A climatology of Laminae in ozonesonde data in the upper troposphere and lower stratosphere at La Reunion Island

Fabrice Chane-Ming, Franck Molinaro, and Jean Leveau

Laboratoire de Physique de l’Atmosphère, Université de la Réunion, France

Philippe Keckhut, Alain Hauchecorne and Sophie Godin

Service d'Aéronomie du CNRS, Verrières Le Buisson - France


FIGURES


Abstract

Introduction

Large and small-scale structures such as planetary and gravity waves are recognized to be major sources of the variability

of atmospheric long-lived trace constituents resulting in both horizontal and vertical exchanges in the atmosphere (Ehhalt et al., 1983; Pfister et al., 1986). In the case of gravity waves, Chiu and Ching (1978) first proposed an analytical estimation of the linear response of trace gases to interpret small-scale density variations of the neutral or ionospheric layer structures. For example, ozone reveals to be a good indicator of gravity wave motions in the troposphere and the stratosphere (Gruzdev and Elanskiy, 1984). But the signatures of gravity wave motions which are extracted from vertical profiles of ozone or temperature can be contaminated by those of laminar structures produced by horizontal large-scale advection. The variability of trace gases induced by gravity waves is observed to be highly correlated with perturbations of potential temperature as opposed to that of quasi-horizontal motions (Holton, 1987).

In fact, normalized perturbations of ozone mixing ratio can be simply linked to those of temperature in the linear case when gravity waves are present (Teitelbaum et al., 1996).

__ and ‘ denote the unperturbed background and perturbations, q is the potential temperature.

Eckermann et al. (1998) formalized the complete analytical response of vertical minor constituent profiles of arbitrary shape or broadly of conserved quantities to adiabatic gravity-wave displacements. Gibson-Wilde et al. (1997) observed simultaneously two small-scale structures on a vertical ozone profile around the same height range in the stratosphere. Using a relation similar to equation 1, the observed structures were identified as resulting from two processes of different nature: a synoptic-scale quasi-horizontal advection and gravity waves.

In this study, these two types of vertical short-scale structures are examined in the upper troposphere and the lower stratosphere (UT/LS) using radiosonde data at La Reunion Island (20.8 S, 55.3 E). The identification and the characterization are based on wavelet techniques (Chane-Ming et al., 2000a).

Radiosonde Data

Measurements are provided by Väisälä RS80 radiosondes launched at the La Reunion Island since September 1992. Radiosondes are equipped with RS80 Electrochemical Concentration Cell (ECC) ozonesonde. The precision is less than 0.2 °C in temperature and 1.5 nb in ozone partial pressure up to the lower stratosphere (Komhyr et al., 1995). A precision of ± 3 % in the ozone mixing ratio is obtained in the stratosphere below 10 mb. Vertical profiles of temperature and partial pressure of ozone are sampled every 150m.

The observational site is located in the vicinity of the subtropical barrier on both sides depending on the season. The island is under tropical conditions during summer (December-February) and under subtropical conditions during winter (June-August). This enables us to study gravity waves produced by convection and the subtropical jet during summer and winter respectively, in addition to possible transport of air masses across the subtropical barrier.

Results

The continuous wavelet transform (CWT) with Morlet wavelet is applied to vertical perturbation profiles of temperature and partial pressure of ozone from September 1992 to December 1997. Statistical distributions of spectral lines deduced from CWT show evidence of dominant modes with vertical wavelengths of 1-7 km from the ground up to 27 km in vertical perturbation profiles of temperature and ozone (Chane-Ming et al., 2000b). Localized multi-scale structures are observed in the troposphere and the stratosphere during Austral summer and winter. Structures with 1-5 km vertical wavelengths have similar characteristics in both ozone and temperature during Austral summer and winter. Therefore these structures are examined through some case studies and a climatology of laminae induced by gravity waves and large scale advection from September 1992 to December 1998.

The CWT is applied to vertical profiles of perturbations of temperature and ozone on January 10, 1995 in Austral summer. Frames of CWT amplitudes so-called scalograms show evidence of a quasi-monochromatic structure of 2 km vertical wavelength in the upper troposphere and the stratosphere on both ozone and temperature profiles (Figure 1). Wavelike structures of 1.2-2.4 km vertical wavelengths are extracted from vertical profiles of ozone and temperature perturbations with the multi-resolution analysis (MRA) (Chane-Ming et al., 2000a) and compared using equation 1 (Figure 2a).

a b

Figure 1- Scalograms (central panel) of vertical perturbation profiles of (a) temperature and (b) ozone with the Morlet wavelet on January 10, 1995 - the perturbation profiles (right panel) and the normalized power spectral density with the standard FFT (top panel).

a b

Figure 2- Normalized perturbations of the temperature and the ozone mixing ratio (black and gray curves - first panel) on (a) January 10, 1995, and (b) June 29, 1993, for vertical wavelengths of 1.2-2.4 km. The temperature perturbation profile has been multiplied by the factor R(z) - Sign of R(z) is plotted on the first panel (broken line) and the 3 km correlation between perturbations on the second panel.

Similar amplitudes and phase are observed at heights of 10-21 km with values of correlation superior to 0.7. This indicates the presence of a gravity wave.

Chane-Ming et al. (2000b) present another case of gravity wave structure observed in the UT/LS in Austral winter with 3-4 km vertical wavelengths.

In other cases, values of correlation are weak and structures are attributed to the horizontal advection. In the case study of June 29, 1993, the amplitude and phase relationships between the normalized perturbations are not verified in the middle and upper troposphere where strong amplitudes and weak values of correlation are observed at heights of 5-15 km (Figure 2b).

Back-trajectories show evidence of a quasi-horizontal motion of air parcels at heights of 14 km (Figure 3).

Figure 3- Back-trajectories indicate quasi-horizontal motions of air parcels at 14 km height above La Reunion Island on June 29, 1993.

The nature and the frequency of occurrence of these structures are analyzed to provide some quantitative information about the role of such structures in horizontal and vertical transport processes and in mixing on both sides of the tropopause and the subtropical barrier.

Vertical short-scale structures are automatically extracted and identified using a method based on the MRA and similar values of parameters proposed by Grant et al. (1998).

Laminar structures induced by gravity waves are localized in the UT/LS in Austral summer and in the middle troposphere up to the lower stratosphere in Austral winter with vertical wavelengths of 2.4 km and 2.4-4.8 respectively (Figure 4a, 4b). The frequency of occurrence of gravity waves are 1.3 times more important in winter (refer to Table 2 in Chane-Ming et al. (2000b)).

Structures induced by horizontal advection are 2 times more important in winter and are mostly detected at heights of 13-15 km independently of the season with vertical wavelengths of 2.4-4.8 km (Figure 4c, 4d).

a

b

c

d

Figure 4- Climatology of laminae induced by (a and b) gravity waves and (c and d) large-scale advection during Austral summer and winter from September 1992 to December 1998.

Conclusions

The identification and the quantification of vertical short-scale structures induced by gravity waves and the large-scale advection were carried out on high-resolution radiosonde data in the UT/LS at La Réunion Island.

The observations suggest horizontal and vertical transport of air masses across the subtropical barrier and the tropopause. Moreover, gravity waves seem to play an important role in mixing, more particularly in winter when advective structures are also dominant. The detection of vertical short-scale structures is here limited to heights of 21-25 km due to weak values of R(z).

Acknowledgements

The authors are grateful to F. Posny, G. Bain and J. M. Metzger and other people in the laboratory for the radiosonde data. Thanks to J. Harris for back-trajectories. This work was financially supported by La Region Réunion and CNRS/INSU.

References

Chane-Ming, F., F. Molinaro, J. Leveau, P. Keckhut, and A. Hauchecorne, Analysis of gravity waves in the tropical middle atmosphere over La Reunion Island (21°S, 55°E) with lidar using wavelet techniques, Ann. Geophys., 2000a.

Chane-Ming, F., F. Molinaro, J. Leveau, P. Keckhut, A. Hauchecorne, and S. Godin, Vertical short-scale structures in the upper tropospheric-lower stratospheric temperature and ozone at La Reunion Island (20.8°S, 55.3°E), J. Geophys. Res., 2000b. (in press)

Chiu, Y.T., and B.K. Ching, the response of atmospheric and ionospheric layer structures to gravity waves, Geophys. Res. Lett., 5, 539-542, 1978.

Eckermann, S., D. E. Gibson-Wilde, and J. T. Bacmeister, Gravity wave perturbations of minor constituents: A parcel advection methodology, J. Atmos. Sci., 55, 3521-3539, 1998.

Ehhalt, D. H., E. P. Röth, and U. Schmidt, On temporal variance of stratospheric trace gas concentration, J. Atmos. Chem., 1, 27-51, 1983.

Gibson-Wilde, D. E., R. A. Vincent, C. Souprayen, S. Godin, A. Hertzog, and S. D. Eckermann, Dual lidar observations of mesoscale fluctuations of ozone and horizontal winds, Geophys. Res. Lett., 24, 1627-1630, 1997.

Grant, W. B., R. B. Pierce, S. J. Oltmans, and E. V. Browell, Seasonal evolution of total and gravity wave-induced laminae in ozonesonde data in the tropics and subtropics, Geophys. Res. Lett., 1998.

Gruzdev, A. N., and N. F. Elanskiy, Ozone observations in the region of mountain lee waves, Izvestiya, Atmos. Oceanic Phys., 20, 632-639, 1984.

Holton, J. R., The production of temporal variability in trace constituent, Proceedings of the NATO Advanced Research Workshop on Transport Processes in the Middle Atmosphere, D. Reidel Puplishing Company, 1987.

Komhyr, W. D., R. A. Barnes, G. B. Brothers, J. A. Lathrop, and D. P. Opperman, Electrochemical concentration cell ozonesonde performance evaluating during STOIC 1989, J. Geophys. Res., 100, 9231-9244, 1995.

Pfister, L., W. Starr, R. Graig, and M. Loewenstein, Small-scale motions observed by aircraft in the tropical lower stratosphere: Evidence for mixing and its relationship to large-scale flows, J. Atmos. Sci., 43, 3210-3225, 1986.

Teitelbaum, H., M. Moustaoui, J. Ovarlez, and H. Kelder, The role of atmospheric waves in the laminated structure of ozone profiles at high latitude, Tellus, 48A, 422-455, 1996.


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