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| Titel |
Surface Longwave Cloud Radiative Forcing of Stratus Clouds at the BSRN site Payerne, Switzerland |
| VerfasserIn |
S. Wacker, J. Gröbner, D. Nowak, L. Vuilleumier, N. Kämpfer |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028736
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| Zusammenfassung |
| This study presents surface longwave downward radiation fluxes and surface longwave cloud
radiative forcing results for single-layered overcast stratus situations at the BSRN site
Payerne, Switzerland, obtained by using high quality BSRN surface observations and the
Radiative Transfer Model (RTM) MODTRAN. The stratus cases were selected using
synoptic observations, radiosonde profiles and ceilometer data. A total of 31 atmospheric
profiles of pressure, temperature and humidity derived from radiosondes launched
at Payerne have been applied to the MODTRAN model for the selected stratus
cases during the winter periods 2000 to 2005. Results show a very good agreement
of 0.2 Wm-2mean difference between model and observations with a standard
deviation of 1.2 Wm-2. Outliers caused by overestimated computed surface longwave
downward fluxes can be observed only if the stratus layer does not completely cover the
sky.
The validated results were then applied to derive the surface longwave cloud radiative
forcing CRFLW. As used in this study, CRFLW is the difference between the longwave
downward radiation flux at the surface for the stratus situation, FLWstr, and the
corresponding expected clear-sky flux, FLWcs:
CRFLW = FLW str - FLW cs
(1)
We used model results for both, clear-sky and cloudy longwave downward fluxes to derive
CRFLW. Results revealed a mean CRFLW of 80.6 Wm-2 with a standard deviation of
2.4 Wm-2. Using the modeled clear-sky and stratus surface longwave downward
fluxes with uncertainties of 2 Wm-2 for each, we estimate an expected CRFLW
uncertainty in the order of 3 Wm-2. However, second-order effects due to different
temperature and especially humidity profiles for clear-sky fluxes are not taken into
account: according to equation [1], CRFLW is the difference between the surface
fluxes under a stratus cloud and clear-sky conditions during the stratus period. Since
cloudy and clear-sky conditions do not appear at the same time, the determination of
the clear-sky fluxes are an additional source of uncertainty in deriving CRFLW. |
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