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| Titel |
The effect of clouds on shortwave radiation over the Mediterranean basin based on recent satellite data |
| VerfasserIn |
Christos Papadimas, Nikolaos Hatzianastassiou, Christos Matsoukas, Ilias Vardavas |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056787
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| Zusammenfassung |
| Clouds are an important element of the climate system as they have a big impact
on the Earth’s energy balance. More specifically, they affect both shortwave and
longwave radiation producing the “albedo” and the “greenhouse” effects, respectively,
thus cooling and warming the Earth, while they modify the radiative cooling and
heating profiles in the atmosphere. The role of clouds for the climate system can be
described using the concept of radiative forcing, which is appropriately estimated with
models.
In the present study, the cloud radiative effect (CRE) on shortwave (SW) radiation is
investigated over the broader Mediterranean basin that is one of the most climatically
sensitive regions of our planet. More specifically, the temporal and spatial variability of CRE
is examined on a gridded 2.5ºx2.5º equal angle latitude-longitude and mean monthly
basis, for the 7-year period (March 2000 - February 2007), by using a deterministic
spectral radiation transfer model and quality cloud data taken from the ISCCP and
MODIS satellite databases. The model input data for various surface and atmospheric
parameters, are mainly taken from the MODerate resolution Imaging Spectroradiometer
(MODIS) of NASA (National Aeronautics and Space Administration), the National
Centers for Environmental Prediction – National Center for Atmospheric Research
(NCEP/NCAR) Global Reanalysis project, and the International Satellite Cloud
Climatology Project (ISCCP-D2). The CRE computations are performed in the
spectral range 0.2–10 μm. The model computes the cloud effect on the SW radiation
budget of the Earth-atmosphere system, namely at the top of atmosphere (TOA
reflected solar radiation, CRETOA), within the atmosphere (absorbed solar radiation,
CREatmab), and at the surface (downwelling and absorbed solar radiation, CREsurf and
CREsurfnet).
The ISCCP-D2 cloud data include: cloud amount, cloud-top pressure, cloud-top
temperature, liquid water path, and optical depth for total clouds. ISCCP provides cloud
amount and cloud-top temperature separately for low-, mid-, and high-level clouds,
as well as cloud amount, cloud-top temperature, cloud optical depth, and cloud
albedo separately for ice and liquid water phase clouds. Given the criticism received
recently by ISCCP cloud products, specifically cloud amount (Ac), in this work
qualitative similar data that were made available from MODIS are also used. A
detailed inter-comparison has been performed between Ac from the two databases
emphasizing on assessing spatio-temporal patterns in the area indicated by them. The
computed relative difference between MODIS and ISCCP annual mean Ac for
the broader Mediterranean basin equals 1.05%, and the correlation coefficient is
equal to 0.93. Also, both data sets indicate an increase in the regional mean Ac
over the period 2000-2007, equal to 8.8% based on MODIS and 6.9% based on
ISCCP.
According to the model results, the annual regional mean CRETOA equals -35.3±1.4
Wm-2, (with local values ranging from -56 to -17 Wm-2) indicating thus a “planetary”
cooling of the study region caused by clouds. Clouds also increase the atmospheric
absorption of SW radiation (CREatmab) by 10.8±0.4 Wm-2 (values up to 21 Wm-2)
inducing thus a significant warming of the region’s atmosphere. As a result, clouds are found
to decrease the downwelling and absorbed SW radiation at surface (CREsurf and
CREsurfnet) by -54.2±2.1 and -46.1±1.8 Wm-2, respectively (with local values as large as
-83 and -75 Wm-2, respectively), inducing thus an important surface radiative
cooling. The regional mean cloud SW radiative effect at surface is found to have
increased during the first 7 years of this century, having thus produced a cloud
dimming
CREs are maximum over areas with larger clouds amounts, namely the northern
parts of the study region. Therefore the existence of clouds in the region, actually
strengthens the north-to-south gradient of surface solar radiation and hence temperature. |
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