Why use NWP for GSICS? It is crucial for climate and very desirable for NWP that we understand the characteristics of satellite radiance biases Simultaneous Nadir Overpasses have a limited time and space sampling By comparing the radiances against those simulated from a NWP model more can be learnt about the nature of the biases as the space and time matchup window can be relaxed using double differences with the NWP model as a transfer standard. The Met Office model 6hr forecast background fields are used for this study which assimilate many satellite and in situ observations.

HIRS/SEVIRI/AATSR stats over ocean © Crown copyright Met Office

HIRS/SEVIRI/AATSR stats over ocean © Crown copyright Met Office

© Crown copyright Met Office

© Crown copyright Met Office

© Crown copyright Met Office

Dependence on scene temperature Key For HIRS (left): NOAA 17 NOAA 19 METOP-A For AATSR (below): Nadir view Forward view ………. Dependence on scene temperature 01/01/2011-07/08/2011

Summary The magnitude of the drift of the radiance of the SEVIRI 13.4µm channel as compared by SNOs with other sensors is confirmed by the NWP O-B plots. By Aug 2011 it was -2.7K. The biases seen in 2010 are close to those seen in 2009 in most cases. Exceptions are NOAA-17 HIRS (increased noise) The O-B bias of the 13.4µm channels on SEVIRI, IASI and HIRS (on METOP) are all significantly different from each other (instrument related). In contrast for the 6.2µm water vapour channels the biases are all similar (model related). The bias for SEVIRI channel 11 on Meteosat-9 is 1.2K larger than for the same channel on Meteosat-8. This inconsistency between the same instrument on different platforms is a concern for climate monitoring. The O-B bias becomes more positive with increasing scene temperature for all instruments except AATSR which uses 2 internal black bodies for its calibration. Message for instrument providers? The changes in bias with scene temperature shows the limitation of the polar SNO method. © Crown copyright Met Office