Speaker
Description
The ROM SAF second reprocessing (RE2) specific humidity data set is based on radio occultation (RO) refractivity, retrieved by the Metop, COSMIC-1, GRACE, and CHAMP missions. The estimated uncertainties and error correlations of both RO observations and background temperature and specific humidity are determining for the quality of the specific humidity product derived through a one-dimensional variational (1D-Var) procedure. Hence, close attention must be given to ensure that the used error covariance matrices reflect the true uncertainty and error correlations of these input data. We apply new procedures, based on a generalized Three Cornered Hat (3CH) method, for empirical estimation of uncertainties and error covariance matrices for both refractivity profiles and background temperature and specific humidity profiles. The error covariance matrices are estimated by combining GRUAN radiosonde relative humidity and temperature with RO refractivities and ERA5 temperature and humidity forecast. It is also demonstrated how positive biases are reduced by allowing negative specific humidity in the 1D-Var solution.
The characteristics of the ROM SAF RE2 specific humidity product are evaluated by comparison to GRUAN radiosondes. The tropospheric water vapour and information content is discussed in terms of prior fractions, averaging kernels, and cumulative degrees of freedom in different latitude bands.
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