Occurrence and detection of impact multipath simulations of bending angle

In the moist tropical lower troposphere, atmospheric refractivity fields are strongly affected by water vapour and have complicated, non-spherically symmetric structures. Strong horizontal gradients of refractivity could make the simulated bending angle by raytracing a multivalued function of the impact parameter, which is called impact multipath for brevity. In this study, we first show such occurrences of impact multipath in the tropical lower troposphere using the National Centers for Environmental Prediction/Global Forecast System analysis as input to a raytracing operator for COSMIC radio occultations (ROs) in March and April 2017. An up-to-600 m lift in altitude for the impact parameter is observed for simulated RO rays in the presence of a strong horizontal gradient of refractivity over 250 km distances from the perigee, rendering the simulation bending angles multivalued functions of impact parameter. A quality-control procedure is then developed to effectively identify the large variations of parameter along the simulated rays and impact multipath simulations while keeping the simulated rays below and above the multivalued rays. Many more RO data in the boundary layer could be used for data assimilation in numerical weather prediction by not removing the RO data below the impact multipath levels in the moist tropical lower troposphere.