53.6� S. Diurnal and semidiurnal modulations were observed
in GW variances over all sites. Semidiurnal modulation with
downward phase propagation was observed at lower latitudes
mainly near the equinoxes. Diurnal modulations occur
mainly near solstice and, except for the zonal component at
Cariri (7� S), do not exhibit downward phase propagation. At
a higher latitude (SAAMER, 53.6� S) these modulations are
only observed in the meridional component where we can
observe diurnal variation from March to May, and semidiurnal,
during January, February, October (above 88 km) and
November. Some of these modulations with downward phase
progression correlate well with wind shear. When the wind
shear is well correlated with the maximum of the variances
the diurnal tide has its largest amplitudes, i.e., near equinox.
Correlations exhibiting variations with tidal phases suggest
significant GW-tidal interactions that have different characters
depending on the tidal components and possible mean
wind shears. Modulations that do not exhibit phase variations
could be indicative of diurnal variations in GW sources.
[Show abstract][Hide abstract] ABSTRACT: Sporadic sodium (Nas) layers, occurring in roughly the same
height range as ionospheric sporadic-E layers, were first detected by
lidar some 30 yr ago. Nas layers have a typical thickness of
a few hundred meters to a few km, with peak atom concentrations several
times that of the background layer. Despite a great deal of excellent
work over the past decades, the source of Nas layers is still
not altogether clear, partly as a result of our incomplete knowledge of
Nas layer characteristics. In this paper we concentrate on
some typical case studies chosen from the ~127 h of sporadic sodium
layer observations made at a time resolution of 1.5 s at Yanqing
(115.97° E, 40.47° N), Beijing, China. This is a much better
time resolution than what has been employed in most earlier
measurements. The results show that the Nas layer peak
heights are dispersed at slightly different although adjacent heights.
When averaged over several minutes, as has been the case with most
earlier measurements, the height scatter results in an apparent layer
thickness of a few km. We conclude, therefore, that these dispersed
peaks at different but adjacent heights constitute the 5 min
Nas layer. Similar to the observations of sporadic-E-ion (Es)
layers and meteor rate, we observe quasi-periodic fluctuations on a
timescale on the order of several minutes in the peak height and the
peak density of sporadic layers, which is a universal feature but
concealed by the lower temporal resolution previously adopted. Spatially
localized multiple scatterers and multiple thin layers with similar
apparent movement in Nas layers are also found. We discuss
the possible formation mechanism by the direct deposition of large
swarms of micrometeoroids and demonstrate a typical example of meteor
trails evolving into a Nas layer, which suggests that this
mechanism might indeed occur.