GNSS-RO Residual Ionospheric Error (RIE): A New Method and Assessment

Abstract

Global Navigation Satellite System (GNSS) radio occultation (RO) observations play an increasingly important role in monitoring climate changes and numerical weather forecasts in the upper troposphere and stratosphere. Because the magnitudes of the RO bending angle are small at these altitudes, quantifying and removing residual ionospheric error (RIE) are critical to accurately retrieve atmospheric temperature and refractivity. Yet, RIEs remain poorly characterized in terms of the global geographical distribution and its variations with the local time and altitude influenced by the solar cycle and solar geomagnetic disturbances. In this study we developed a new method to determine RIE from the RO excess phase measurement on a profile-by-profile basis. The method, called the φₑₓ-gradient (dφₑₓ/dhₜ) method, is self-sufficient and based on the vertical derivative of the RO excess phase (φₑₓ) with respect to tangent height (hₜ), which can be applied to individual RO bending angle observations for RIE correction. In addition to the RIE in bending angle measurements, RIEs can also be found in the RO φₑₓ measurements in the upper atmosphere where an exponential dependence is expected. RIEs are likely to impact the RO temperature retrieval by inducing a small-scale variance that is solar-cycle-dependent. We found that the RIE values derived from the dφₑₓ/dhₜ method can be both positive and negative, which is fundamentally different from the κ method that produces only positive RIE values. The new algorithm reveals a latitude-dependent diurnal variation with a larger daytime negative RIE (up to ∼ 3 µrad) in the tropics and subtropics. Based on the observed RIE climatology, a local-time-dependent RIE representation is used to evaluate its impacts on reanalysis data. We examined these impacts by comparing the data from the Goddard Earth Observing System (GEOS) data assimilation (DA) system with and without the RIE. The RIE impact on GEOS DA temperature is mainly confined to the polar regions of the stratosphere. Between 10 and 1 hPa the temperature differences are ∼ 1 K and exceed ∼ 3–4 K in some cases. These results further highlight the need for RO RIE correction in modern DA systems.