Radiometric biases in AMSU-A resulting from local oscillator drifts and shifts: Improved radiative transfer modelling and impacts in the met office NWP system

Improvements in the radiative transfer modelling for the lower tropospheric channels of AMSU-A (channels 6-8) have been achieved through developing more accurate estimates of the central frequencies for the channel passbands. Shifts and drifts in the center frequencies caused by instability in the local oscillator have been diagnosed for NOAA-15, -18, -19 and Metop A. The effect of correcting for these deviations is assessed through analysis of departure statistics using theMet Office global Numerical Weather Prediction (NWP) system. The form and magnitude of the residual bias in AMSU-A before and after implementation of the improvements to the radiative transfer model are also investigated.