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| Titel |
Evaluating the capabilities and uncertainties of droplet measurements for the fog droplet spectrometer (FM-100) |
| VerfasserIn |
J. K. Spiegel, P. Zieger, N. Bukowiecki, E. Hammer, E. Weingärtner, W. Eugster |
| 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 ; 5, no. 9 ; Nr. 5, no. 9 (2012-09-20), S.2237-2260 |
| Datensatznummer |
250003089
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| Publikation (Nr.) |
 copernicus.org/amt-5-2237-2012.pdf |
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| Zusammenfassung |
| Droplet size spectra measurements are crucial to obtain a quantitative
microphysical description of clouds and fog. However, cloud droplet size
measurements are subject to various uncertainties. This work focuses on the
error analysis of two key measurement uncertainties arising during cloud droplet
size measurements with a conventional droplet size spectrometer (FM-100):
first, we addressed the precision with which droplets can be
sized with the FM-100 on the basis of the Mie theory. We deduced error
assumptions and proposed a new method on how to correct measured size distributions for these
errors by redistributing the measured droplet size distribution using a
stochastic approach. Second, based on a literature study, we summarized
corrections for particle losses during sampling with the FM-100. We applied
both corrections to cloud droplet size spectra measured at the high alpine
site Jungfraujoch for a temperature range from 0 °C to 11 °C.
We showed that Mie scattering led to spikes in the droplet size
distributions using the default sizing procedure, while the new stochastic
approach reproduced the ambient size distribution adequately. A detailed
analysis of the FM-100 sampling efficiency revealed that particle losses were
typically below 10% for droplet diameters up to 10 μm. For larger
droplets, particle losses can increase up to 90% for the largest droplets
of 50 μm at ambient wind speeds below 4.4 m s−1 and even to
>90% for larger angles between the instrument orientation and the wind
vector (sampling angle) at higher wind speeds. Comparisons of the FM-100 to
other reference instruments revealed that the total liquid water content
(LWC) measured by the FM-100 was more sensitive to particle losses than to
re-sizing based on Mie scattering, while the total number concentration was
only marginally influenced by particle losses. Consequently, for further
LWC measurements with the FM-100 we strongly recommend to consider (1) the
error arising due to Mie scattering, and (2) the particle losses, especially
for larger droplets depending on the set-up and wind conditions. |
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