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| Titel |
An improved algorithm for polar cloud-base detection by ceilometer over the ice sheets |
| VerfasserIn |
K. Van Tricht, I. V. Gorodetskaya, S. Lhermitte, D. D. Turner, J. H. Schween, N. P. M. van Lipzig |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 7, no. 5 ; Nr. 7, no. 5 (2014-05-06), S.1153-1167 |
| Datensatznummer |
250115749
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| Publikation (Nr.) |
 copernicus.org/amt-7-1153-2014.pdf |
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| Zusammenfassung |
| Optically thin ice and mixed-phase clouds play an important role in polar
regions due to their effect on cloud radiative impact and precipitation.
Cloud-base heights can be detected by ceilometers, low-power backscatter
lidars that run continuously and therefore have the potential to provide
basic cloud statistics including cloud frequency, base height and vertical
structure. The standard cloud-base detection algorithms of ceilometers are
designed to detect optically thick liquid-containing clouds, while the
detection of thin ice clouds requires an alternative approach. This paper
presents the polar threshold (PT) algorithm that was developed to be
sensitive to optically thin hydrometeor layers (minimum optical depth
τ ≥ 0.01). The PT algorithm detects the first hydrometeor layer
in a vertical attenuated backscatter profile exceeding a predefined threshold
in combination with noise reduction and averaging procedures. The optimal
backscatter threshold of 3 × 10−4 km−1 sr−1 for
cloud-base detection near the surface was derived based on a sensitivity
analysis using data from Princess Elisabeth, Antarctica and Summit,
Greenland. At higher altitudes where the average noise level is higher than
the backscatter threshold, the PT algorithm becomes signal-to-noise ratio
driven. The algorithm defines cloudy conditions as any atmospheric profile
containing a hydrometeor layer at least 90 m thick. A comparison with
relative humidity measurements from radiosondes at Summit illustrates the
algorithm's ability to significantly discriminate between clear-sky and
cloudy conditions. Analysis of the cloud statistics derived from the PT
algorithm indicates a year-round monthly mean cloud cover fraction of 72%
(±10%) at Summit without a seasonal cycle. The occurrence of
optically thick layers, indicating the presence of supercooled liquid water
droplets, shows a seasonal cycle at Summit with a monthly mean summer peak of
40 % (±4%). The monthly mean cloud occurrence frequency in summer
at Princess Elisabeth is 46% (±5%), which reduces to 12%
(±2.5%) for supercooled liquid cloud layers. Our analyses
furthermore illustrate the importance of optically thin hydrometeor layers
located near the surface for both sites, with 87% of all detections below
500 m for Summit and 80% below 2 km for Princess Elisabeth. These
results have implications for using satellite-based remotely sensed cloud
observations, like CloudSat that may be insensitive for hydrometeors near
the surface. The decrease of sensitivity with height, which is an inherent
limitation of the ceilometer, does not have a significant impact on our
results. This study highlights the potential of the PT algorithm to extract
information in polar regions from various hydrometeor layers using
measurements by the robust and relatively low-cost ceilometer instrument. |
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