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 degrees beamwidth) observations at Gadan ki (13.5 degrees N, 79.2 degrees E), India, during the period from Septembe r 1995 to August 1999 are used to study monthly, seasonal and annual median s of vertical eddy diffusivity, K in the troposphere, lower stratosphere an d mesosphere. First, the spectral width contribution due to non-turbulent e ffects has been removed for further analysis and the monthly, seasonal medi ans of K are calculated. The monthly median of K in the troposphere shows m aximum and minimum in June-July and November-December, respectively. In gen eral, 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 wi nter and summer. In general, the maximum and minimum values of the annual m edian of K (in logarithmic values) in the troposphere are found to be 0.25 and -1.3 m(2)s(-1) respectively. In the mesosphere, the monthly median of K shows maximum and minimum during June-July and November-December, respecti vely, similar to the lower atmosphere. The value of K in the mesosphere bec omes larger and it increases with height up to 75 km and again decreases ab ove that hei ht. The maximum values are seen during the summer, followed by equinoxes and a minimum during the winter. In general, the maximum and min imum values of K (in logarithmic values) are found to be 0.7 and 0.3 m(2)s( -1) respectively, in the mesosphere. A comparison of Doppler spectral param eters in different beam directions shows anisotropy in both signal-to-noise ratio (SNR) and spectral widths in the mesosphere, whereas it shows isotro py in SNR and anisotropy in the spectral widths in troposphere and lower st ratosphere.