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An adaptive buddy‐check algorithm is presented that adjusts tolerances for suspect observations, based on the variability of surrounding data. The algorithm derives from a statistical hypothesis test combined with maximum‐likelihood covariance estimation. Its stability is shown to depend on the initial identification of outliers by a simple background check. The adaptive feature ensures that the final quality‐control decisions are not very sensitive to prescribed statistics of first‐guess and observation errors, nor on other approximations introduced into the algorithm.

The implementation of the algorithm in a global atmospheric data assimilation is described. Its performance is contrasted with that of a non‐adaptive buddy check, for the surface analysis of an extreme storm that took place over Europe on 27 December 1999. The adaptive algorithm allowed the inclusion of many important observations that differed greatly …