Study of the quasi-two-day wave during summer over Santa Maria, Brazil using meteor radar observations

摘要

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 (29.7°S, 53.7°W), 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.