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| Titel |
Water droplet calibration of the Cloud Droplet Probe (CDP) and in-flight performance in liquid, ice and mixed-phase clouds during ARCPAC |
| VerfasserIn |
S. Lance, C. A. Brock, D. Rogers, J. A. Gordon |
| 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 ; 3, no. 6 ; Nr. 3, no. 6 (2010-12-14), S.1683-1706 |
| Datensatznummer |
250001369
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| Publikation (Nr.) |
 copernicus.org/amt-3-1683-2010.pdf |
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| Zusammenfassung |
| Laboratory calibrations of the Cloud Droplet Probe (CDP) sample area and
droplet sizing are performed using water droplets of known size, generated at
a known rate. Although calibrations with PSL and glass beads were consistent
with theoretical instrument response, liquid water droplet calibrations were
not, and necessitated a 2 μm shift in the manufacturer's
calibration. We show that much of this response shift may be attributable to
a misalignment of the optics relative to the axis of the laser beam.
Comparison with an independent measure of liquid water content (LWC) during
in-flight operation suggests much greater biases in the droplet size and/or
droplet concentration measured by the CDP than would be expected based on the
laboratory calibrations. Since the bias in CDP-LWC is strongly concentration
dependent, we hypothesize that this discrepancy is a result of coincidence,
when two or more droplets pass through the CDP laser beam within a very short
time. The coincidence error, most frequently resulting from the passage of
one droplet outside and one inside the instrument sample area at the same
time, is evaluated in terms of an "extended sample area" (SAE),
the area in which individual droplets can affect the sizing detector
without
necessarily registering on the qualifier. SAE is calibrated with
standardized water droplets, and used in a Monte-Carlo simulation to estimate
the effect of coincidence on the measured droplet size distributions. The
simulations show that extended coincidence errors are important for the CDP
at droplet concentrations even as low as 200 cm−3, and these errors are
necessary to explain the trend between calculated and measured LWC observed
in liquid and mixed-phase clouds during the Aerosol, Radiation and Cloud
Processes Affecting Arctic Climate (ARCPAC) study. We estimate from
the simulations that 60% oversizing error and 50% undercounting error can
occur at droplet concentrations exceeding 400 cm−3. Modification of the
optical design of the CDP is currently being explored in an effort to reduce
this coincidence bias. |
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