Partially scanned interferogram methodology applied to IASI for the retrieval of CO, CO2, CH4 and N2O.

The technique of partially scanned interferograms is applied to the retrieval of trace gases from Infrared Atmospheric Sounding Interferometer (IASI) observations. For the specific case of CO, CO2, CH4 and N2O, we show that this methodology allows us to retrieve trace gases column abundances at an unprecedented accuracy at the level of the single IASI footprint. The technique consists in transforming the IASI spectra back to the interferogram domain where we identify small regions that are mostly sensitive to single gas species. The retrieval is then performed by directly applying Least Squares estimation to these small segments of interferometric radiances. One of the main advantages of the technique is that it allows the efficient use of the information contained in all the IASI channels that are available in the absorption bands of a specific gas species. The retrieval technique has been applied to IASI radiances measured over the Mediterranean sea during the month of July 2010, one of the hottest months on record. Results have been validated against ground-based measurements. We have also carried out a comparison with Atmospheric Infrared Radiometer Sounder data and IASI retrievals obtained with usual variational approaches in the spectral domain.

Simulation of uplooking and downlooking high-resolution radiance spectra with two different radiative transfer models.

Measurements of up-looking spectral radiances measured during the Convection and Moisture Experiment and down-looking spectral radiances measured at one of the Atmospheric Radiation Measurement sites are compared with simulations with use of two different line-by-line models. Simulations are performed in tightly controlled conditions to verify the behavior of the models. Spectra computed at higher samplings are used to study the spectral structure of the differences between simulations and measurements. A revised list of water vapor spectroscopic parameters is used to test the impact of improved spectroscopic data on the accuracy of the line-by-line calculations. The sensitivity of the results to errors that result from uncertainties in the input atmospheric temperature and humidity profiles is also investigated.

Homomorphism between cloudy and clear spectral radiance in the 800-900-cm(-1) atmospheric window region.

The sensitivity of a new algorithm for cloud detection over a sea surface has been assessed on the basis of extensive simulations of clear and cloudy radiance spectra, including water and ice and low- and high-altitude clouds. The new algorithm makes use of autocorrelation and cross correlation between an observed spectrum and either a synthetic or a laboratory spectrum and can be used to determine quantitatively the degree of homogeneity of two spectra in the 800-900-cm(-1) region (11.11-12.5 microm). The scheme is intended for high-spectral-resolution observations and could form the basis for an operational stand-alone cloud-detection algorithm for next-generation sounding spectrometers. Application of the scheme to real observations is presented and discussed.