Interaction between bias correction and quality control

An interaction between the quality control (QC) and the bias correction of satellite radiances has been identified. If the bias correction is recalibrated intermittently, or if it is adaptive, a feedback process is possible. Indeed, the bias is calculated over a population of quality-controlled observations. Since QC usually acts upon bias-corrected observed-minus-first-guess departures, the value of the bias correction influences the next population that passes the QC, and so on. Two situations that can trigger a feedback are described: residual outliers that have not been detected by the QC; and an asymmetric QC that selects a sub-population of the good dataset. In both cases, the bias correction is strongly influenced by the feedback, and the performance of the QC is also degraded. The use of a new metric for the bias correction (called the 'pseudo-mode'), approximating the mode of the radiance-departure distribution, significantly reduces the feedback due to outliers and asymmetric QC. The variational bias correction scheme VarBC, which updates the bias inside the analysis, also constrains the feedback triggered by an asymmetric QC, but it has limited skill for outliers in a population of infrared window channel observations. A combination of VarBC with the pseudo-mode benefits from the advantages of both approaches. The bias correction is less sensitive to the QC, and more robust with respect to residual outliers. Copyright (C) 2007 Royal Meteorological Society.