Spatially varying fields are required in the form used by the model, for example at each intersection point on a regular grid of longitude and latitude circles, and initial data must be valid at a single time that corresponds to the present or the recent past.
In addition to initializing operational forecasts, the analyses themselves are a valuable tool for subsequent meteorological and climatological studies.
A reanalysis project involves reprocessing observational data spanning an extended historical period using a consistent modern analysis system, to produce a dataset that can be used for meteorological and climatological studies.
As a precursor to a revised extended reanalysis product to replace ERA-40, ECMWF released ERA-Interim, which covers the period from 1979 to 2019.
[13] Degradation, replacement, or modification of instruments (e.g. satellites), as well as changes in methods of observation (e.g., surface, aloft) may create error.