GPS radio occultation and the role of atmospheric pressure on spaceborne gravity estimation over Antarctica

Dedicated satellite gravity missions are anticipated to significantly improve the current knowledge of the Earth’s mean gravity field and its time variable part–climate sensitive gravity signals. They could be measured by the Gravity Recovery and Climate Experiment (GRACE) twin-satellite with sub-centimeter accuracy in terms of column of water movement near the Earth’s surface with a spatial resolution of several hundred kilometers or larger, and a temporal resolution of one month or weeks. To properly recover the time variable gravity signals from space, the gravity measurements require the atmospheric pressure contribution to be accurately modeled and removed. The sparse coverage of measurements makes the weather products less accurate in the southern hemisphere, especially over the Southern Ocean and Antarctica. The asynoptic observation from GPS radio occultation could achieve dense spatial coverage even in remote regions. In this research, we investigate the potential use of GPS occultation to improve the pressure modeling over Antarctica. Atmospheric pressure profiles are retrieved and validated against ECMWF, NCEP and radiosonde observations. Our results show that occultation can provide compatible observations especially in the upper atmosphere. Large standard deviations and biases are found near the ground and in the Antarctic region. GPS occultation in the polar regions is less affected by multipath problem and can penetrate down near the surface. Through an experiment using a 1-D variational (1DVar) approach, we show ii that the high vertical accuracy of GPS occultation can be propagated down to reduce the uncertainty of surface pressure, indicating that GPS occultation can be expected to have positive impact on the pressure modeling over data-sparse areas after obtaining adequate number of observations (e.g., from Constellation Observing System for Meteorology, Ionosphere & Climate (COSMIC)). We also find that the retrieved profiles could be different due to various assumptions and retrieval algorithms. Pressure uncertainty degrades the GRACE recovered gravity change. We study the uncertainty of pressure modeling on various temporal scales. Global analysis models show large differences in the Antarctic region. The surface topography may introduce additional biases if it is not well treated. The atmospheric tides are non-negligible and need to be properly considered. The real magnitude of the mismodeled and un-modeled errors in the analysis is hard to evaluate, especially in Antarctica. We simulate the errors sensitive to GRACE using the differences between two global analysis models. Most of the very long wavelength errors are well captured by GRACE. Their changes in the form of short-period variation increase the errors of the middle to high degree spherical harmonic coefficients. After de-aliasing, middle to high degree coefficients are noticeably improved. The Inverted Barometer (IB) assumption decreases the amplitude of the aliasing error, and the pattern of the RMS difference is slightly changed over land by neglecting the large variations in the Southern Ocean. Our result using more recent ECMWF and NCEP operational analyses shows reduced aliasing effects, which indicates that two models are becoming increasingly close to each other. The model correlation and IB assumption may underestimate the true aliasing error.