Spaceborne climate change monitoring by GNSS occultation sensors

The Global Navigation Satellite System (GPS and GLONASS, generically GNSS) based radio occultation technique is an active limb sounding technique exploiting quasi-horizontal transatmospheric sat-to-sat links. GNSS occultation sensors probe refractive properties of the atmosphere (and ionosphere) and profiles of associated fundamental atmospheric variables such as temperature and humidity can be retrieved with high quality.

The method bears great utility for climate change studies in providing an unique combination of global coverage, high vertical resolution and accuracy, long-term stability, and all-weather capability. A domain of highest strength of the method is the profiling of the thermal structure (i.e., temperature profiling) in the upper troposphere and stratosphere. As indications exist that the changing thermal structure in this domain is a particularly sensitive indicator of climate change and anthropogenic climatic impacts, respectively, we expect that occultation sensors can furnish unique data to globally monitor this changing structure with unprecedented accuracy.

We undertook to rigorously test the climate change monitoring and climate change detection capability of GNSS occultation sensors by an integrated analysis involving: (i) realistic modeling of both the neutral atmosphere (T42L39-GCM up to the mesopause) and ionosphere (empirical, solar-activity dependent 4D model) over the time span 1995 to 2020, (ii) realistic observation system simulations of observables (phases and amplitudes) of a small GNSS receiver constellation (6 satellites) over this time period, (iii) a state-of-the-art occultation data processing chain for temperature profile retrieval in the upper troposphere/stratosphere (core region 8 to 40 km) for establishing a significant set of realistic simulated measurements over the period, (iv) a multivariate statistical analysis of 1996-2020 temporal trends in latitude-height slices of both the "true" states from the atmospheric modeling and the "measured" states from the database of retrieved temperature profiles, and (v) an assessment, by methods of statistical inference, of whether and to what degree the GNSS occultation observing system was able to detect trends in the atmospheric evolution (i.e., climate change) from 1996 to 2020.

This ambitious study is expected to be still in progress at the time of the meeting and the latest status will thus be reported. We will discuss in some detail how we actually implemented steps (i) to (v) above, with emphasis on critical scientific and technical areas. Furthermore, we plan to present preliminary results on the climate trend analysis.