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| Titel |
Noctilucent clouds and the background atmosphere above ALOMAR |
| VerfasserIn |
Jens Fiedler, Gerd Baumgarten, Uwe Berger, Peter Hoffmann, Natalie Kaifler, Franz-Josef Lübken |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052592
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| Zusammenfassung |
| Noctilucent clouds (NLC) are the visible manifestation of ice particles persistently present in
the polar summer mesopause region. Their formation is a rather complicated physical process
depending on atmospheric background parameters, such as temperature and water vapor,
which are hardly to measure directly at the altitudes of interest. This makes NLC an attractive
tracer for dynamic processes in the atmosphere. Cloud parameters show variabilities at
different time scales, ranging from minutes to decades, including tidal and solar cycle
variations. Between 1997 and 2010 NLC have been observed by the ALOMAR
Rayleigh/Mie/Raman (RMR) lidar in Northern Norway at 69° N, 16° E. During a total of
4340Â measurement hours NLC were detected for more than 1850Â hours on 440 different
days, which is the largest NLC data base acquired by lidar. Collocated MF-radar
measurements and calculations with the Leibniz-Institute Middle Atmosphere (LIMA-)
model are used to characterize the background atmosphere. Temperature as well as horizontal
winds at 83 km altitude show distinct differences during NLC compared to the
absence of NLC. On seasonal mean it is colder and the winds have been stronger
westward when NLC are detected. The wind separation is a robust feature as it
shows up in measurement as well as model and it is consistent with the current
understanding that lower temperatures support the existence of ice particles. For the
whole 14-years data set there is no statistically significant relation between NLC
occurrence and solar activity. The reason is that between 2005 and 2008 cloud
occurrence and Lyman-α radiation were almost in-phase, contrary to the other years.
On the other hand NLC occurrence and temperature at 83 km show a significant
anti-correlation, which suggests that the thermal state plays a major role for the existence of
ice particles and dominates the pure Lyman-α influence on water vapor during
certain years. NLC above ALOMAR are strongly influenced by atmospheric tides.
Depending on the parameter, diurnal and semidiurnal amplitudes and phases have
different and partly strong year-to-year variations. Most striking features are shown by
the NLC brightness: The semidiurnal amplitude has very low values during solar
maximum and is over a period of 10 years significantly anti-correlated to Lyman-α
radiation. The diurnal phase increases monotonic over the whole time series by
more than 90°. In general, amplitudes as well as phases of NLC parameters are not
constant over the years. They rather vary in a different manner, amplitudes can
change by a factor of more than 3 and phases show changes up to 7 hours. Such
year-to-year variability can impact time series generated from observations at fixed local
times. |
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