Estimating the refractivity bias of Formosat-7/COSMIC-II GNSS Radio Occultation in the planetary boundary layer

Abstract. FORMOSAT-7/COSMIC-2 radio occultation (RO) measurements are promising for observing the deep troposphere and providing critical information on the Earth's planetary boundary layer (PBL). However, refractivity retrieved in the low troposphere can have severe bias under certain thermodynamic conditions. This research examines the characteristics of bias in the low troposphere and presents methods for estimating the region-dependent bias using regression models. The results show that the bias has characteristics that vary with land and oceans. With substantial correlation between local spectral width (LSW) and bias, the LSW-based bias estimation model can explain the general pattern of the refractivity bias but with deficiencies in measuring the bias in the ducting regions and certain areas over land. The estimation model involving the relationship with temperature and specific humidity can capture the bias of large amplitude associated with ducting. Finally, a minimum variance estimation that combines the benefits of the individual estimation provides the most accurate estimation of the refractivity bias.