Wave-optics uncertainty propagation and regression-based bias model in GNSS radio occultation bending angle retrievals

摘要

A new reference occultation processing system (rOPS) will include a GlobalNavigation Satellite System (GNSS) radio occultation (RO) retrieval chainwith integrated uncertainty propagation. In this paper, we focus onwave-optics bending angle (BA) retrieval in the lower troposphere and introduce(1) an empirically estimated boundary layer bias (BLB) model then employed toreduce the systematic uncertainty of excess phases and bending angles in about thelowest 2 km of the troposphere and (2) the estimation of(residual) systematic uncertainties and their propagation together withrandom uncertainties from excess phase to bending angle profiles. Our BLBmodel describes the estimated bias of the excess phase transferred from theestimated bias of the bending angle, for which the model is built, informedby analyzing refractivity fluctuation statistics shown to induce such biases.The model is derived from regression analysis using a large ensemble ofConstellation Observing System for Meteorology, Ionosphere, and Climate(COSMIC) RO observations and concurrent European Centre for Medium-RangeWeather Forecasts (ECMWF) analysis fields. It is formulated interms of predictors and adaptive functions (powers and cross products of predictors),where we use six main predictors derived from observations: impact altitude,latitude, bending angle and its standard deviation, canonical transform (CT)amplitude, and its fluctuation index. Based on an ensemble of test days,independent of the days of data used for the regression analysis to establishthe BLB model, we find the model very effective for bias reduction and capableof reducing bending angle and corresponding refractivity biases by about afactor of 5. The estimated residual systematic uncertainty, after the BLBprofile subtraction, is lower bounded by the uncertainty from the (indirect) useof ECMWF analysis fields but is significantly lower than the systematicuncertainty without BLB correction. The systematic and random uncertaintiesare propagated from excess phase to bending angle profiles, using aperturbation approach and the wave-optical method recently introduced byGorbunov and Kirchengast (2015), starting with estimated excess phaseuncertainties. The results are encouraging and this uncertainty propagationapproach combined with BLB correction enables a robust reduction andquantification of the uncertainties of excess phases and bending angles inthe lower troposphere.