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Blizzard
No, almeno per il dataset dell'Hadley Centre (HADCRUT2) sono utilizzati proprio i dati delle stazioni, interpolati, nella maniera descritta sotto, mediante un modello climatico:
What are the basic raw data used?
Over land regions of the world over 3000 monthly station temperature time series are used. Coverage is denser over the more populated parts of the world, particularly, the United States, southern Canada, Europe and Japan. Coverage is sparsest over the interior of the South American and African continents and over the Antarctic. The number of available stations was small during the 1850s, but increases to over 3000 stations during the 1951-90 period. For marine regions sea surface temperature (
SST) measurements taken on board merchant and some naval vessels are used. As the majority come from the voluntary observing fleet, coverage is reduced away from the main shipping lanes and is minimal over the Southern Oceans. Maps/tables giving the density of coverage through time are given for land regions by Jones and Moberg (2003) and for the oceans by Rayner
et al. (2003). Both these sources also extensively discuss the issue of consistency and homogeneity of the measurements through time and the steps that have made to ensure all non-climatic inhomogeneities have been removed.
How are the land and marine data combined?
Both the component parts (land and marine) are separately interpolated to the same 5º x 5º latitude/longitude grid boxes. The combined versions (HadCRUT2 and HadCRUT2v) take values from each component and weight the grid boxes where both occur (coastlines and islands). The weighting method is described in detail in Jones
et al. (2001). Land temperature anomalies are infilled where more than four of the surrounding eight 5º x 5º grid boxes are present, as discussed in Jones
et al. (2001). Infilling doesn't take place when the box is ocean, except when it covered by sea ice based on 1961-90 average conditions.
How accurate are the hemispheric and global averages?
Annual values are approximately accurate to +/- 0.05°C (two standard errors) for the period since 1951. They are about four times as uncertain during the 1850s, with the accuracy improving gradually between 1860 and 1950 except for temporary deteriorations during data-sparse, wartime intervals. Estimating accuracy is a far from a trivial task as the individual grid-boxes are not independent of each other and the accuracy of each grid-box time series varies through time (although the variance adjustment has reduced this influence to a large extent). The issue is discussed extensively by Folland
et al. (2001a,b) and Jones
et al. (1997). Both Folland
et al. (2001a,b) references extend discussion to the estimate of accuracy of trends in the global and hemispheric series, including the additional uncertainties related to homogeneity corrections.
Per maggiori info:
http://www.cru.uea.ac.uk/cru/data/tem2/
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