Quantile-Conserving Ensemble Filters for All-sky Infrared Radiance Assimilation

All-sky satellite radiance assimilation faces the challenge of non-Gaussian error distributions, which can be exacerbated by fine model resolutions with better resolved physical processes. Compared to the ensemble adjustment Kalman filter (EAKF) that assumes Gaussian error distributions, the nonlinear rank histogram filter (RHF) uses a rank histogram distribution to represent the observation prior. The rank histogram filter with probit transform regression (QCF_RHF) further allows nonlinear regression updates by performing regression of observation increments onto state variables in a transformed space. Performance of the three ensemble filters is investigated in an observing system simulation experiment with non-cycled assimilation of synthetic Geostationary Interferometric Infrared Sounder (GIIRS) all-sky infrared radiances, with model resolution increasing from 7.5 km to 1.5 km and then 300 m. Single observation assimilation experiments demonstrate the advantages of QCF_RHF over EAKF and RHF for both Gaussian and non-Gaussian error distributions, especially for the common all-sky situation with cloudy observations but mostly clear-sky ensemble priors. Consistent results are obtained for the analyses assimilating all synthetic observations from GIIRS channel 1025 and subsequent ensemble forecasts. For state variables of specific humidity, temperature and wind speed, QCF_RHF produces improved analyses and better captures posterior distributions, and smaller forecast errors till 1-h lead time than EAKF and RHF. The advantages of QCF_RHF over EAKF and RHF become more prominent as the horizontal grid spacing decreases from 7.5 km to 1.5 km and then 300 m due to more non-Gaussian error distributions and nonlinear forward operators.