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| Titel |
A comparative study of the response of modeled non-drizzling stratocumulus to meteorological and aerosol perturbations |
| VerfasserIn |
J. L. Petters, H. Jiang, G. Feingold, D. L. Rossiter, D. Khelif, L. C. Sloan, P. Y. Chuang |
| 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 ; 13, no. 5 ; Nr. 13, no. 5 (2013-03-05), S.2507-2529 |
| Datensatznummer |
250018469
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| Publikation (Nr.) |
 copernicus.org/acp-13-2507-2013.pdf |
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| Zusammenfassung |
The impact of changes in aerosol and cloud droplet concentration
(Na and Nd) on the radiative forcing of
stratocumulus-topped boundary layers (STBLs) has been widely studied. How
these impacts compare to those due to variations in meteorological context
has not been investigated in a systematic fashion for non-drizzling overcast
stratocumulus. In this study we examine the impact of observed variations in
meteorological context and aerosol state on daytime, non-drizzling overcast
stratiform evolution, and determine how resulting changes in cloud properties compare.
Using large-eddy simulation (LES) we create a model base case of daytime
southeast Pacific coastal stratocumulus, spanning a portion of the diurnal
cycle (early morning to near noon) and constrained by observations taken
during the VOCALS (VAMOS Ocean-Atmosphere-Land Study) field campaign. We
perturb aerosol and meteorological properties around this base case to
investigate the stratocumulus response. We determine perturbations in the
cloud top jumps in potential temperature θ and total water mixing
ratio qt from ECMWF Re-analysis Interim data, and use a set of
Nd values spanning the observable range. To determine the cloud
response to these meteorological and aerosol perturbations, we compute
changes in liquid water path (LWP), bulk optical depth (τ) and cloud
radiative forcing (CRF).
We find that realistic variations in the thermodynamic jump properties can
elicit a response in the cloud properties of τ and shortwave (SW) CRF that are on
the same order of magnitude as the response found due to realistic changes in
aerosol state (i.e Nd). In response to increases in
Nd, the cloud layer in the base case thinned due to increases
in evaporative cooling and entrainment rate. This cloud thinning somewhat
mitigates the increase in τ resulting from increases in
Nd. On the other hand, variations in θ and
qt jumps did not substantially modify Nd. The
cloud layer thickens in response to an increase in the θ jump and
thins in response to an increase in the qt jump, both resulting
in a τ and SW CRF response comparable to those found from perturbations
in Nd. Longwave CRF was not substantially altered by the
perturbations we tested.
We find that realistic variations in meteorological context can elicit a
response in CRF and τ on the same order of magnitude as, and at times
larger than, that response found due to realistic changes in aerosol state.
We estimate the limits on variability of cloud top jump properties required
for accurate observation of aerosol SW radiative impacts on stratocumulus,
and find strict constraints: less than 1 K and 1 g kg−1 in the
early morning hours, and order 0.1 K and 0.1 g kg−1 close to
solar noon. These constraints suggest that accurately observing aerosol
radiative impacts in stratocumulus may be challenging as co-variation of
meteorological properties may obfuscate aerosol-cloud interactions. |
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