Seasonal variability of vertical eddy diffusivity in the middle atmosphere: 1. Three‐year observations by the middle and upper atmosphere radar

Abstract

The vertical eddy diffusivity K due to atmospheric turbulence with spatial scales of 10ø-102 m has been computed from the echo power spectral width observed by the middle and upper atmosphere radar for almost every month from January 1986 to December 1988. The method of analysis follows Lilly et al. [1974], $ato and Woodman [ ], and Hocking [1983a[ , 1988, and the contamination due to beam broadening, vertical shear, and transience has been removed. Although observations for horizontal wind speeds larger than approximately 40 m/s, such as occur 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 troposphere and lower stratosphere (6-20 km altitude) and for the mesosphere (60-82 km altitude). The monthly median of K shows a local maximum near the tropopause jet stream altitude. It becomes larger in the mesosphere, increasing gradually with height. Maxima of K are observed in 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. effects of seasonal and meridional variations [Johnson and Wilkins, 1965; Justus, 1973; McElroy et al., 1974; Shimazaki and Ogawa, 1974; Crutzen, 1974; Johnston et al., 1976; Ogawa and $himazaki, 1975; Blum and $chuchardt, 1978; Massie and Hunten, 1981; Allen et al., 1981; $trobel et al., 1987]. Such an ad hoc description has also been used in dynamical models of gravity-wave dissipation [Matsuno, Paper number 94JD00911. 0148-0227/94/94JD-00911 $05.00 1982]. Chemically deduced values of K are often affected by the lifetime of each constituent and include not only the true diffusion effect due to microscale turbulence but also an advection effect due to the meridional circulation [Strobel, 1989; Mcintyre, 1989] and/or planetary waves [cf. Matsuno, 1980; Holton, 1981]. Since Lindzen [1981] proposed his well-known parameterization scheme for K due to purely monochromatic internal gravity waves, modelers have incorporated it into their models [e.g., Holton, 1982; Garcia and Solomon, 1985].Recently, mesosphere-stratosphere-troposphere (MST) and mesosphere/lower-thermosphere (MLT) radars have provided a powerful measurement technique for determination of K over a quite broad altitude range, with far better temporal resolution than previously afforded with the other techniques. There are two main procedures which may be used to infer K. Firsfly, we may estimate characteristics of the target scattering the radio wave from the radar echo power intensity [Gage et al., 1980;Balsley and Garello, 1985;Sato et al., 1985]. We then use relationships between the refractive index gradient and the turbulence parameters [e.g., Tatarski, 1971] to infer K, but the accuracy of this method decreases when temperature and humidity are not known with high resolution. Alternatively, we can e...

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SMLTM simulations of the diurnal tide: comparison with UARS observations

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