Tidal wind oscillations in the tropical lower atmosphere as observed by Indian MST Radar

Abstract

Abstract. Diurnal tidal components in horizontal winds measured by MST radar in the troposphere and lower stratosphere over a tropical station Gadanki (13.5 • N, 79.2 • E) are presented for the autumn equinox, winter, vernal equinox and summer seasons. For this purpose radar data obtained over many diurnal cycles from September 1995 to August 1996 are used. The results obtained show that although the seasonal variation of the diurnal tidal amplitudes in zonal and meridional winds is not strong, vertical phase propagation characteristics show significant seasonal variation. An attempt is made to simulate the diurnal tidal amplitudes and phases in the lower atmosphere over Gadanki using classical tidal theory by incorporating diurnal heat sources, namely, solar radiation absorption by water vapour, planetary boundary layer (PBL) heat flux, latent heat release in deep convective clouds and short wave solar radiation absorption by clouds. A comparison of the simulated amplitudes and phases with the observed ones shows that agreement between the two is quite good for the equinox seasons, especially the vertical structure of the phases of the meridional wind components.

First results from the CAWSES-India Tidal Campaign

“…During this period, the MST radar at Gadanki sampled the altitude region 3-25 km in 4-5 day diurnal cycles. The largest radar derived tidal amplitudes estimated during this period were in the range 2-3 m/s and are in agreement with the earlier report by Sasi et al (2001). An interesting outcome from combining the results from the rocket and MST radar derived data sets is noted in this work.…”

Meteor wind radar observations of tidal amplitudes over a low-latitude station Trivandrum (8.5°N, 77°E): Interannual variability and the effect of background wind on diurnal tidal amplitudes

“…Their study also shows that zonal wavenumber 5 is prominent throughout the year for all the seasons, whereas zonal wavenumber À3 is absent during summer season. A simulation study by Sasi et al (2001) shows that the nonmigrating component of zonal wavenumber 5 is more prominent than zonal wavenumber À3 in the low-latitude lower atmosphere. Tsuda and Kato (1989) has also shown a similar zonal wavenumber distribution with the dominance of zonal wavenumber 5 for planetary boundary layer heat flux, which is a source for nonmigrating tidal components.…”

Diurnal Variations in Lower-Tropospheric Wind over Japan Part I: Observational Results using the Wind Profiler Network and Data Acquisition System (WINDAS)

“…The main excitation sources of tides are the absorption of solar radiation by tropospheric water vapor and stratospheric ozone (e.g., Chapman and Lindzen 1970), as well as diurnally varying latent heat release in the tropics (Hamilton 1981;Williams and Avery 1996a;Forbes et al 1997;Forbes 2002, 2003). Planetary-boundary-layer sensible heating (Tsuda and Kato 1989) and solar heating of cloud particles (Sasi et al 2001) are also possible excitation sources. Diurnal and semidiurnal tides have contrasting physical characteristics (e.g., vertical wavelength, latitudinal structure, and vertical propagation).…”

“…Few studies have provided a broader viewpoint of ''diurnal wind variations in the troposphere,'' including local wind systems. These few studies were based mainly on tropical observations, either intensive radiosonde campaigns (Tsuda et al 1994;Alexander and Tsuda 2008) or atmospheric radars (Williams et al 1992;Chen et al 1996;Williams and Avery 1996b;Sasi et al 1998Sasi et al , 2001Riggin et al 2002), and investigated the diurnal and semidiurnal wind components by focusing on atmospheric tides and local wind systems.…”