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METOP IMS RAL computation process

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The RAL Infrared Microwave Sounder (IMS) retrieval scheme jointly retrieves height-resolved temperature, water vapour, and ozone, as well as surface spectral emissivity, effective cloud fraction, effective cloud ice fraction, and effective cloud top height, from three instruments on board the MetOp platform: the Infrared Atmospheric Sounding Interferometer (IASI), the Advanced Microwave Sounding Unit (AMSU), and the Microwave Humidity Sounder (MHS). The IMS scheme is a modified version of the EUMETSAT 1DVar IASI L2 retrieval, resulting from the study “Optimal Estimation Method retrievals with IASI, AMSU and MHS measurements” (R. Siddans et al., 2015). The IMS scheme differs from the EUMETSAT L2 scheme through the addition of microwave channels from AMSU and MHS to the measurement vector, and the addition of surface spectral emissivity and cloud to the state vector. There are also differences in the prior assumptions made by the retrieval and RTTOV coefficients used.

The IMS scheme measurement vector comprises the same subset of 139 IASI channels as the EUMETSAT L2 scheme (700-1900 cm-1), all (functioning) AMSU channels, and all MHS channels. The forward model used is RTTOV (Matricari, 2009) with v9 coefficients to allow the modelling of spatial and seasonal variation and trends in greenhouse gases.

The IMS retrieval state vector is expressed in terms of PC weights of a climatological global zonal mean covariance matrix, with 28 PC weights for the atmospheric temperature profile, 18 for the water vapour profile, 10 for the ozone profile and 20 for surface spectral emissivity. In the output files, profiles are reported on the 101 level RTTOV fixed pressure grid.

A climatological zonal mean prior, scaled to match NWP, is used for temperature, water vapour and ozone, and is interpolated linearly to the latitude of each measurement.

For the retrieval of surface spectral emissivity, PCs were computed from the global covariance of the RTTOV implementation of the University of Wisconsin global infrared land surface emissivity database (Borbas and Ruston, 2010) and RTTOV microwave spectral emissivity models. Prior covariances for surface spectral emissivity are defined for the first 6 patterns using the combined RTTOV infrared and microwave atlases. Spectral correlations between the infrared and microwave emissivities are included (derived from the RTTOV representation of the spatial covariance of infrared and microwave emissivity).

Initial cloud screening of IASI scenes is performed by implementing a threshold on the brightness temperature difference between observations and cloud-free simulations (based on ECMWF) at 11 microns. Scenes containing partial, thin or very-low cloud are processed by the retrieval and characterised by the retrieved cloud variables.

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