,l I cm~o l Sol Vol. 3 t, Suppl. I, pp $ 5 9 ~ S 5 9 1 , 2 0 0 0

Pergamon

www.elsevier.com/locate/j aerosci

Session 7A -Atmospheric aerosols: optical properties I

Lidar Backscatter Signatures of Noctilucent Clouds at Mid-Latitudes

M. ALPERS, M. GERDING, J. HOFFNER, and J. SCHNEIDER

Leibniz-Institut fiir Atmospharenphysik SchloSstr. 6, D-18225 Ostseebad Kfihlungsborn, Germany

Keywords: LIDAR, NOCTILUCENT CLOUDS, OPTICAL PROPERTIES.

INTRODUCTION

At polar latitudes in summer sometimes aerosol clouds appear at about 81-85 km altitude, just below the cold polar summer mesopause. At latitudes between about 50 ° and the polar circle also at local midnight the sun is just a few degrees below the horizon and illuminates the atmospheric range higher than about 80 km. Therefore these aerosols appear as bright cirrus-like clouds of white or light-blue color at the nightly poleward horizon (Noctilucent Clouds = NLCs). Main appearance period at polar latitudes is from about 30 days before until about 60 days after the summer solstice, while the period is reduced at mid- latitudes. The particles forming these clouds are assumed to consist of water ice. Multi-wavelength lidar observations of NLCs at polar and mid-latitudes confirmed the assumption of the homogeneous spherical shape of these particles for several NLC events [von Cossart et al., 1999; Alpers et al., 2000]. The lidar returns fit well to lognormal distributions of spherical water ice particles, but in a few cases the lidar returns seem to lack the infrared wavelengths.

METHODS

Due to the special atmospheric conditions at the summer mesopause three assumptions can be introduced for the determination of NLC aerosol particle properties: (1) The NLC particles consist of water ice. (2) The NLC particles are spherical. (3) The particles size distribution follows a monomodal lognormal function. With these assumptions the NLC particle size distribution is characterized by only three parameters: (1) The median radius rm, (2) the distribution width •and (3) the total particle number density N. Therefore a minimum of three independent observation parameters, e.g. three lidar wavelengths, is necessary for the complete determination of the particle size distribution. The use of more than three lidar wavelengths allows cross checks of the robustness of the results or tests of the water ice assumption. We calculate Mie backscatter cross sections of water ice particles as a function of median radius rm and distribution width G using the algorithm of Bohren and Huffman (1983) and compare the theoretical data set with the relative backscatter signals of the different lidar wavelengths. The methods are described in detail by von Cossart et al. (1999) and Alpers et al. (2000). At the site of the Leibniz-Institut ftir Atmosph~irenphysik in Ktihlungsborn, Germany (54°N, 12°E) since 1996 three different lidar systems are in operation: (1) A Rayleigh-Mie-Raman (RMR) lidar (Alpers et al., 199.9) with 2 wavelengths for the investigation of aerosol particle properties between 2 and 90 km altitude, (2) a two-wavelengths resonance lidar system for the investigation of mesospheric metal layers (Alpers et al., 1996), and (3) a potassium resonance lidar for temperature measurements at 80-110 km altitude (von Zahn and Hrffner, 1996). Therefore at Ktihlungsborn simultaneous and common volume soundings of noctilucent clouds with up to five wavelengths between the near ultraviolet and the near infrared were possible.

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CONCLUSIONS

The backscatter signatures of most of the NLC events observed at polar and middle latitudes fit well to the assumption of water ice particles with lognormal size distributions. All wavelengths show significant backscatter signals with signal variations matching the wavelength dependence of water ice spheres. Typical values are rm = 20-50 nm, (~ = 1.4-1.6 and N = 80-260 cm 3 (von Cossart et al., 1999; Alpers et al., 2000). Fig. la shows an example for such a "regular" NLC event, observed on June 13/14, 1998 at Kiahlungsbom (54°N). A few NLC events seem to differ from this regular case due to their spectral backscatter signature lacking the infrared wavelengths. On July 6/7, 1997 we observed a NLC above Ktihlungsbom, which definitely did not produce significant backscattering at the infrared wavelength 770 nm (Fig. lb). Also at polar latitudes there are a few NLC events, where the infrared wavelength 1064 nm seems to be suppressed (von Cossart, private communication), but due to the special problems of daylight lidar observations at the polar latitudes 83 . . . . . . . . I . . . . . . . . . , : ' l' ; ~ r in summer these events are not ........ ~,~, n - 423nm

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statistically significant. The absence of backscattering at 770 nm or 1064 nm wavelength definitely does not fit to the spectral signature of homogeneous water ice spheres. Variations of size distribution width or type (e.g. bimodal lognormal or other distributions) can not explain the lidar results. Very different aerosol particle shapes or compositions must be responsible for the spectral behavior of these rare NLC events. So far we did not find a material or particle shape, which could exist under the atmospheric conditions of the upper summer mesosphere, and which can explain the observations.

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Fig. 1. Backscatter profdes (30 min integration) of NLC lidar observations at 54°N. The signals are normalized to Rayleigh backscattering at 40 km altitude. (a) "Regular" NLC event (June 13/14, 1998) with a significant signal at 770 nm wavelength. (b) Unusual NLC event (July 6/7, 1997) with no significant backscatter signal at 770 nm wavelength. The enhanced signals above the NLC layer (423 nm and 770 nm) are caused by resonance backscattering on free Ca and K atoms, respectively.

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ACKNOWLEDGEMENTS

The author thanks Ronald Eixmann, Cord Fricke-Begemann, and J6m Oldag for their assistance during the NLC observations. Torsten K6pnick was involved substantially in the realization of the IAP lidar systems. The observations were granted by the Deutsche Forschungsgemeinschaft and the Leibniz-Institut ftir Atmospharenphysik, Ktihlungsbom.

REFERENCES

Alpers, M, J. H6ffner, and U. von Zahn (1996). Upper atmosphere Ca and Ca + at mid-latitudes: First simultaneous and common-volume lidar observations, Geophys. Res. Lett. 23, 567.

Aipers, M, R. Eixmann, J. H6ffner, T. K6pnick, J. Schneider, and U. von Zahn (1999). The Rayleigh- Mie-Raman lidar at lAP Ktihlungsborn, J. Aerosol Sci. 30, 637.

Alpers, M, M. Gerding, J. H6ffner, and U. von Zahn (2000). NLC particle properties from a 5-color lidar observation at 54 N, J. Geophys. Res., in press.

Bohren, C.F., and D.R. Huffman (1983). Absorption and scattering of light by small particles (John Wiley & Sons, Inc., New York).

von Cossart, G., J. Fiedler, and U. von Zahn (1999). Size distributions of NLC particles as determined from 3-color observations of NLC by ground-based lidar, Geophys. Res. Lett. 26, 1513.

von Zahn, U., and J. H6ffner (1996). Mesopause temperature profiling by potassium lidar, Geophys. Res. Lett. 23, 141.