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| Titel |
Generalizing cloud overlap treatment to include solar zenith angle effects on cloud geometry |
| VerfasserIn |
F. Di Giuseppe, A. M. Tompkins |
| 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 |
250022747
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| Zusammenfassung |
| Shortwave radiative transfer depends on the cloud field geometry as viewed from the
direction of the sun. To date, the radiation schemes of large-scale models only consider a
zenith view of the cloud field, and the apparent change in the cloud geometry with
decreasing solar zenith angle is neglected. A simple extension to an existing cloud
overlap scheme is suggested to account for this for the first time. It is based on the
assumption that at low sun angles the overlap between cloud elements is random for an
unscattered photon. Using cloud scenes derived from radar retrievals at two European
sites, it is shown that the increase of the apparent cloud cover with a descending
sun is reproduced very well with the new scheme. Associated with this, there is a
marked reduction in the mean radiative biases averaged across all solar zenith angles
with respect to benchmark calculations. The scheme has been implemented into
both the ECMWF global forecast model and the COSMO regional model. Using
imposed sea surface temperatures, it shows that the impact on the radiative statistics
is significant. In global integrations the feedback on the large-scale dynamics is
minimal. |
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