Most operational Numerical Weather Prediction (NWP) centers have the capability to assimilate satellite radiance data directly using variational Data Assimilation Systems (DAS). The direct radiance assimilation method has led to significant improvements in forecast skill, with the NOAA AMSU-A instrument being the pathfinder in the radiance assimilation efforts. Necessary components of the direct assimilation method are the routine monitoring of the departures from the observed radiances (OB) and those computed using radiative transfer and a short term forecast from the NWP model, termed the background (BK), and bias correction schemes. For temperature sounding radiances, the measurement uncertainty requirements for forecast skill improvement are very demanding. Uncertainties of the bias corrected departures must be of the order 0.2 K (MW) to 0.4 K (IR) or less in order to improve the analysis and skill of the subsequent NWP forecast. This uncertainty requirement is often at or below the sensor calibration error and noise level (NEÄT), so that the routine monitoring of the patterns of the global departures can often detect problems with sensor data such as increased channel noise, channel failures, calibration drift, field of view intrusion and other calibration anomalies. For examples, the NWP DAS-based radiance monitoring capability employed for the DMSP SSMIS Cal/Val efforts was vital in understanding the spatial and temporal departure patterns, unveiling the underlying causes of the observed SSMIS calibration anomalies. Results of the success of the NWP DAS in detecting calibration anomalies with the SSMIS will be presented.

Prior to adding a new sensor to the operational data stream at NWP centers, several weeks or months of passive radiance monitoring is performed, where the departures are monitored using the DAS but not assimilated into the operational analysis. NWP trial forecasts using the new sensor radiances are also performed and bias correction schemes are developed as needed to support the assimilation of the new sensor data of as part of the pre-operational sensor checkout. The day-one capability of the NWP centers to monitor global patterns of the OB-BK sensor departures is part of the routine preparation for assimilating new sensor data and will continue to play an important role in the Cal/Val efforts for future satellite sensor systems.