Title: Variational assimilation of GPS radio-occultation observations in rainy conditions

Author: François Vandenberghe (NCAR)
Thomas Auligné (JCSDA)
Gael Descombes (NCAR)

Assimilation of GPSRadio-Occultation (GPSRO) observations typically requires the integration of the GPS phase along the ray path between the emitting GPS satellite and the low earth orbit receiving satellite. Evaluation of the atmospheric refractivity along the ray path is therefore part of any GPSRO observation operator. At GPS frequency, the atmospheric refractivity is function of pressure, temperature and water vapor. The dependenceon water vapor, however, vanishes in saturated atmosphere. This introduces a switch in the GPSRO observations operator. The problem is therefore twofold:

1) Find those rainy regions along the ray path,
2) Mitigate the non-linear effects of saturation during the transition into those regions.

To address 1) and to improve the representation of the cloud field in the model,satellite radiance in cloudy regions can be jointly assimilated with GPSRO data. This approach brings additional non-linearity in the minimization due to the scattering caused byhydrometeors in theradiative transfer model calculations. Various strategies, such as adaptive variational bias correction and re-linarizations are proposed. 2) has been traditionally addressed using smoothing operators.

We describe our strategies to overcome non-linearities and present our recent efforts to jointly assimilate cloudy radiances and GPSRO observations in rainy conditions with the Gridpoint Statistical Interpolation (GSI) hybrid variational/ensemble data assimilation system.

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

Curator: Nikki Privé Last Updated: Feb 9 2015