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.

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SUMMARYQuantitative theoretical estimates are made of the reflection and radiative damping of the Kelvin and Rossby-gravity (RG) waves propagating upward in the 20-90 km altitude region. An atmosphere with realistic temperature, wind and cooling rate coefficient values is approximated by a model of vertically stacked, thin, homogeneous layers. The amplitude reflection coefficients are shown for different wave periods ( r ) and zonal wavenumbers ( k J . Combining the attenuation due to radiative cooling with the growth due to density decrease, the net changes of wave amplitudes with height in the 20-90 km region are shown for various wave modes.For Kelvin waves the reflected wave energy is generally less than 15% of the incident wave energy, but the reflected wave amplitude can be 20%-30% of the incident wave amplitude for many modes, at 20 km height. In contrast, many RG wave modes suffer strong or total reflection at various heights if they are not severely damped at lower heights. Calculations for January and July wind conditions reveal a good deal of sensitivity of RG wave reflection to wind conditions. Radiative damping is found to be severe in the mesosphere for Kelvin waves with T s 6-8 days, depending on wind conditions; it is severe even in the lower stratosphere for RG waves with T > 4 days. Considerable changes in the damping rates and wave amplitudes at various heights take place from January to July owing to wind changes. Kelvin waves in the 2-8-day period range and RG waves in the 1.5-3-day range can propagate into the upper mesosphere/lower thermosphere without large attenuation, depending upon wind conditions.It is shown that our theoretical results can explain several observed characteristics of equatorial waves in the stratosphere and mesosphere, including the recent results on wave disturbances in satellite-measured temperatures. Further modelling studies based on our methodology can lead to a good physical insight and quantitative interpretations of the SAO-and the QBO-generation processes.

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