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Cloud top inference for hyperspectral infrared radiance assimilation

Numerical weather prediction (NWP) centers are now actively involved in new avenues of research and development created by the availability of hyperspectral infrared radiances. The main goal is to improve NWP analyses and subsequent forecasts, taking maximum advantage of the rich information content present in these new data. The AIRS instrument (Atmospheric Infrared Radiance Sounder), operational since 2002, provides 2378 channels covering the infrared spectrum at unprecedented spectral resolution. Most of the globe is covered in a period of about 12 h. These data are distributed in real time to several NWP centers in the form of subsets of 281 or 324 channels with one pixel (~14 km field of view) every ~50 km. As a first step toward assimilation, the main task is the monitoring of the data which implies to identify those radiances which are not affected by clouds. This is the object of this paper. The MSC 3D-var system (soon 4D-var) allows the relatively straightforward assimilation of radiances. The current work has views toward the assimilation of cloudy radiances and such research requires as a foundation a good characterization of cloud amount and height.