Direct assimilation of PC data for global NWP

The operational ECMWF 4D-Var has been adapted to allow the direct assimilation of principal component (PC) scores derived from high spectral resolution infrared sounders. The primary aim of this development is towards an efficient use of the entire measured spectrum that could not be achieved by traditional radiance assimilation. We present a system that uses 50 PC scores derived from 305 IASI channels obtained by augmenting the 191 operational IASI channels with additional surface, ozone, and water vapour channels. The new scheme has been extensively tested in a full data assimilation system that uses all operational observations (satellite and conventional). Testing over a three-month summer period suggests that the quality of the analyses produced by the assimilation of 50 IASI PCs is almost identical to that obtained when the operational 191 IASI radiances are assimilated. The verification of forecasts launched from these test analyses further confirms that there is no loss of skill from the assimilation of IASI PCs compared to that of radiances. This result is all the more important in light of the fact that while the use of PC data is currently restricted to fully clear spectra, in the operational radiance assimilation system the use of IASI data extends to channels unaffected by clouds and to fully overcast scenes. In addition, the 50 PC score system based on 305 radiances uses ~20% less computer resources (during the 4D-var minimization) compared to the system that assimilates 191 radiances. This figure represents a significant saving inside the time critical processing path for NWP centres, but could potentially be improved even further by changing the setting of the tunable accuracy of the PC based fast radiative transfer model (PC_RTTOV) used for the assimilation of PC scores. To summarise, the results obtained from the direct assimilation of IASI PC scores are extremely significant and encouraging. They demonstrate the viability of an alternative route to radiance assimilation for the exploitation of data from high spectral resolution infrared sounders in NWP. Progress in this area is very timely - at the time of writing there were four such instruments in space (IASI on METOP-A and B, AIRS on AQUA and CrIS on NPP). Work is now urgently needed to take this system forward to a stage where it can be considered as an option for the safe and efficient operational exploitation of these crucial instruments.