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Ann. Geophys., 20, 1863-1868, 2002
www.ann-geophys.net/20/1863/2002/
© European Geosciences Union 2002
First observation of one noctilucent cloud by a twin lidar in two different directions
G. Baumgarten1, F.-J. Lübken1, and K. H. Fricke2
1Leibniz-Institut für Atmosphärenphysik, Kühlungsborn, Germany
2Physikalisches Institut der Universität Bonn, Bonn, Germany
Correspondence to: F.-J. Lübken (luebken@iap-kborn.de)
Abstract. In the early morning
hours of 14 July 1999, a noctilucent cloud (NLC) was observed simultaneously by
the two branches of a twin lidar system located at the ALOMAR observatory in
northern Norway (69° N). The telescopes of the two lidars were pointing
vertical (L^) and off the zenith
by 30° (L30°). The two lidars detected an enhancement in the
altitude profile of backscattered light (relative to the molecular background)
for more than 5 h, starting approximately at 01:00 UT. These measurements
constitute the detection of one NLC by two lidars under different directions
and allow for a detailed study of the morphology of the NLC layer. A
cross-correlation analysis of the NLC signals demonstrates that the main
structures seen by both lidars are practically identical. This implies that a
temporal evolution of the microphysics within the NLC during its drift from one
lidar beam to the other is negligible. From the time delay of the NLC
structures, a drift velocity of 55–65 m/s is derived which agrees nicely with
radar wind measurements. During the observation period, the mean NLC altitude
decreases by ~0.5 km/h (=14 cm/s) at both observation volumes. Further-more,
the NLC is consistently observed approximately 500 m lower in altitude at L30°
compared to L^. Supplementing
these data by observations from rocket-borne and ground-based instruments, we
show that the general downward progression of the NLC layer through the night,
as seen by both lidars, is caused by a combination of particle sedimentation by
4–5 cm/s and a downward directed vertical wind by 9–10 cm/s, whereas a tilt
of the layer in drift direction can be excluded.
Key words. Atmospheric composition and structure
(cloud physics and chemistry; aerosols and particles) Meteorology and
atmospheric dynamics (middle atmosphere dynamics)
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