Title: Assimilation of transformed retrievals obtained from IASI measurements

Author: Laura Stewart (Met Office UK)

The last decade has seen the assimilation of retrievals in NWP replaced by the direct assimilation of radiances, hence mitigating the effect of background dependent biases and pre-processing errors. However in recent years, the advent of hyperspectral sounders and the vast amount of data available from them has renewed interest in the assimilation of retrieval products.

In a recent paper 'On the equivalence between radiance and retrieval assimilation' by Migliorini, the assimilation of a retrieval product was shown to be equivalent to the direct assimilation of the radiances used in the retrieval. The conditions required for this equivalence were (i) the observation operator is approximately linear in the region of the state space centred at the retrievals and (ii) the prior information should not underrepresent the variability of the state. Assimilation of the retrieval product was beneficial because it removed the need for a complex observation operator (the observation operator for the retrieval is linear) and could include information from a high number of instrument channels in a small quantity of data.

In this work we investigate the application of this method of retrieval assimilation in an operational framework. Simulated IASI radiances are processed in the Met Office 1D-Var system (used as a pre-processor of satellite observations before they are assimilated in the full incremental 4D-Var system) to generate (i) retrievals of temperature and humidity, and (ii) retrieval products whose assimilation should be equivalent to that of the original radiances provided the required conditions are satisfied. The retrieval products are assimilated using the standard OI equations and the differences between the two retrieval profiles are calculated under different specifications of information content, the use of different prior information, and different representations of cloud.

GMAO Head: Steven Pawson Global Modeling and Assimilation Office NASA Goddard Space Flight Center

Curator: Nikki Privé Last Updated: Feb 9 2015