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Ann. Geophys., 19, 975-984, 2001
www.ann-geophys.net/19/975/2001/
© European Geosciences Union 2001
Seasonal variation of vertical eddy diffusivity in the troposphere, lower stratosphere and mesosphere over a tropical station
D. Narayana Rao, M. V. Ratnam, T. N. Rao, and S. V. B. Rao
Department of Physics, Sri Venkateswara University, Tirupati 517 502, India
Abstract. Long-term VHF radar (53
MHz with 3° beam-width) observations at Gadanki (13.5° N, 79.2° E), India,
during the period from September 1995 to August 1999 are used to study monthly,
seasonal and annual medians of vertical eddy diffusivity, K in the
troposphere, lower stratosphere and mesosphere. First, the spectral width
contribution due to non-turbulent effects has been removed for further analysis
and the monthly, seasonal medians of K are calculated. The monthly
median of K in the troposphere shows maximum and minimum in June-July
and November-December, respectively. In general, large values of K are
seen up to 10 km and then decrease with height. Larger values of K are
observed during monsoon and post-monsoon than in winter and summer. In general,
the maximum and minimum values of the annual median of K (in logarithmic
values) in the troposphere are found to be 0.25 and - 1.3 m2 s-1
respectively. In the mesosphere, the monthly median of K shows maximum
and minimum during June-July and November-December, respectively, similar to
the lower atmosphere. The value of K in the mesosphere becomes larger
and it increases with height up to 75 km and again decreases above that height.
The maximum values are seen during the summer, followed by equinoxes and a
minimum during the winter. In general, the maximum and minimum values of K
(in logarithmic values) are found to be 0.7 and 0.3 m2 s-1,
respectively, in the mesosphere. A comparison of Doppler spectral parameters in
different beam directions shows anisotropy in both signal-to- noise ratio (SNR)
and spectral widths in the mesosphere, whereas it shows isotropy in SNR and
anisotropy in the spectral widths in troposphere and lower stratosphere.
Key words. Meteorology and
atmospheric dynamics (middle atmosphere dynamics; turbulence; waves and tides)
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