Assessing passband errors in MW Sounding Data Roger Saunders2 Bill Bell2, Qifeng Lu1, Carole Peubey3, Niels Bormann3, Steve English3, Alan Geer3, Enza Di Tomaso3 and Richard Renshaw2 National Satellite Meteorological Center, CMA, Beijing Met Office ECMWF Thanks to: Michail Tretyakov

FY-3A MWTS & AMSU channels FY-3 & AMSU-A are heterodyne radiometers FY-3: 4 channels. free running LOs AMSU-A:14 channels. channels 9  phase locked channels 1-8 free running MWTS Channels Channels < 57 GHz . BW ~ 300 MHz Required frequency stability : ~ 5MHz for channels < 57 GHz * * Peubey et al, ECMWF Tech memo 643 (based on simulations of forecast degradations resulting from frequency drift)

MWS Biases - CASE 1 FY-3A MWTS Brightness temperature biases (state dependent) found to be due to pass band uncertainties (30-55Hz errors in pre-launch measurements) Due to n change in LO micro-cavity [Lu et al, JTECH, 2011] Mechanism: vertical shift in weighting functions results in a state dependent bias.

Optimisation of pass band centre frequency estimates Pass band centres: design spec. measured optimised Shifts exist relative to pre-launch measurements Residual biases for ch 3 and 4 © Crown copyright Met Office

MWTS Radiometer Non-linearity In general the response of a MW radiometer will be slightly non-linear wrt the measured scene temperature. If perfect linearity is assumed between the 2 calibration points (cold space and a warm target) then an error (bias) results

MWTS Radiometer Non-linearity Adjust passband: variance reduced Corrected for non-linearity: leaves locally unbiased data

Improved FY-3A MWTS Data Quality First Guess Departures / K

Improved AMSU-A Data Quality First Guess Departures / K (obs – model equivalents, based on T+9 hour forecasts)