SEASONAL AND INTERANNUAL VARIABILITIES OF

Adv. SpaceRes.Vol. 14, No. 9, pp. (9)277-(9)280, 1994

Pergamon

Copyright© 1994COSPAR Printedin GreatBritain.All fightsreserved. 0273-1177/94 $6.00+ 0.00

S E A S O N A L AND I N T E R A N N U A L VARIABILITIES OF V E R T I C A L EDDY DIFFUSIVITY O B S E R V E D BY THE MU R A D A R M. D. Yamanaka, S. Kurosaki, S. Fukao, H. Hashiguchi,T. Tsuda and S. Kato Radio Atmospheric Science Center, Kyoto University, Uji, Kyoto 611, Japan

ABSTRACT The vertical eddy diffusivity in the troposphere, lower stratosphere and mesosphere has been observed by the MU radar every month since January 1986. Semiannual variations are dominantly detected in the mesosphere. Annual variations with maxima in winter dominates in the lower stratosphere, while annual maxima in the mesosphere exist in summer. Quasi-biennial variabilities are also analyzed. These observational results are interpreted by the gravlty-wave breaking theory. INTRODUCTION The vertical eddy diffusivity K

= ~elN ~

(1)

(e: kinetic energy dissipation rate, N: the VKisiil~-Brunt frequency, and/~ ~. 0.3) is one of the most important parameters needed to model the middle atmosphere. Recently, MST/MLT radars have provided a powerful measurement technique for determination of K over a quite broad altitude range/1,2/, with far higher vertical and temporal resolution than previously afforded with the other techniques. Many of those observations so far made are for short periods mainly in the mesopanse region, and thus the seasonal and interannual variabilities of K are quite controversial. We are preparing to publish a two-part study/3,4/, in which we have described a climatology of K in the middle atmosphere, based on MU (Middle and Upper atmosphere) radar (35ON, 136OE) observations during 1986-88 and gravity-wave bre~kl, g theory. In this paper, we extend the observational period for six years until 1991, and show an updated climatology of K. M U R A D A R OBSERVATIONS The MU radar detects atmospheric turbulence with spatial scales from a half radar wavelength (3 m) to a vertical resolution (150 or 600 m). It" due to such turbulence can be computed from the echo power spectral (half-power half) width o' observed by the MU radar. Observations have been carried out for about 100 h each month during January 1986-December 1991. The method of analysis follows Hocking/2,5,6/: e ~ 0.3N~ 2

(2)

The contamination in ¢ due to beam broadening, vertical shear and transience has been removed. Observations for horizontal wind speeds larger than approximately 40 m/s, such as occur near the tropopanse 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 small-senle vertical variability of K is smoothed by taking a median of results each month, since altitudes of turbulence layers and patches distribute evenly over the whole observed altitude range during a time interval > 1 day. We have estimated K also (i) from the refractivity turbulence structure constant C,2 (extracted from the radar signal-to-noise ratio) through e ~ 1.7N'M-3(C2.) a/2, (3) where M is the vertical radio refraction index gradient calculated from temperature and humidity profiles (Gage et al./7/); or (ii) approximatcly from the vertical shear [O'd/~z[ through ~, L21~/Ozl '

where/; = 10 m (VanZandt et al./8/).

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K (,.,.,2/~) Fig. I Verticalprofiles of the annual medians of vertical eddy diffusivity K observed by the MU radar during 1986-91 in (a) the mesosphere and (b) the upper troposphere and lower stratosphere, compared with standard model profiles/9,10/. SEASONAL VARIABILITY The monthly median of K shows a maximum reaching 10Zm2/s near the tropopause jet stream altitude, and takes a minimum less than 10°m2/s in the lower stratosphere (Figs. l(b) and 2(b)). K has a striking annual variability with maximum in winter and amplitude reaching about an order of magnitude, although observations near the tropopause jet stream in winter are less reliable as mentioned before. The maximum seems to be reasonable, since the mean vertical shear maximizes and hence the mean dynamic stability minimizes near the tropopause jet stream in winter. We have confirmed this seasonal variability by calculating of e also by (4), based on routine daily observations in the nearby meteorological station. The magnitude of K in the troposphere is smaller every season than the values required by model studies /10/, except for a narrow altitude range during winter. It is considered by modelers that advection and diffusion due to synoptic- and/or planetary-scale waves may be more important causes of mixing in the troposphere and stratosphere than eddy diffusion. Such a large-scale quasi-horizontal mixing is considered to take place along the isentropes, so that this effect becomes quite large in the troposphere, whereas it is not so strong in the stratosphere where the isentropes are almost horizontal. Seasonal variations of K calculated by (3) for the troposphere do not indicate a winter maximum as obtained by (2). This inconsistency is considered to be mainly due to large contribution of the water vapor and precipitation particles to the radar wave refraction, and partly due to difference of turbulence scales detected by each methods. The magnitude of K in the lower stratosphere is quite close to (in summer) or slightly larger than (in winter) that estimated from modeling studies/10/. This implies that the vertical transport process in the lower stratosphere is mainly due to turbulence of observable scales of the MU radar, which appears frequently as thin sporadic turbulence layers, especially in winter above the strong tropopausal jet stream, and is quite different from the patchy turbulence observed in the troposphere. The sporadic (and also localized) nature of turbulence layers is dependent upon the generation mechanism, that is, breaking of gravity waves of which the dominant temporal, horizontal and vertical scales are several hours, 10a - 10a k m and 1 - 2 kin, respectively. This nature of the lower stratospheric turbulence may explain the large differences in values of K deduced from the foregoing studies. Broadly-distributed long-life constituents such as ozone and carbon dioxide are governed by K with values of the magnitude observed here but short-life constituents such as nitrogen dioxide may be affected by K of much smaller value induced by smaller eddies. K becomes larger in the mesosphere (,~ 101m2/s), increasing gradually with height (Fig. l(n)). Senfian-

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