The assimilation of Cross‐track Infrared Sounder radiances at ECMWF

Cross-track Infrared Sounder (CrIS) is a Michelson interferometer providing radiance data on 1,305 channels in the wavenumber range 650–2550 cm. CrIS was launched into an afternoon orbit in October 2011 and it shares the Suomi-NPP satellite platform with the Advanced Technology Microwave Sounder (ATMS), Visible Infrared Radiometer Suite (VIIRS), and Ozone Mapping and Profiler Suite (OMPS). CrIS samples the infrared spectrum in three distinct bands with nominal spectral resolutions 0.625, 1.25, and 2.5 cm in long-wave, mid-wave, and short-wave bands, respectively, The field-of-view (FOV) diameter is 13.5 km in nadir and global coverage is produced every 12 hours. We aim at operational assimilation of CrIS radiances in the global Numerical Weather Prediction (NWP) system of the European Centre for Medium-range Weather Forecasts (ECMWF). Past experience gathered from experimental and operational assimilation of hyper-spectral radiances, including those from the Atmospheric Infrared Sounder (AIRS; McNally et al. (2006)) and Infrared Atmospheric Sounding Interferometer (IASI; Collard and McNally (2009)), is applied as far as possible: in experiments carried out so far, the assimilation of CrIS radiances is restricted to cloud-free channels over sea and sea ice, and the emphasis is put on efficient use of temperature sounding channels in the 15μm CO2 absorption band. In contrast to the operational assimilation of AIRS and IASI radiances, however, we deploy an aggressive approach by assimilating a large number of channels that are adjacent to each other in the observed infrared spectrum. Together with a relatively aggressive specification of observation error standard deviation this is hoped to compensate for the effect of poorer spectral resolution of CrIS instrument. A non-diagonal observation error covariance matrix is applied to account for the effect of the signal apodization. This paper is structured as follows: we start by discussing the trade-off between instrument noise and spectral resolution in Section 2. Section 3 documents the baseline assimilation system that has produced a good impact on forecast system performance in recent experiments run using a slightly out-of-date version of the Integrated Forecasting System (IFS) at ECMWF. Section 4 describes further improvements to the assimilation system and results from experiments using the currently-operational version of the IFS. Planned future experiments are discussed in Section 5.