 |
| Titel |
Global analysis of cloud field coverage and radiative properties, using morphological methods and MODIS observations |
| VerfasserIn |
R. Z. Bar-Or, O. Altaratz, I. Koren |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 11, no. 1 ; Nr. 11, no. 1 (2011-01-11), S.191-200 |
| Datensatznummer |
250009065
|
| Publikation (Nr.) |
 copernicus.org/acp-11-191-2011.pdf |
|
|
|
|
|
| Zusammenfassung |
| The recently recognized continuous transition zone between detectable clouds
and cloud-free atmosphere ("the twilight zone") is affected by
undetectable clouds and humidified aerosol. In this study, we suggest to
distinguish cloud fields (including the detectable clouds and the
surrounding twilight zone) from cloud-free areas, which are not affected by
clouds. For this classification, a robust and simple-to-implement cloud
field masking algorithm which uses only the spatial distribution of clouds,
is presented in detail. A global analysis, estimating Earth's cloud field
coverage (50° S–50° N) for 28 July 2008, using the Moderate
Resolution Imaging Spectroradiometer (MODIS) data, finds that while the
declared cloud fraction is 51%, the global cloud field coverage reaches
88%. The results reveal the low likelihood for finding a cloud-free pixel
and suggest that this likelihood may decrease as the pixel size becomes
larger. A global latitudinal analysis of cloud fields finds that unlike
oceans, which are more uniformly covered by cloud fields, land areas located
under the subsidence zones of the Hadley cell (the desert belts), contain
proper areas for investigating cloud-free atmosphere as there is 40–80%
probability to detect clear sky over them. Usually these golden-pixels, with
higher likelihood to be free of clouds, are over deserts. Independent global
statistical analysis, using MODIS aerosol and cloud products, reveals a
sharp exponential decay of the global mean aerosol optical depth (AOD) as a
function of the distance from the nearest detectable cloud, both above ocean
and land. Similar statistical analysis finds an exponential growth of mean
aerosol fine-mode fraction (FMF) over oceans when the distance from the
nearest cloud increases. A 30 km scale break clearly appears in several
analyses here, suggesting this is a typical natural scale of cloud fields.
This work shows different microphysical and optical properties of cloud
fields, urging to separately investigate cloud fields and cloud-free
atmosphere in future climate research. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|