Variational Cloud-Clearing with CrIS Data at NCEP

An inline “cloud-clearing” algorithm has been developed recently at NCEP to utilize the observed radiances from the CrISinstrument in partly cloudy areas by removing cloud radiative effects. With the assumption that the surface, atmospheric state and cloud formation characteristics are the same within one field of regard and only cloud fraction varies among adjacent pixels, the clear-column radiances can be reconstructed by minimizing the difference between the radiances with cloud radiative effects removed and simulated clear-sky radiances from radiative transfer model using the GSI first guess fields. The above process differs from other existing cloud-clearing systems in that it is integrated to the GSI system itself. The reconstructed clear-column radiances are estimated in each GSI outer loop and then assimilated together with all other observations in the inner loop.

Progresses have been made on this algorithm and will be discussed at the conference. Results will be shown from several analysis experiments performed to test the impacts of different algorithm configurations on the reconstructed clear-column radiance results. Finally, the impact of the CrIS cloud-cleared radiances on the global analysis and forecast skills will be reported at the conference.