VERTICAL EDDY DIFFUSIVITY IN THE LOWER AND MIDDLE ATMOSPHERE - A CLIMATOLOGY BASED ON THE MU RADAR OBSERVATIONS DURING 1986-1992

Wind velocity variance due to turbulence in the inertial subrange (spa tial scale: 10(0)-10(2) m) has been estimated from a seven-year (1986- 1992) database of the echo power spectral width observed by a VHF (46. 5 MHz) Doppler radar (the MU radar) in Japan. We study a climatology o f the vertical eddy diffusivity K in the tropo-stratosphere (altitude: 5-20 km) and mesosphere (60-90 km) which has been calculated from the observed wind velocity variance. We cofirm completely the seasonal va riabilities of K suggested by Fukao er al. (1994) based on a three-yea r database: an annual variation with winter maximum in the tropo-strat osphere, and a semi-annual variation with solstice maxima (plus a weak annual variation with summer maximum) in the mesosphere. In this pape r we further point out some interannual variabilities of K with a domi nant period of about two years, and discuss an overestimation of the m edian value of K near the tropopausal jet stream.