Variational bias correction for Mode-S aircraft derived winds

A variational bias correction (VarBC) technique is proposed within the three-dimensional variational (3D-Var) assimilation system of the Météo-France convective scale model AROME to remove biases present in horizontal wind components derived from aircraft Mode-S data. A set of two predictors derived from the geometry between ground and air speeds has been chosen in order to correct the aircraft speed and heading. Data collected from a set of 8 ADS-B antennas located over France for two periods in 2018 (April and October) have been used to assess the VarBC. Thanks to the availability of anchoring data (radiosoundings, AMDAR aircraft reports), the adaptive bias correction scheme is very efficient for improving the quality of Mode-S derived winds to a level that is similar to those from AMDAR and Mode-S bias corrected by the KNMI. The heading bias correction is the most important one in order to reach background departure values around 2.5 m/s. The behaviour of the VarBC is assessed more precisely for specific aircraft where its capacity to adapt to rapid changes due for example to aircraft maintenance is demonstrated. Moreover, when examining separately predictors for both wind components they appear to converge most of the time towards similar speed and heading corrections. Additional quality controls have allowed to reach background departure values of 2.2 m/s while increasing by a factor of ten the number of aircraft observations to be assimilated in the AROME-France domain.