The Expected Impacts of NPOESS Infrared Sounder Radiances on Operational Numerical Weather Prediction and Data Assimilation Systems

The NPOESS infrared (IR) and microwave (MW) sensor suite (CrIMSS) consists of the Cross-track Infrared Sounder (CrIS) and the Advanced Technology Microwave Sounder (ATMS). CrIMSS is scheduled to fly on the NPOESS Preparatory Project (NPP), as well as NPOESS operational flight units C1 and C3. ATMS is similar to the current operational Advanced Microwave Sounding Unit (AMSU) and the Microwave Humidity Sounder (MHS), and will be discussed in a companion paper in this session. CrIS is a Fourier Transform Spectrometer and will provide 159 shortwave IR channels in the 3.92-4.64 µm range, 433 medium wave IR channels in the 5.71-8.26 µm range, and 713 longwave IR channels in the 9.14-15.38 µm range. CrIS has a nominal nadir field of view of 14 km and a swath width of 2200 km. The heritage sensors for CrIS are the NASA AIRS (Advanced Infrared Sounder; launched in 2002) and MetOp-A IASI (Infrared Atmospheric Sounding Interferometer; launched in 2006). These high quality, high spectral resolution sounders, because of their numerous overlapping weighting functions, represent a significant improvement in the effective vertical resolution over previous IR sounders.

Most NWP (numerical weather prediction) centers have demonstrated significant increases in forecast skill, using standard verification metrics, due to assimilation of AIRS and IASI radiance assimilation. In some cases, the impact from the addition of IASI assimilation to the operational observational data suite was as large as any other single sensor implementation. AIRS is perhaps the first IR sensor where the NWP centers actively shared information and algorithms during the implementation phase concerning channel selection, cloud detection, and other assimilation issues. Daily monitoring using the DA and NWP systems, with statistics posted to websites, provided valuable feedback to the other NWP centers and instrument teams. The experience gained implementing AIRS radiance assimilation, along with prior preparations for day-1 monitoring of IASI radiances, allowed for rapid and effective implementation of IASI. An overview of similar preparations that are underway for day-1 monitoring of CrIS radiances will be presented. These pre-launch activities should pave the way for rapid implementation of CrIS radiance assimilation.

One future challenge will be to more fully utilize AIRS, IASI and CrIS radiances. Currently, only a few hundred channels (at most) are assimilated, and the observations are heavily thinned horizontally. The use of cloud-affected radiances is still imperfect and most centers chose to assimilate only when a channel peaks above the top of the cloud. Further radiance screening is performed over land and sea-ice where assimilation is limited to channels with negligible contribution from the surface. Assimilation of water vapour information in general is problematic, and this is true for AIRS and IASI water vapour channel radiance assimilation as well. This presentation will outline the reasons why the current operational use of the data is limited, and present a summary of the research in progress aimed towards greater utilization of the data. It is important to recognize that while a relatively small proportion of the AIRS and IASI data are being assimilated operationally, these observations still represent a significant fraction of the 1-5 million observations typically assimilated within a 6-hr assimilation window.

Finally, a brief summary of the current status and future potential of AIRS, IASI and CrIS for atmospheric constituent information for environmental monitoring and assimilation will be presented.