Abstract. Turbulent kinetic energy dissipation rates (ε) and eddy diffusion coefficients (K z ) in the tropical mesosphere over Gadanki (13.5 • N, 79.2 • E), estimated from Doppler widths of MST radar echoes (vertical beam), observed over a 3-year period, show a seasonal variation with a dominant summer maximum. The observed seasonal variation of ε and K z in the mesosphere is only partially consistent with that of gravity wave activity inferred from mesospheric winds and temperatures measured by rockets for a period of 9 years at Trivandrum (8.5 • N, 77 • E) (which shows two equinox and one summer maxima) lying close to Gadanki. The summer maximum of mesospheric ε and K z values appears to be related to the enhanced gravity wave activity over the lowlatitude Indian subcontinent during the southwest monsoon period (June -September). Both ε and K z in the mesosphere over Gadanki show an increase with an increase in height during all seasons. The absolute values of observed ε and K z in the mesosphere (above ∼80 km) does not show significant differences from those reported for high latitudes. Comparison of observed K z values during the winter above Gadanki with those over Arecibo (18.5 • N, 66 • W) shows that they are not significantly different from each other above the ∼80 km altitude.

Abstract. Tidal activity in the Mesospheric Lower Thermosphere (MLT) region over Trivandrum (8.5 • N, 77 • E) is investigated using the observations from newly installed SKiYMET Meteor Radar. The seasonal variability and vertical propagation characteristics of atmospheric tides in the MLT region are addressed in the present communication.The observations revealed that the diurnal tide is more prominent than the semi/terdiurnal components over this latitude. It is also observed that the amplitudes of meridional components are stronger than that of zonal ones. The amplitude and phase structure shows the vertical propagation of diurnal tides with vertical wavelength of ∼25 km. However, the vertical wavelength of the semidiurnal tide showed considerable variations. The vertical propagation characteristics of the terdiurnal tide showed some indications of their generating mechanisms. The observed features of tidal components are compared with Global Scale Wave Model (GSWM02) values and they showed a similar amplitude and phase structure for diurnal tides. Month-to-month variations in the tidal amplitudes have shown significant seasonal variation. The observed seasonal variation is discussed in light of the variation in tidal forcing and dissipation.

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SUMMARYUsing altitude pro les of temperature in the range 4 to 70 km derived from Mesosphere-StratosphereTroposphere radar and lidar observations at Gadanki (13.5 ± N, 79.2 ± E) from 18 January 1999 to 5 March 1999, characteristics of equatorial waves are studied. Two-dimensional Fourier-transform analysis of the temperature pro les is carried out to identify the periodicities and their vertical wave numbers. From the characteristics obtained, equatorial slow Kelvin waves with periodicities 15.7 d, 9.4 d, 7.8 d and 6.7 d are identi ed in the troposphere and stratosphere regions and among these 7.8 d and 6.7 d periodicities are found to penetrate into the mesosphere. Equatorial waves with smaller periodicities in the range 5.2 d to 3.6 d are also observed. The vertical ux of horizontal momentum (zonal) of the identi ed slow Kelvin-wave periodicities in the altitude region 4-25 km is estimated. It is found that equatorial waves modulate tropical tropopause temperature and altitude.