On the Assimilation of Advanced Infrared Sounder Radiances in Cloudy Skies

Accurate cloud detection is very important for assimilating advanced infrared (IR) sounder radiances in numerical weather prediction (NWP) models. Most operational centers use IR sounder stand-alone cloud detection approach which could bring some cloudy contaminated radiances in assimilation. Collocated high spatial resolution imager (e.g., MODIS, VIIRS) cloud mask are used for IR sounder (e.g., AIRS, CrIS) sub-pixel cloud detection, impact study has been conducted for three typical hurricanes, Sandy (2012), Irene (2011) and Ike (2008), with WRF (Weather Research and Forecasting) as the forecast model and GSI (Gridpoint Statistical Interpolation) as the assimilation system. Results indicate that forecasts of both hurricane track and intensity are substantially improved when collocated high spatial resolution MODIS cloud mask is used for AIRS cloud detection, and VIIRS cloud mask is used for CrIS cloud detection. Furthermore, based on the IR sounder sub-pixel cloud detection with imager cloud mask, the assimilation of thermodynamic information under partially cloudy skies is also conducted by using the cloud-cleared IR radiances. Since the cloud-clearing method obtains clear equivalent radiances, the same clear radiance assimilation approach can be applied directly to the cloud-cleared IR radiances. By assimilating the cloud-cleared AIRS radiances in cloudy skies for the three hurricanes mentioned above, the 48 and 72 hours temperature forecast standard deviation (STD) are reduced by 0.1- 0.3 K between 300 and 850 hPa. The substantial improvement in reducing track forecasts error in the range of 10 to 50 km was achieved. The IR sounder sub-pixel cloud detection and cloud-clearing with collocated high spatial resolution imager data could reduce the usage of cloud contaminated IR radiances to be treated as clear in assimilation, and also enhance the assimilation of thermodynamic information in partially cloudy skies. The methodologies can be applied to operational assimilation of advanced IR sounder radiances from AIRS, CrIS and IASI.