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| Titel |
Challenges in the Development of a Self-Calibrating Network of Ceilometers. |
| VerfasserIn |
Maxime Hervo, Frank Wagner, Ina Mattis, Holger Baars, Alexander Haefele |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104310
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| Publikation (Nr.) |
 EGU/EGU2015-3732.pdf |
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| Zusammenfassung |
| There are more than 700 Automatic Lidars and Ceilometers (ALCs) currently operating in
Europe. Modern ceilometers can do more than simply measure the cloud base height. They
can also measure aerosol layers like volcanic ash, Saharan dust or aerosols within the
planetary boundary layer. In the frame of E-PROFILE, which is part of EUMETNET, a
European network of automatic lidars and ceilometers will be set up exploiting this new
capability.
To be able to monitor the evolution of aerosol layers over a large spatial scale, the
measurements need to be consistent from one site to another. Currently, most of
the instruments do not provide calibrated, only relative measurements. Thus, it is
necessary to calibrate the instruments to develop a consistent product for all the
instruments from various network and to combine them in an European Network like
E-PROFILE.
As it is not possible to use an external reference (like a sun photometer or a Raman
Lidar) to calibrate all the ALCs in the E-PROFILE network, it is necessary to use a
self-calibration algorithm. Two calibration methods have been identified which are
suited for automated use in a network: the Rayleigh and the liquid cloud calibration
methods
In the Rayleigh method, backscatter signals from molecules (this is the Rayleigh signal) can
be measured and used to calculate the lidar constant (Wiegner et al. 2012). At the wavelength
used for most ceilometers, this signal is weak and can be easily measured only during
cloud-free nights. However, with the new algorithm implemented in the frame of the
TOPROF COST Action, the Rayleigh calibration was successfully performed on a CHM15k
for more than 50% of the nights from October 2013 to September 2014. This method was
validated against two reference instruments, the collocated EARLINET PollyXT lidar and the
CALIPSO space-borne lidar. The lidar constant was on average within 5.5% compare to the
lidar constant determined by the EARLINET lidar. It confirms the validity of the
self-calibration method. For 3 CALIPSO overpasses the agreement was on average
20.0%. It is less accurate due to the large uncertainties of CALIPSO data close to the
surface.
In opposition to the Rayleigh method, Cloud calibration method uses the complete
attenuation of the transmitter beam by a liquid water cloud to calculate the lidar constant
(O’Connor 2004). The main challenge is the selection of accurately measured water clouds.
These clouds should not contain any ice crystals and the detector should not get into
saturation. The first problem is especially important during winter time and the
second problem is especially important for low clouds. Furthermore the overlap
function should be known accurately, especially when the water cloud is located at a
distance where the overlap between laser beam and telescope field-of-view is still
incomplete.
In the E-PROFILE pilot network, the Rayleigh calibration is already performed
automatically. This demonstration network maked available, in real time, calibrated ALC
measurements from 8 instruments of 4 different types in 6 countries. In collaboration with
TOPROF and 20 national weathers services, E-PROFILE will provide, in 2017, near real time
ALC measurements in most of Europe. |
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