Seasonal and interannual variabilities of vertical eddy diffusivity observed by the MU radar

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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nual variations, reaching approximately an order of magnitude, are dominantly detected there, and annual maxima of K exist in summer (Fig. 2(a)). This seasonal variability is consistent with a similar one observed for gravity wave activity (and the resultant momentum flux)/21/, and another one calculated in a chemicaldynamical coupling model including gravity-wave breaking effect/22/. These results support the hypothesis that middle-atmospheric turbulence is mainly induced by breaking of internal gravity waves. The seasonal variability mentioned above is also consistent to estimations of K in the homopause (or turbopanse) region by /23/, which suggests that the homopause level is also controlled by gravity-wave breaking. The median of K in the mesosphere is smaller than some 'standard' values required for chemical transport / 9 / and wave dissipation /11/, although chemical models of shorter llfe-time species /12,13,14/ and recent iu-sltu measurements/15,16/also show such smaller values. Each sampled values of K with smaller-scale variability or inhomogenelty due to breaking gravity wave characteristics are seldom close to the 'standard' values. In the mesosphere, turbulence layer several times thicker than in the lower stratosphere/17/is generated by waves with vertical wavelengths of longer than the order of 1 k i n / 1 8 / , and eddies with dimensions of the order of I km may induce a larger value of I( than that observed here in a too small sampling volume. Indeed MF-band radar observations employing more coarse resolution (2 kin) than the present study have shown much larger values of It" in the mesopanse region/2,5,6/. The low reliability of N also may induce an underestimation of K by using (2), and advective transport due to a large-scale circulation may contribute dominantly to the 'standard' values /19,20/. Although radar observations are possible only in daytime in the mesosphere, we consider that the values of/t" in the nighttime should be similar, since breaking gravity wave behavior observed by lidars at night/24,25/is consistent with those observed by radars. INTERANNUAL VARIABILITY Tile seasonal variabilities mentioned above have been suggested in the threc-yenr observations/3/, and here they are confirmed for the five years. Thus the interannual variabilities are quite small, as shown already in Fig. 1. However, we have found out, although.the detailed examinations are now going on, a variation of 2-3 year period in the lower stratosphere (Fig. 3(b)). Similar but weaker variations with some vertical phase shifts are also seen in tile mesosphere (Fig. 3(a)). Since interannual variations of K may produce those of distributions of atmospheric constituents such as ozone mid carbon dioxide, they must be very importmit also for the global environmental problem.

(9)280

M.D. Yamanaka et al.

Variations of 2-3 year period are also seen in horizontal wind data, which affected more or less by the equatori,'d quasi-biennial oscillation (QBO), and these may induce directly an interannual variability of K through the stratospheric stability. Actually K seems to be stronger when the vertical shear is large. Furthermore, if some gravity waves are generated in the equatorial region and propagated to the observed area, else if some gravity wave sources or selection mechanisms are affected by QBO, K may have a 2-3 year variation induced by gravity wave activity. This may be also interesting to understand the mechanism of semiannual variability observed for K (and gravity wave activity) in the mesosphere. THEORETICAL

INTERPRETATION

K observed in the middle atmosphere is considered to be induced by layered turbulence associated with gravity wave breaking. We apply Lindzen's/26/parameterization ~. (.N'~'~./2m3.).

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to a model of "quasi-monochromatic" gravity wave /27,4/ with vertical wavenumber mo ~ (2~rll.Skm) • e x p ( - z / 3 4 k m ) and intrinsic frequency &, ~, Lf]/0.3 (jr: the Coriolis parameter), with calculating Ja'd/az[, N and the density scale height H from the CIRA86 zonal-mean zonal wind and temperature. The calculated seasonal and interannual variabilities of K are rather small (as predicted ad hoc in earlier chemical and dynamical models) in comparison with its vertical variation (increasing upward by a scale height of about 11 kin). Semiannual variation dominates to annual variation in the altitude region from the upper stratosphere to the lower thermosphere. K takes weak maxims (as seen in Fig. 2(a)) near the mesopause in s-miner and near the tropopauso in winter. Weak maxima are also predicted in winter near the stratopaase and in summer in the middle stratosphere. A weak quasi-biennial variability of K is also predicted. We find that these characteristics are mainly due to an upward decrease of m. and a weak dependence on [~r~/az[. The median of K plotted in Fig. 2(a) in the mesosphere is about an order of magnitude smaller than the calculated values (reaching ,~ 10~m2/s), which can be explained by an observational limit of the MU radar for large eddies generated by long-wavelength gravity waves. CONCLUSIONS We conclude that middle-atmospheric eddy transport is mainly governed by gravity wave-brenking turbulence and its seasonal and interannual variabilities are within about an order of magnitude. The parameterization (5) is derived without considering any special wave sources, so that it can be applied also for the atmosphere over the ocean. Although the statistical behavior of the quasi-monochromatic field is empirically formulated in the present study, a theoretically improved treatment will be addressed in future investigations. Ac.lmo~ledgmen'.. We thank Wayne Hocking and Toru Sato for their valuable comments in the earlier stage of this study. The MU radar belongs to and is operated by the Radio Atmospheric Science Center, Kyoto University. REFERENCES

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