Seasonal variation of vertical eddy diffusivity in the troposphere, lower stratosphere and mesosphere over a tropical station

Long-term VHF radar (53 MHz with 3 beamwidth) 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 m2s−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 m2s−1, respectively, in the mesosphere. A comparison of Doppler spectral parameters in different beam directions shows anisotropy in both signalto-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.