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| Titel |
Barbados Cloud Observatory: The detection of weak Radar reflectivity signals below shallow cumulus clouds |
| VerfasserIn |
Marcus Klingebiel, Lutz Hirsch, Bjorn Stevens |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250141139
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| Publikation (Nr.) |
 EGU/EGU2017-4613.pdf |
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| Zusammenfassung |
| The Barbados Cloud Observatory (BCO) is located on the east coast of the island of Barbados
(13∘ 09’ N, 59∘ 25’ W), where it is exposed to the relatively undisturbed easterly trade winds.
The position of the BCO is well suited for studies of shallow cumulus clouds, which have a
significant impact on Earth’s radiation budget and on the energy and water cycles. In the
presented study, measurements from a K-band cloud radar, with an exceptional sensitivity,
and a ceilometer are used to analyze the lower environment of shallow cumulus
clouds. Below their nominal base, which is well defined by the determination of
the lifting condensation level and the ceilometer measurements, the cloud radar
detects a weak reflectivity signal. This signal is observable down to 250 m below
the cloud base and shows an intensity between -50 dBZ and -65 dBZ. Artifacts,
caused by the radar instrument itself (e.g. Bragg-Scattering) or by objects other than
hydrometeors, cannot produce this weak reflectivity signal. We suggest that small
raindrops (400 µm diameter or larger) develop inside the upper part of the cloud and
fall into the region below the cloud, where they cause the weak radar reflectivity
signal. These drops must have a very low number concentration (1 drop/100 m3)
since they are not visible to the human eye. Radiosonde data and satellite images
indicate that the occurrence of the weak radar reflectivity signal is connected to the
humidity conditions in higher altitudes (up to 2500 m). A high relative humidity
at this altitude allows for a larger vertical extension of a cloud and thus makes it
easier for raindrops to develop inside the cloud and then fall below cloud base.
This phenomenon is interesting, because the appearance of droplets with a low
number concentration below shallow cumulus clouds could be a precursor of surface
precipitation. |
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