|Title||Characterising channel center frequencies in AMSU-A and MSU microwave sounding instruments|
|Year of Publication||2013|
|Authors||Lu, Q, Bell, W|
|Secondary Title||Technical Memorandum|
Passive microwave observations from the Microwave Sounding Unit (MSU) and the Advanced Microwave Sounding Unit-A (AMSU-A) have been exploited widely for numerical weather prediction (NWP), atmospheric reanalyses and for climate monitoring studies. The treatment of biases in these observations, both with respect to models as well as between satellites, has been the focus of much effort in recent years. In this study we present evidence that shifts, drifts and uncertainties in pass band center frequencies are a significant contribution to these biases. Center frequencies for AMSUA channels 6-14 and MSU channel 3 have been analysed using NWP fields and radiative transfer models, for a series of operational satellites covering the period 1979 - 2012. AMSU-A channels 6 (54.40 GHz), 7 (54.94 GHz) and 8 (55.50 GHz) on several satellites exhibit significant shifts and drifts relative to nominal pass band center frequencies. No significant shifts were found for AMSU-A channels 9-14, most probably as a consequence of the active frequency locking of these channels. For MSU channel 3 (54.96 GHz) most satellites exhibit large shifts, the largest for the earliest satellites. For example for the first MSU on the TIROS-N satellite the analysed shift is 68 MHz over the lifetime of the satellite. Taking account of these shifts in the radiative transfer modelling significantly improved the fit between model and observations, eliminates the strong seasonal cycle in the model observation misfit and significantly improves the bias between NWP models and observations. The study suggests that, for several channels studied, the dominant component of the model-observation bias results from these spectral errors, rather than radiometric bias due to calibration errors.