Abstract
Clouds are of high importance for the climate system but they still remain one of its principal uncertainties. Remote sensing techniques applied to satellite observations have assisted tremendously in the creation of long-term and homogeneous data records; however, satellite data sets need to be validated and compared with other data records, especially ground measurements. In the present study, the spatiotemporal distribution and variability of Total Cloud Cover (TCC) from the Satellite Application Facility on Climate Monitoring (CM SAF) Cloud, Albedo And Surface Radiation dataset from AVHRR data—edition 2 (CLARA-A2) and the International Satellite Cloud Climatology Project H-series (ISCCP-H) is analyzed over Europe. The CLARA-A2 data record has been created using measurements of the Advanced Very High Resolution Radiometer (AVHRR) instrument onboard the polar orbiting NOAA and the EUMETSAT MetOp satellites, whereas the ISCCP-H data were produced by a combination of measurements from geostationary meteorological satellites and the AVHRR instrument on the polar orbiting satellites. An intercomparison of the two data records is performed over their common period, 1984 to 2012. In addition, a comparison of the two satellite data records is made against TCC observations at 22 meteorological stations in Europe, from the European Climate Assessment & Dataset (ECA&D). The results indicate generally larger ISCCP-H TCC with respect to the corresponding CLARA-A2 data, in particular in the Mediterranean. Compared to ECA&D data, both satellite datasets reveal a reasonable performance, with overall mean TCC biases of 2.1 and 5.2% for CLARA-A2 and ISCCP-H, respectively. This, along with the higher correlation coefficients between CLARA-A2 and ECA&D TCC, indicates the better performance of CLARA-A2 TCC data.
Highlights
Clouds play a vital role in the Earth’s climate system, being one of its main drivers, while in turn, they are affected by climate changes
Average values of the Total Cloud Cover (TCC) were calculated for the European region for the 29-year period of 1984–2012, which is the common period for the two satellite data records, CLARA-A2 and International Satellite Cloud Climatology Project H-series (ISCCP-H)
The general geographical pattern regarding the two satellite data sets appears quite similar, with a gradual increase in cloud cover from the South to the North-West. Minimum values for both data records are located over the region of North Africa and the South Mediterranean basin with cloud cover values of approximately 25% and 30% for CLARA-A2 and International Satellite Cloud Climatology Project (ISCCP)-H respectively
Summary
Clouds play a vital role in the Earth’s climate system, being one of its main drivers, while in turn, they are affected by climate changes. An in-depth evaluation and intercomparison of the various cloud datasets is necessary, given the systematic errors or artifacts that arise from the different instruments used and retrieval algorithms applied in each satellite data record, which could mislead our understanding of the climatic role of clouds. Surface observations of TCC and cloud type, despite their problems and limitations (e.g. scarcity and inhomogeneity) [10,11] have been performed since the second half of the 19th century [12] and enable us to study long-term cloud trends. Such surface data should be included in the framework of comparisons among satellite cloud data
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