Large Scale Uncertainties in Regional Sea Level Related to the Estimation of the Orbit Solution
Esselborn, Saskia; Schöne, Tilo; Rudenko, Sergei
GFZ Helmholtz Centre Potsdam, GERMANY
As part of the ESA-CCI sea level project new orbit solutions for ERS-1, ERS-2, Topex and Envisat have been estimated at GFZ. This study aims at quantifying the error of regional sea level variability and trends related to the actual choice of the orbit solution. We have calculated gridded radial orbit differences from several state-of-the-art orbit solutions for every cycle of Envisat, Jason-1, ERS-1, ERS-2, and Topex, respectively. The orbit solutions are originating from the original geophysical data records (GDR-C) on the one hand and from different centers (eg. GFZ, ESOC, GSFC) on the other hand and are integrated in GFZ's Altimeter data system (ADS Central). Regional RMS values of the radial orbit differences range between 3 and 9 mm. Even though the global mean sea level is quite insensitive to the choice of the (state-of-the art) orbit solution there are considerable large scale differences between orbit solutions currently available. The temporal variability of the radial orbit differences is large scale and seems to be connected to shifts of the geocenter. It can be described as the sum of a long term trend (up to 2.5 mm/year) and variability with peaks at frequencies of 120, 180 and 365 days. The long term trend of the radial orbit differences can be explained to a large extent by the different treatment of time variable Earth gravity for the orbit calculation. The annual signal is of large scale nature as well and reaches ~10% of the value of the annual sea level signal. Even though regional RMS values of the radial orbit differences are less than 1cm they add to the error level of regional sea level trend estimations and might be miss-interpreted as inter-ocean sea level variability or even mass exchange due to their large-scale nature.