[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; 53:257-265. · 1.18 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: Features of the terdiurnal tide in the mesosphere and lower thermosphere (MLT) are investigated using four years (2005–2008) of meteor radar wind data from a southern hemisphere equatorial station, São João do Cariri (7.4S, 36.5W). Prominent existence of the terdiurnal tide is found to be a permanent feature of the MLT over the present site. It exhibits considerable inter-annual variability with smaller amplitude during 2007. Maximum monthly mean amplitudes of the terdiurnal tide are found to be $ 11 and 14 m/s in the zonal and meridional winds, respectively. The meridional wind shows sharp amplitude enhancement during fall equinox and a broader peak during late winter and spring equinox. Phase profiles indicate large vertical wavelength and possible wave evanescence. The generation mechanism of the terdiurnal tide has been investigated and it is inferred that nonlinear interaction between the diurnal tide and semidiurnal tide/gravity waves can partially excite the terdiurnal tide during fall equinox and solar heating is believed to be the primary source at the present latitude throughout the year.
Journal of Atmospheric and Solar-Terrestrial Physics 08/2013; 104:87-95. · 1.42 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 07/2013; · 1.42 Impact Factor
[show abstract][hide abstract] ABSTRACT: Characteristics of the 3.5-day ultra-fast Kelvin (UKF) wave are investigated with 4 years of meteor radar wind observations over Cachoeira Paulista (22.7S, 45W). The UFK signature manifests itself as discrete bursts spanning over various times of the year with generally less amplitude during fall equinox and early winter. Daily as well as seasonal mean amplitude of the UFK component is found to be considerably smaller than that observed previously from equatorial stations. Estimated vertical wavelengths of the UFK wave come out to be larger than 40 km. Comparison of the present results with another Brazilian equatorial station implies latitudinal invariance of the wave as well as transient local dynamical influence to its variability. High zonal acceleration caused by momentum flux convergence due to the UFK wave just after equinoxes is believed to be responsible for driving the westerly phase of the mean zonal wind immediately after the equinoxes and hence the mesospheric semiannual oscillation to some extent. There is a clear signature of modulation of the UFK wave amplitude by various harmonics of the annual oscillation.
Journal of Atmospheric and Solar-Terrestrial Physics 05/2013; · 1.42 Impact Factor
[show abstract][hide abstract] ABSTRACT: The composite day analysis is a widely known tool that presents advantages for all-sky interferometric meteor radars when measuring mean winds and tides. In contrast, problems happen if this technique is applied to Hocking’s (2005) analysis of gravity wave (GW). This is because any daily variability of the prevailing winds, like tide daily variability, will be seen as a GW perturbation. In this work, we describe how a simple change in this procedure makes possible to use a composite day in Hocking’s (2005) analysis allowing inferring GW momentum fluxes from the simple all-sky meteor radars. Also, we explain how a modified composite day can be constructed to test its ability to measure gravity wave momentum fluxes. Tests results for specified mean, tidal, and GW fields, including tidal amplitudes and its phase propagation, suggest that the modified composite day allows characterization of monthly mean profiles of the GW momentum fluxes, with good accuracy at least at the altitudes where the meteor counts are large (from 89 to 92.5 km). We also show in this work that the variances values from Hocking’s (2005) analysis have a contribution of tidal propagation. For this reason, an empirical method is suggested here to overcome this problem by using a simple simulation model, making possible to extract reliable GW variances values from the Hocking’s analysis. Test results make us very confident, because they show that our technique is able to remove the tide-induced false variances from Hocking’s analysis.
AGU Meeting of Americas 2013, Cancun, Mexico; 05/2013
[show abstract][hide abstract] ABSTRACT: Using data obtained from LIDARs installed at São José dos Campos (23.2°S, 45.9°W) and Wuhan (30.5°N, 114.4°E), we have observed the main characteristics of Sporadic Sodium Layers (Nas), including full width at half maximum, peak height, intensity, preferred time of occurrence and vertical motion. These characteristics, observed during the period from 1996 to 2008, were used to make a comparison of seasonal, annual and nocturnal variation, typical peak height, width and speed of descent between the two locations. The results show good agreement between the two locations, leading us to believe that the same mechanism is responsible for the formation of Nas observed in Brazil (low latitude) and China (medium latitude). Besides the study with LIDAR data, we analyzed meteor radar and ionosonde data from the Brazilian sector in order to investigate the possible contribution of ionospheric sporadic layers (Es) and wind shear in the formation of Nas. In the near future, all-sky images of the sodium airglow emission will also be used in an attempt to observe the horizontal dimensions and motion direction of Nas clouds. In this paper we show the results obtained so far and discuss the possible mechanisms for the generation of this phenomenon.
AGU Meeting of the Americas, Cancun, Mexico; 05/2013
[show abstract][hide abstract] ABSTRACT: The advantages of using a composite day analy-sis for all-sky interferometric meteor radars when measur-ing mean winds and tides are widely known. On the other hand, problems arise if this technique is applied to Hock-ing's (2005) gravity wave analysis for all-sky meteor radars. In this paper we describe how a simple change in the proce-dure makes it possible to use a composite day in Hocking's analysis. Also, we explain how a modified composite day can be constructed to test its ability to measure gravity wave momentum fluxes. Test results for specified mean, tidal, and gravity wave fields, including tidal amplitudes and gravity wave momentum fluxes varying strongly with altitude and/or time, suggest that the modified composite day allows charac-terization of monthly mean profiles of the gravity wave mo-mentum fluxes, with good accuracy at least at the altitudes where the meteor counts are large (from 89 to 92.5 km). In the present work we also show that the variances measured with Hocking's method are often contaminated by the tidal fields and suggest a method of empirical correction derived from a simple simulation model. The results presented here greatly increase our confidence because they show that our technique is able to remove the tide-induced false variances from Hocking's analysis.
[show abstract][hide abstract] ABSTRACT: Meteor radar observations at São João do Cariri (7.4°S, 36.5°W), Cachoeira Paulista (23°S, 45°W) and Santa Maria (29.7°S, 53.7°W) have permitted estimates to be made of winds and temperature in the mesosphere and lower thermosphere (MLT). Using horizontal winds the lunar semidiurnal tide was determined from January 2005 to December 2008 for these three sites. In January 2006 an unusual enhancement was observed in the lunar tide amplitude at these stations, for meridional and zonal components. During this period, sudden stratospheric warming (SSW) events were observed in the northern hemisphere. Meridional and zonal winds observed in Brazil showed evidence of SSW effects in the MLT region. Moreover, the mesospheric temperatures at Cachoeira Paulista and Santa Maria presented cooling and a semimonthly oscillation, which followed the new–full–new moon phases. These results suggest that the enhancement of the lunar tide could be a response for the SSW in the MLT.
Journal of Atmospheric and Solar-Terrestrial Physics 12/2012; s 90–91:97–103. · 1.42 Impact Factor
[show abstract][hide abstract] ABSTRACT: Characteristics of the quasi-two-day wave (QTDW) have been investigated during late summer of 2005, 2006 and 2007 using meteor wind observations over Santa Maria (29.71S, 53.71W), Brazil. Significant inter-annual variability is evident in the derived wave parameters. Maximum wave amplitude is found to be 18 and 42 m/s for zonal and meridional wind, respectively. Mean vertical wavelength shows the values of $ 62 and 74 km for zonal and meridional components, respectively. In general high QTDW amplitude is observed when diurnal tide amplitude is lower suggesting strong wave-tide interaction and enhancement of the QTDW at the expense of the diurnal tide energy. Furthermore, the amplitude of the QTDW reveals a clear modulation of 7–8 days periodicity suggesting nonlinear interaction among planetary waves and a probable reason for the observed QTDW variability. Comparison of the present site with the other two Brazilian low latitude sites reveals overall latitudinal invariance except during a few local and/or transient events. There is an indication of wave ducting in the mesosphere and lower thermosphere through which the QTDW can extend latitudinally. Although the response of the QTDW to the mean background wind sometimes exhibits meridional (northward) acceleration, there is no evident signature in the zonal background wind. Both mechanisms suggested so far for exciting the QTDW, i.e. (i) baroclinic/barotropic instability and (ii) Rossby-gravity normal modes are found to be consistent with the present findings.
Journal of Atmospheric and Solar-Terrestrial Physics 11/2012; · 1.42 Impact Factor
[show abstract][hide abstract] ABSTRACT: Three meteor radars of the SkiYmet type have been installed in Brazil covering low, tropical and
sub-tropical latitudes. The �rst at Cachoeira Paulista(22.7 S, 45.0 W) started in march 1999,
the second at Cariri(7.4 S, 36.5 W) in May, 2005, and the last one at Santa Maria( 29.7 S, 53.8
W) in December, 2005. Coincident periods of measurements permitted the determination of the
Mean Winds, Planetary Waves, Tides and Gravity Wave Variances for these di�erent latitudes
and their comparison. Amplitude and phase structures are similar for Cachoeira Paulista and
Santa Maria, but di�er from the near-equatorial site Cariri. Also the Lunar Semidiurnal Tides
have been studied at the three sites for the period January 2005 to December 2008. Amplitudes
between 1 and 8 m/s were determined with the meridional winds being larger than the zonal
in the three sites. It is found that northern hemisphere SSW's a�ect the QTDW , and the
solar and lunar tides at southern low latitudes. Wind measurements have been used also as
subsidiary data in the studies involving the sodium layer and the mesospheric airglow though
Lidar, Photometers and Imagers.
[show abstract][hide abstract] ABSTRACT: The dynamics of the equatorial mesosphere have been investigated during austral summers of 2004–2005, 2005–2006 and 2006–2007 from meteor radar measurements obtained at São João do Cariri, Brazil (7.4°S, 36.5°W), together with stratospheric northern hemisphere polar parameters. Some recent studies have demonstrated coupling between high latitude northern hemisphere major Sudden Stratospheric Warming (SSW) events and mesospheric–ionospheric disturbances, including the equatorial region. Here we have analyzed the tides, quasi-two-day planetary wave and local mean zonal wind variability in the equatorial upper Mesosphere and Lower Thermosphere (MLT) region during three austral summers and found that upper mesospheric dynamics at 7°S has been affected when a remarkable major SSW event took place in January 2006. The more intense tides and quasi-two-day wave amplitudes during the major SSW event are suggestive of an association between them and high planetary wave activity in the northern stratospheric winter.
Journal of Atmospheric and Solar-Terrestrial Physics 04/2012; · 1.42 Impact Factor
[show abstract][hide abstract] ABSTRACT: In the present work we have used Hocking’s (2005) technique to infer the monthly mean gravity wave (GW) momentum fluxes and their seasonal variability over Sao Joao do Cariri (7°S; 36°W). We also have used numerical simulations to test the ability of Skyimet meteor radars to measure GW momentum fluxes. From theses tests we can conclude that, provided allowance is made for contamination by tides, we can measure GW momentum fluxes with reasonable accuracy on a monthly basis, at least at altitudes at which meteor counts are relatively high, typically from 87 to 94.5 km, with greater accuracies near the peak of the meteor distribution in altitude. We have analyzed the data from June 2004 to December 2008 and the results are encouraging. We observed mainly annual variability in the component, being positive from March to August. On the other hand, we observe mainly a 4 month oscillation in the component, being positive from February to April, July to August, and October to November.
[show abstract][hide abstract] ABSTRACT: A sodium lidar, capable of measuring temperature in the 80–100 km region, has been in operation at São José dos Campos (23° S, 46 W) since March 2007. Good quality data have been obtained for late autumn, winter and spring, but weather conditions make it extremely difficult to make measurements from mid-November to mid- February. We find the temperature structure to be strongly modulated by tides and gravity waves, but average profiles typically show a primary mesopause height close to 100 km with temperatures around 180 K, and a tendency for a secondary minimum of about 185 K to occur close to 90 km. Vertical temperature gradients greater than 50 K/km are sometimes seen even on profiles averaged over several hours. The strongest gradients are always positive and are frequently associated with strong gradients in sodium concentration. On the other hand, we frequently see rapid changes in the temperature profile, suggesting that models and non-local temperature measurements, as made by satellite radiometers, for example, are of little use in applications such as the analysis of gravity wave propagation seen in airglow images.
Advances in Space Research 01/2011; · 1.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: In Simonich et al. (1979) the authors published a paper dealing principally with the seasonal variation of the sodium layer for 344 nights of data from 1972 to 1977. In this paper we will consider the annual average profile from 1980 to 2008 (29 years), for a total of 1254 nights of data. We first took all the data and for each day averaged the abundances and the sodium profiles to get an average for the night. These daily averages were subsequently averaged to get an average profile and abundance for a given month. Comparing the yearly sodium variation thus derived with fig. 2 of Simonich et al. we note that the semiannual behavior with maxima in May and August has been replaced by an annual variation with maxima in August and October with the minimum of 2.5 x 109 m-3 remaining in December. The amplitude of the maxima is 4 x 109 m-3 which is the same as observed for the August peak in the cited paper. The May peak observed in Simonich et al. was slightly larger with amplitude of 5 x 109 m-3. The abundance variation was similar for the present analysis and the cited work for the first half of the year. In the second half there are two peaks corresponding to the two maxima observed in the density (months 8 and 10) and the minimum was in December in both. The amplitudes are slightly smaller than those of Simonich et al. In 2006 we received a new narrow band laser which is capable of making temperature measurements. Since this laser can be synchronized exactly to the D2a hyperfine line we have excellent calibration of our signal. We have analyzed 182 nights of data varying from 1 night in January to 31 in August for the years 2007/8. The annual variation is somewhat similar to the first analysis. There are peaks of about 3.5 x 109 m-3 in January, May, August and October. January has only one day of data so it is probably just a coincidence. The reason for the May peak is unknown even though there were 21 days of data analyzed. The last two are consistent with the same peaks in the first analysis although the amplitudes are slightly smaller 3 x 109 m-3 as opposed to 4 x 109 m-3 in the first analyses. Comparing the abundances of these two analyses we find them to be quite similar with an average deviation of 0.42 x 1013 m-2. The centroid height for the 2007/8 data is also quite similar to that for the data analyzed above. The major difference is that it is consistently less than that for all the data. The average difference was 0.45 km. less. This might be due to the much narrower bandwidth (> 0.28 pm) of the new laser as compared to that which was used earlier (~8 to 10 pm). The effect of this narrow bandwidth is to get scattering from the coldest part of the mesosphere. The lower centroid could thus be due to a possible long term cooling of the Mesopause. Simonich, D.M., B.R. Clemesha, and V.W.J.H. Kirchhoff, The Mesospheric Sodium Layer at 23 S: Nocturnal and Seasonal variations, J. Geophys. Res., 84, 1543-1550, 1979.
[show abstract][hide abstract] ABSTRACT: Internal gravity waves propagating up from the lower atmosphere are thought to be the main mechanism responsible for transporting energy and momentum to the MLT region. In order to study the behavior of the wind variance caused by gravity waves, we use Hocking’s technique for analyzing meteor radar data. This method is a generalization of the 2-beam technique of Vincent and Reid (1983) designed to infer gravity wave momentum fluxes and variances from SKYiMET meteor radar data. The meteor radar at Cachoeira Paulista (22.7°S, 45.0°W) has been measuring mesospheric winds since 1999, although the equipment has been out of commission since 2008. Thus we are able to study the monthly mean variances for approximately ten years of data. We observe that the meridional variances are always stronger than the zonal one, and there are semidiurnal modulations in both components. In addition, the maximum amplitude of this modulation coincides with the maximum gradient of the background winds, confirming earlier studies related by Clemesha et al. (2009).
AGU Meeting of Americas 2010, Foz do Iguaçu, Brazil; 08/2010
[show abstract][hide abstract] ABSTRACT: It is well known that the upward propagation of internal gravity waves from the lower atmosphere to the mesosphere plays an important role in the dynamics and energy balance of this region. Hocking (2005) developed a technique to calculate gravity wave momentum flux using meteor radar data. This technique is a generalization of the 2-beam technique of Vincent and Reid (1983). Hocking’s technique uses radial velocity variances, from 80 to 100 km, which are mainly caused by gravity waves, to determine the gravity wave momentum fluxes. We apply this technique to data from a SKiMET meteor radar located at Santa Maria (29.7°S, 53.7ºW) during 2005. The data were analyzed in 3-km/2-h bins centered on 82, 85, 88 km etc. and 1, 3, 5 UT etc., generating monthly means. It was found that the meridional variances showed a fairly constant behavior throughout the year, with maximum at around 90 km. The zonal and vertical variances were less consistent.
The monthly means of the horizontal momentum flux, u´v´, showed an oscillatory behavior with phase decreasing with increasing altitude and similar behavior was observed in the v’w’ component. Although the behavior of u’w’ was observed to be oscillatory, its phase did not show altitude propagation. In order to study the features of gravity wave activity in different latitude these results will be compared with two other radars located at São João do Cariri (7.3°S, 36.4°W) and Cachoeira Paulista (22.7°S, 45.0°W) for the same period.
[show abstract][hide abstract] ABSTRACT: Mesopause region temperature measurements made with a sodium resonance lidar show two unexpected features: (1) Strong positive temperature gradients are often associated with strong gradients in the sodium concentration and (2) positive temperature gradients are generally much stronger than negative ones. Although the structures we see frequently appear to be associated with gravity waves or tides, the asymmetrical temperature oscillations cannot be explained as the result of simple wave propagation. We suggest that strong positive temperature gradients correspond to regions of high atmospheric stability, where eddy diffusion is inhibited, permitting the build-up of strong gradients in temperature and minor constituent mixing ratio.
Journal of Atmospheric and Solar-Terrestrial Physics 01/2010; 72(9):740-744. · 1.42 Impact Factor