Situation-dependent observation errors for AMSU-A tropospheric channels in the ECMWF forecasting system

• Main Authors: Heather Lawrence, Niels Bormann, Enza Di Tomaso, Stephen English
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Sea ice
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1068.2459; http://cimss.ssec.wisc.edu/itwg/itsc/itsc19/program/papers/5p_05_lawrence.pdf

ABSTRACT The Advanced Microwave Sounding Unit-A (AMSU-A) is a key satellite instrument used in numerical weather prediction systems around the world. In order to assimilate AMSU-A data, observation errors must be defined. In the ECMWF system each AMSU-A channel is currently assigned a constant observation error. However the observation errors should include forward model error, which will depend on the state of the atmosphere and surface. Here we present a study where observation errors for the surface-and cloud-sensitive channels were allowed to vary with surface type (land, sea, sea-ice, snow cover), sensitivity to the surface (surface-to-space transmittance) and the liquid water path, in order to account for errors in the forward model due to emissivity errors and undetected cloud/precipitation. We developed the new observation errors and ran assimilation trials using these new situation-dependent values. We also investigated extending the coverage with these new observation errors by relaxing screening over high orography and by relaxing the cloud screening. First results showed that the new observation errors had a neutral impact on forecast scores when no new data were added but there was a reduction in the standard deviation of background departures for ATMS when the AMSU-A cloud screening was relaxed, which was encouraging.