A Bending Angle Raytracing Operator with Limited Ray Length for COSMIC-2 RO Data Assimilation in the Tropics

The upcoming COSMIC-2 will provide unprecedented quantity and quality of GPS RO data in the tropics. The horizontal gradients of refractivity in the tropics are substantially stronger than middle and high latitudes. A non-local raytracing observation operator of bending angle would provide more accurate model simulations than any local observation operators of either bending angle or refractivity. There exist two major challenges for assimilation of RO bending angle using a non-local raytracing observation operator: (i) The simulated bending angle as function of impact parameter could be multi-valued, while RO bending angle observations processed by wave-optics methods do not have such issue; and (ii) The raytracing operator requires an integration over a ray path from outside of the top of atmosphere, into the perigee and away from the perigee to go out of the top of the atmosphere, making the parallelization of the RO data assimilation difficult for operational application. In this study, a new quality control procedure is proposed to eliminate only the multi-valued bending angle simulations while useful RO data below the problematic vertical layer are kept. Being more feasible for parallel implementation, the raytracing operator with limited ray length produces a set of bending angle simulations of good accuracy when compared with those from the non-local raytracing operator. The bending angle raytracing operator with limited ray length and the new quality control are incorporated into the NCEP GSI assimilation system and their impacts on forecasts in the tropics are evaluated. Results will be presented at the conference.