Effect of small‐scale ionospheric variability on GNSS radio occultation data quality

Global Navigation Satellite Systems (GNSS) radio occultation (RO) measurements are sensitive to thin ionization layers and small-scale ionosphere structures. To evaluate error bounds and possible biases in atmospheric retrievals, we characterized ionospheric irregularities encountered in the affected profiles by analyzing the L1 signal-to-noise ratio (SNR) variability at E layer altitudes (from 90km to 130km). New metrics to analyze statistical effects of small-scale ionospheric irregularities on refractivity retrievals are proposed. We analyzed refractivity (N) retrievals with Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) ROs in 2011. Using refractivity from European Centre for Medium-Range Weather Forecasts (ECMWF) analysis (N-ECMWF) as the reference data set, we studied statistical properties of the fractional refractivity bias (N) defined by the difference (N-ECMWF-N)/N-ECMWF and averaged in the altitude range from 20 to 25km for each individual profile. We found that (1) persistently larger variability of the L1 SNR as measured by the interquartile range (IQR) existed when the occultation tangent point was in the 90km to 110km altitude range than at higher E layer altitudes; (2) the upper limits on the fractional refractivity bias for COSMIC ROs are 0.06% (for daytime local time), 0.1% (for nighttime local time), and similar to 0.01% (for all local times); (3) distributions of N are non-Gaussian (leptokurtic); (4) latitudinal distributions of small and large N for different levels of ionospheric variability show large tails (N-ECMWF>N) occurring around the Himalaya and the Andes regions, which are possibly due to biases in ECMWF analysis. We conclude that the refractivity bias due to small-scale irregularities is small below 25km altitude and can be neglected.