Handling clouds in assimilating high spectral resolution infrared radiances

Accurate cloud detection is very important for infrared (IR) radiance assimilation; a better cloud detection could reduce the data rejection and improve the assimilation. Although the operational centers are using IR sounder data alone for cloud detection, the collocated high spatial resolution imager data could help improve sounder sub-pixel cloud detection and characterization. Besides better sounder cloud detection with collocated imager, assimilating cloudy radiances will be very important in order to take full advantage of IR sounder thermodynamic information in the tropical cyclone (TC) inner core region. Usually only clear IR channels (not affected by clouds) are used in most of data assimilation systems, and cloud contaminated channels have not been used effectively due to difficulties in modeling clouds in both forecast and radiative transfer models. An optimal imager/sounder cloud-clearing technique has been developed at CIMSS (Li et al. 2005), and can be used to retrieve clear column radiances through combining collocated multi-band imager (e.g., MODIS) IR clear radiances and the sounder (e.g., AIRS) cloudy radiances, and no background information is needed in MODIS/AIRS cloud-clearing. Although only ~13% of the AIRS footprints are found clear, additional 21% of the AIRS footprints can be cloud cleared successfully with MODIS. The imager/sounder cloud-clearing is similar to AMSU/AIRS cloud-clearing for deriving the clear equivalent radiances, but retains the original IR sounder resolution, which is critical for regional NWP applications. We have investigated the assimilation of IR sounder radiances by better cloud detection and cloud-clearing using AIRS/MODIS as example, for hurricane Sandy (2012) forecast experiments with WRF/GSI. Initial results of impact on better sounder sub-pixel cloud detection and cloud-clearing with high spatial resolution imager are presented; the technique can be applied to process CrIS/VIIRS and IASI/AVHRR for radiance assimilation in NWP.