Cloud characteristics and channel selection for IASI radiances in the meteorological sensitive areas.

Année de publication

2004

The cloudiness in simulated Infrared Atmospheric Sounding Interferometer<br> (IASI) pixels deduced from the Advanced Very-High-Resolution Radiometer<br> (AVHRR) satellite imager is studied specifically in meteorologically <br>sensitive areas during the Fronts and Atlantic Storm-Track Experiment. <br>It is found that few clear AVHRR observations are located in the IASI <br>pixels in these regions, which are covered by high-level and low-level <br>clouds. The IASI channel selection is then studied in the context of the<br> sensitive areas for the pixels with low-level clouds. The Entropy <br>Reduction (ER) method, which was previously studied in a general <br>context, is compared with two other channel selection methods using <br>selection criteria based on adjoint sensitivity: the sensitivity to <br>observations and the so-called Kalman-filter sensitivity. It is found <br>that, even though the sensitive' methods give slightly better results <br>than the ER one, the latter performs quite robustly and at a lower <br>computational cost. The robustness to the specification of the <br>background-error covariance matrix is then studied. It is shown that <br>channel selection based on the ER method is particularly robust to the <br>specification of the background-error covariance matrix, but the <br>analysis step itself requires an accurate determination of the <br>background-error covariance matrix. In addition it is shown that an <br>independently computed constant channel set gives comparable results to <br>the optimal channel set.<br>Copyright © 2004 Royal Meteorological Society.</div>

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