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| Titel |
Does precipitation susceptibility vary with increasing cloud thickness in marine stratocumulus? |
| VerfasserIn |
C. R. Terai, R. Wood, D. C. Leon, P. Zuidema |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 10 ; Nr. 12, no. 10 (2012-05-24), S.4567-4583 |
| Datensatznummer |
250011168
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| Publikation (Nr.) |
 copernicus.org/acp-12-4567-2012.pdf |
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| Zusammenfassung |
| The relationship between precipitation rate and accumulation mode aerosol
concentration in marine stratocumulus-topped boundary layers is investigated
by applying the precipitation susceptibility metric to aircraft data obtained
during the VOCALS Regional Experiment. A new method to calculate the
precipitation susceptibility that incorporates non-precipitating clouds
is introduced. The mean precipitation rate R over a
segment of the data is expressed as the product of a drizzle fraction f
and a drizzle intensity I (mean rate for drizzling columns). The
susceptibility Sx is then defined as the fractional decrease in
precipitation variable x = {R, f, I} per fractional increase in the
concentration of aerosols with dry diameter >0.1 μm, with cloud
thickness h held fixed. The precipitation susceptibility SR is
calculated using data from both precipitating and non-precipitating cloudy
columns to quantify how aerosol concentrations affect the mean precipitation
rate of all clouds of a given h range and not just the mean precipitation
of clouds that are precipitating. SR systematically decreases with
increasing h, and this is largely because Sf decreases with h while
SI is approximately independent of h. In a general sense, Sf can
be thought of as the effect of aerosols on the probability of precipitation,
while SI can be thought of as the effect of aerosols on the intensity of
precipitation. Since thicker clouds are likely to precipitate regardless of
ambient aerosol concentration, we expect Sf to decrease with increasing
h. The results are broadly insensitive to the choice of horizontal
averaging scale. Similar susceptibilities are found for both cloud base and
near-surface drizzle rates. The analysis is repeated with cloud
liquid water path held fixed instead of cloud thickness. Simple power law
relationships relating precipitation rate to aerosol concentration or cloud
droplet concentration do not capture this observed behavior. |
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