The effect of undetected cloud on IASI Retrievals
COLLARD, A.D.
In Technical Report 256 ("Notes on IASI Performance"), the performance of temperature and humidity retrievals using IASI observations in the clear sky case was investigated. It was shown that, for optimal estimation with a forecast a priori measurement, the measurement accuracy in the troposphere for temperature was around 0.6 K and around 15% for absolute humidity, with averaging kernel widths of 2-3 km and 1-2 km for temperature and humidity respectively. These are all major improvements on the present generation of satellite sounding instruments. In order to achieve these performances, one must be able to determine whether cloud is present in the field of view in order to reject the cloud-contaminated scenes or to somehow allow for the clouds;' effect on the observed radiances. In both cases one should consider the effect of residual cloud in an assumed clear-sky or "cloud-cleared" observation. The retrieval error covariance may be affected not only by direct propagation of the increased observational error, but also by the difference between the assumed and actual observational error covariance matrix resulting in a non-optimal retrieval. Both ...
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