GPD Wet Tropospheric Corrections for Present and Future Altimeter Missions
Author:
Joana Fernandes - (U.Porto)
Co-Investigator(s):
Clara Lazaro (U.Porto)
Telmo Vieira (U.Porto)
Abstract:
The wet tropospheric correction (WTC) of satellite altimeter (SA) range observations, accounting for the path delay due to the presence of water vapour and cloud liquid water in the troposphere, is still a dominant source of uncertainty in SA.
While current WTC retrieval algorithms use the on-board Microwave Radiometers (MWR) measurements to accurately compute the correction over open-ocean, the same retrieval over non-ocean surfaces, in particular over important zones such as coastal, inland water and polar regions, becomes invalid. This called for the development of methods to improve the MWR-derived WTC in these regions and for alternative solutions for missions such as CryoSat-2.
For several years, the University of Porto (UPorto) has been working on the development of methods to determine enhanced WTC over these regions. The GNSS-derived Path Delay Plus (GPD+) is the result of such effort. GPD+ are continuous and consistent wet tropospheric corrections, valid over all surface types, preserving the valid MWR-derived WTC over open ocean and estimating new values whenever these are invalid. The new estimates are obtained by data combination, through space-time objective analysis of all available WTC sources, taking into account the variability of the wet path delay (WPD) field and the white noise associated to each observation type.
This study aims to contribute to the generation of the best possible satellite-derived ocean surface topography products for present (CryoSat-2, Jason-3, Sentinel-3) and future (Sentinel-6/Michael Freilich) satellite altimeters, by means of: i) performing regular assessment and independent calibration/validation of the WTC derived from the MWR aboard each altimeter mission; ii) providing enhanced GPD+ WTC for these missions.
The project team includes two senior researchers with long experience in satellite altimetry and WTC retrieval and a young researcher working on the same topic.
In the scope of this proposal, GPD+ WTC shall be computed for the various SA missions. To ensure long-term stability of the corrections, the large set of radiometers used in the GPD+ estimations are calibrated with respect to the Special Sensor Microwave Imager (SSM/I) and the SSM/I Sounder (SSM/IS), recognized for their stability and independent calibration. In this way, all satellite altimeter missions relevant for the study of the ocean surface topography will have a consistent and inter-calibrated set of wet tropospheric corrections.
By using the baseline MWR WTC for each mission, the WTC from all available observations (Scanning imaging MWR aboard other satellites and GNSS data) and the ECMWF ERA5 model, the WTC for the various SA missions shall be computed for all along-track measurements
Two versions of the GPD+ WTC shall be computed: i) “on-board” or normal GPD+, using all available WTC sources, including the observations from the MWR aboard the respective SA mission; ii) “a la Cryosat” GPD+, using only third-party data. While the former preserves the valid MWR values, the latter is independent from the aboard MWR observations, being a useful validation tool.
The results shall be presented at the OSTST meetings and published in scientific journals.
