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Imager‐assisted cloud detection for assimilation of Infrared Atmospheric Sounding Interferometer radiances

The operational assimilation of Infrared Atmospheric Sounding Interferometer (IASI) radiances at the European Centre for Medium‐range Weather Forecasts (ECMWF) relies primarily on the use of clear data, either in completely cloud‐free locations or restricting the assimilation to channels that are insensitive to underlying cloud. Prior to the data assimilation, cloud‐contaminated channels are identified and rejected in cloud detection, i.e. in a screening process based on observation minus background departure data. Background errors have the potential to confuse the cloud detection. On the one hand, a false alarm occurs when a background error is incorrectly interpreted as a cloud. On the other hand, cloud is missed if the background error compensates for the cloud radiative effect.