The ECMWF (European Centre for Medium-range Weather Forecasts) near-surface data
set for the ISLSCP Initiative II data collection has been derived from the ECMWF 40-year reanalysis,
or ERA40 (Simmons and Gibson, 2000), covering the years 1957 to 2001. The purpose
of ERA40 is to produce an objective analysis of the atmosphere making optimal use of a wide
range of observing systems. A recent version of the ECMWF Numerical Weather Prediction
system (cycle 23r4) is used for the entire analysis period. The advantage of re-analysis over
operational analysis is that no system changes occur that might affect the analysis products,
although there are significant changes in the observations