SEASONAL VARIABILITY OF VERTICAL EDDY DIFFUSIVITY IN THE MIDDLE ATMOSPHERE .1. 3-YEAR OBSERVATIONS BY THE MIDDLE AND UPPER-ATMOSPHERE RADAR

The vertical eddy diffusivity K due to atmospheric turbulence with spa tial scales of 10(0)-10(2) m has been computed from the echo power spe ctral width observed by the middle and upper atmosphere radar for almo st every month from January 1986 to December 1988. The method of analy sis follows Lilly et al. [1974], Sate and Woodman [1982], and Hocking [1983a, 1985, 1988], and the contamination due to beam broadening, ver tical shear, and transience has been removed. Although observations fo r horizontal wind speeds larger than approximately 40 m/s, such as occ ur near the tropopause jet stream in winter, have been omitted because of excessive beam broadening, sufficient numbers of observations have been accumulated to produce a reasonable climatology for the upper tr oposphere and lower stratosphere (6-20 km altitude) and for the mesosp here (60-82 km altitude). The monthly median of K shows a local maximu m near the tropopause jet stream altitude. It becomes larger in the me sosphere, increasing gradually with height. Maxima of K are observed i n winter near the tropopause and in summer in the mesosphere, and the seasonal variability of K reaches approximately an order of magnitude. A semiannual variability is apparent in the mesosphere with minima in the equinoctial seasons.