The mechanism of formation of inversion layers in the mesosphere

Adv Space Res Vol 12, No 10, pp (10)219-(10)223, 1992

0273-1177/92 $15 00 Copyright © 1992 COSPAR

Pnnted m Great Britain All rights reserved

THE MECHANISM OF FORMATION OF INVERSION LAYERS IN THE MESOSPHERE A. Hauchecorne and A Maillard Servtce d'Adronomte du CNRS, BP 3, 91371 Verrtdres le Butsson Cedex, France

ABSTRACT Mesospheric temperature profiles obtained by Rayleigh lidar in the south of France frequently show winter temperature inversions, with an amplitude up to 40 K in a few km. A 2D dynamic model, developed to interpret these observations, showed that the mesospheric inversions were mainly due to the vertical circulation induced by the breaking of gravity waves in the upper mesosphere. OBSERVATIONS

The mean thermal profile of the mesosphere is characterized by a regular temperature decrease from the stratopause around 50 k m up to the mesopause around 90 kin. However, the temperature profiles obtained by Rayleigh lidar in the south of France frequently exhibit near 70 k m a temperature inversion with an amplitude up to 40 K in a depth of few kin, corresponding to a deficit of about 30% on the density profile (Fig. I). It m a y persist for several days and is often observed simultaneously by both Observatory of Haute-Provence (OHP; 44°N, 6°E) and Biscarosse (440N, low) lidars, located 550 k m apart. These inversions have firstbeen observed by sounding rockets, their origin not being understood /1/. More recently, density decreases associated with these inversions have been detected during the reentry of the american space shuttle in the atmosphere/2/. The firststatisticalstudy of this phenomenon was performed by Hauchecorne et al. /3/ on the basis of more than 500 temperature profiles obtained by Rayleigh lidar since 1981. This study has shown that, in the presence of an inversion, the altitude of the secondary temperature m i n i m u m varies from 55-72 k m in winter to 70-83 k m in s u m m e r (Fig. 2), whereas its appearance probability exhibits a semi-annual variation with a winter m a x i m u m sharper than the summer one and a m i n i m u m in M a y more prominent than in September (Fig. 3). This seasonal variation is similar to that observed on the M S T radar echoes in the mesosphere and we suggest that both phenomena are linked to gravity waves breaking in the mesosphere. The persistence for several days of this inversion layer can be explained by the preferential breaking of gravity waves inducing a continuous energy deposit in it. The impact of such inversion layers, corresponding to density "holes" of 20 to 30%, is particularly criticalfor the reentry of a space shuttle in the atmosphere. The analysis of data from the Solar Mesospheric Explorer satellite /4/ showed that these inversions had a global extension at middle altitude in winter and that their amplitude was greater in the Southern hemisphere than in the Northern hemisphere. MODELLING A 2D dynamic model of the middle atmosphere has been developed to explain the phenomenon of mesospheric inversions / 5 / . The initial temperature field is close to radiative equilibrium (Fig. 4), with a winter mesopause much colder than the summer one and a zonal wind u continuously increasing with the altitude. The gravity waves are represented by a parameterization close to that proposed by Lindzen / 6 / with 3 phase velocities C=-20, 0 and 20 m / s . The waves are dissipated as soon as their amplitude u' is larger than u-c, which provokes a zonal wind velocity decrease and induces a meridian and vertical circulation. This model generates within a few computer days a realistic mesopheric circulation with a warm mesopause in winter and a cold one in summer, as well as a weak wind at this point. When the amplitude of the waves injected in the bottom of the model is sufficiently weak, the model produces a mesospheric inversion at the upper latitudes of the winter hemisphere (Fig. 5 and 6), whilst for larger amplitude waves the model reproduces the results of previous models (10)219

(10)220

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/ 7 / , i.e. an inversion of the temperature gradient between the winter pole and the summer pole at the mesopause and a regular temperature decrease between the stratopause and the mesopause (Fig. 7). The main difference between those two cases is that for low wave amplitudes, the gravity wave breaking is situated in the upper mesosphere above 70 km whereas for high amplitudes the breaking occurs in the whole mesosphere. The analysis of different terms in the thermal balance of the mesosphere shows that the formation of temperature inversions is mainly due to the adiabatic warming in the descending branch of the drculaton ceJ1 induced by the breaking of gravity waves, the vertical diffusion and the turbulent energy dissipation having a secondary role.

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Formauonof InversionLayersm the Mesosphere

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(10)222

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CONCLUSION Mesospheric t e m p e r a t u r e inversions are f r e q u e n t l y o b s e r v e d at m i d d l e latitude. Their appearance probability is higher in winter than in other seasons. The characteristics of these inversions indicate that they are generated by the breaking of gravity waves in the upper mesosphere. The wintertime inversion layers have been reproduced in a 2D dynamic model in which it is shown that they are mainly due to the adiabatic warming in the descending part of the circulation cell induced by the breaking of gravity waves.

Formauon of Inversion Layers m the Mesospbere

(10)223

REFERENCES 1. Schmidlin, F. ]., Temperature inversions near 75 km, Geophys. Res. Letters, 3, 173-176 (1976). 2. Champion, K.S.W., Middle atmosphere models and compaeison with shuttle reentry density data, Advances zn Space Research,, 6, 9715-9721 (1986). 3. Hauchecorne, A., M. L. Chanin and R. Wilson, Mesosopheric temperature inversion and gravity wave breaking, Geophys. Res. Letters., 14, 933-936 (1987). 4. Clancy, R. T. and D. W. Rusch, Climatology and trends of mesospheric (58-90 km) temperatures based upon 1982-1986 SME limb scattering profiles, J. Geophys. Res., 94, 3377-3393 (1989). 5 Hauchecorne, A. and A. Maillard, A 2-D dynamical model of mesospheric temperature inversions in winter, Accepted in Geophy$. Res. Letters, 14 (1990). 6. Lindzen, R. S., Turbulence and stress owing to gravity wave and tidal breakdown, ]. Geophys. Res., 86, 9707-9714 (1981). 7. Holton, J. R., The influence of gravity wave breaking on the general circulation for the middle atmosphere, J. Atmos. Sci., 40, 2497-2507 (1983).