An Evaluation of Fengyun-3C Radio Occultation Atmospheric Profiles Over 2015–2018

Jinde Wei,Congliang Liu,Xiaoming Wang,Mi Liao,Weihua Bai,Yan Liu,Ying Li,Kefei Zhang

doi:10.3390/rs12132116

Abstract

Fengyun-3C (FY-3C) is the first Chinese satellite that is capable of using the Radio Occultation (RO) technique to retrieve atmospheric profiles. This research evaluates the quality of FY-3C RO profiles including refractivity, temperature, and specific humidity by comparing with corresponding information from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim) data over the period of 2015–2018. The evaluation is carried out by calculating and analyzing mean systematic differences between FY-3C and ERA-Interim profiles and corresponding standard deviations over a selected spatial and temporal domain. Results show that the FY-3C RO profiles are overall with good agreements with the ERA-Interim data. Global mean refractivity systematic differences are within ±0.2% from 5 to 30 km altitude range with relative standard deviations of less than 2%. Global temperature mean systematic differences vary within ±0.2 K from a 10- to 20-km altitude range with standard deviations of less than 2 K. Global mean specific humidity differences are found to be within ±0.2 g/kg from 2 to 20 km with standard deviations of less than 1 g/kg. FY-3C profiles show visible latitudinal and altitudinal variations, while the seasonal variations are minor. Sampling errors of refractivity and temperature are also found to be larger at higher latitudinal regions due to RO events being less sampled in the polar region.

Highlights

The Global Navigation Satellite Systems (GNSS) Radio Occultation (RO) technique is a robust limb-sounding technique to retrieve vertical profiles, such as refractivity, temperature, and specific humidity of the Earth’s atmosphere [1,2]
The accumulated bending angle can be retrieved by using GNSS observations and the orbits of both GNSS and Low Earth Orbit (LEO) satellites based on the Geometric Optics (GO) and Wave Optics (WO) methods
Below 10 km, strong positive differences are found from 3 to 10 km with values exceeding 2% with standard deviations exceeding 4%. These strong positive differences are consistent with the findings found by Liao et al [42], but with slightly larger magnitudes

Summary

Introduction

The Global Navigation Satellite Systems (GNSS) Radio Occultation (RO) technique is a robust limb-sounding technique to retrieve vertical profiles, such as refractivity, temperature, and specific humidity of the Earth’s atmosphere [1,2]. The RO technique sets specially designed receivers onboard Low Earth Orbit (LEO) satellites to receive GNSS radio signals. As they propagate through the atmosphere, these signals are bent due to the refractivity gradient. The accumulated bending angle can be retrieved by using GNSS observations and the orbits of both GNSS and LEO satellites based on the Geometric Optics (GO) and Wave Optics (WO) methods. The retrieved bending angles from two frequencies of the GNSS satellites are linearly combined to remove first-order ionospheric errors [3]. Profiles are retrieved by using prescribed temperature and humidity information using 1D-Variational (1DVar) or alternative approaches [4,5]

Methods

Results

Discussion

Conclusion