Potential of space-borne GNSS reflectometry to constrain simulations of the ocean circulation

Abstract

The Agulhas current system transports warm and salty water masses from the Indian Ocean into the Southern Ocean and into the Atlantic. The transports impact past, present and future climate on local and global scales. The size and variability, however, of the respective transports are still much debated. In this study, an idealized model based twin experiment is used to study whether sea surface height (SSH) anomalies estimated from reflected signals of the Global Navigation Satellite System (GNSS-R) can be used to determine the internal water mass properties and transports of the Agulhas region. A space-borne GNSS-R detector on the International Space Station (ISS) is assumed and simulated. The detector is able to observe daily SSH fields with a spatial resolution of 1-5 • . Depending on reflection geometry, the precision of a single SSH observation is estimated to reach 3 cm (20 cm) when the carrier phase (code delay) information of the reflected GNSS signal is used. The average precision over the Agulhas region is 7 cm (42 cm). The proposed GNSS-R measurements surpass the radar-based satellite altimetry missions in temporal and spatial resolution but are less precise. Using the estimated GNSS-R characteristics, measurements of SSH are generated by sampling a regional nested general circulation model of the South African oceans. The artificial observations are subsequently assimilated with a 4DVAR adjoint data assimilation method into the same ocean model but with a different initial state and forcing. The assimilated and the original, i.e., the sampled model state are compared to systematically identify improvements and degradations in the model variables that arise due to the assimilation of GNSS-R based SSH observations. We show that SSH and the independent, i.e., not assimilated model variables velocity, temperature and salinity improve by the assimilation of GNSS-R based SSH observations. After the assimilation of 90 days of SSH observations, improvements in the independent variables cover the whole water column. Locally, up to 39 % of the original model state are recovered. Shorter assimilation windows result in enhanced reproduction of the observed and assimilated SSH but are accompanied by an insufficient or wrong recovery of sub-surface water properties. The assimilation of real GNSS-R observations, when available, and consequently the estimation of Agulhas water mass properties and the leakage of heat and salt into the Atlantic will benefit from this model based study.

Lake Level and Surface Topography Measured With Spaceborne GNSS‐Reflectometry From CYGNSS Mission: Example for the Lake Qinghai

“…10.1029/2018GL080976 Saynisch et al (2015), Z. Li, Zuffada, et al (2016), and Xie et al (2018) have shown that these high density sea surface height measurements can significantly improve the mesoscale oceanographic models.…”

“…As a bistatic RA concept, Global Navigation Satellite System‐Reflectometry (GNSS‐R) or PAssive Reflectometry and Interferometry System (Martín‐Neira, ) can perform ocean altimetry along several tracks simultaneously and provide high density of sea surface height measurements. Simulation studies by Saynisch et al (), Z. Li, Zuffada, et al (), and Xie et al () have shown that these high density sea surface height measurements can significantly improve the mesoscale oceanographic models. This passive altimetry concept can be also extended to inland water, which could benefit long‐term global monitoring of inland water.…”

First spaceborne phase altimetry over sea ice using TechDemoSat‐1 GNSS‐R signals

“…This passive wide-swath altimeter concept raises the possibility of resolving mesoscale features in ocean height. Significant impact of GNSS-R sea surface height (SSH) observations into mesoscale oceanographic models has been demonstrated by Saynisch et al [2015] and Li et al [2016] through simulation studies.…”

A Phase-Altimetric Simulator: Studying the Sensitivity of Earth-Reflected GNSS Signals to Ocean Topography

“…Considering a spaceborne GNSS-R setup, the swath of observations could essentially be increased by low elevation data. This effect has already been shown within GNSS-R simulation studies on tsunami detection [14] and ocean model assimilation [15]. Low elevation angles, however, are accompanied by a reduced altimetric sensitivity [16].…”