In ECMWF operations a comprehensive system for the correction of systematic errors (biases) in radiosonde temperature and geopotential is used. It is based on feedback information from the data assimilation and determines corrections, which are a function of the solar elevation, to individual radiosonde observations made by known instrument types. A simplified version of this scheme has been adapted for ERA. Bias corrections are calculated from the preceding twelve months of observation minus first guess statistics for those WMO blocks where homogeneous radiosonde equipment were believed to have been used. The correction is then applied above 250 hPa.

Continuous manual monitoring of analysis feedback information and monthly mean analysis increment fields reveals areas (stations) with large and suspicious increments. Suspicious areas are added to the bias correction scheme, and individual stations with very large first guess deviations are added to the blacklist. The monitoring is also applied to bias-corrected and blacklisted stations so they can be removed from the correction when they improve. There is thus a continuously developing radiosonde bias correction and blacklisting. All information on correction and blacklisting is kept.

The correction has been applied through the whole ERA period, however the corrections calculated from 1980 were used at the start of the 1979 rerun. This assumes that instrument characteristics did not significantly change between these years. The correction is largest in the beginning of the period and decreases clearly towards 1989 and from 1990 only small corrections are applied to a small number of areas.

16.11.2001