THE INFORMATION-CONTENT OF CLEAR-SKY IASI RADIANCES AND THEIR POTENTIAL FOR NUMERICAL WEATHER PREDICTION

The estimation-error covariance matrix associated with the analysis of the atmospheric state is used in one particular meteorological situat ion to examine the potential benefit of radiance data for numerical we ather prediction. The gain of information content obtained from simula ted Infrared Atmospheric Sounding Interferometer (IASI) data is studie d and compared with the current information content present in the TIR OS Operational Vertical Sounder (TOVS) radiances. Nineteen independent items of information on a typical temperature/humidity profile are av ailable from the IASI data, compared to six from the TOVS data. In ter ms of temperature, fine-scale structures associated with a vertical re solution of 1 km are estimated with a 0.7 K error standard deviation. The gain of information for specific humidity is of the same order of magnitude as for temperature. Typically, humidity structures associate d with 1 km vertical resolution are estimated with a relative error of 16%. The projection of the analysis-error covariance matrix on atmosp heric-error structures (relevant for numerical weather prediction) giv es a measure of the impact of the use of radiances on the observabilit y of such structures. The results indicate that IASI data would be a d ecisive source of information for the analysis of such structures. Fin ally, a preliminary sensitivity study suggests that the degradation du e to radiance noise associated with possible modifications of the IASI instrument, hardly affects the quality of the analysis.