Mean sea surface and geoid gradient comparisons with TOPEX altimeter data

Abstract

Cycles 4 to 54 of TOPEX data have been analyzed through comparisons with the mean sea surface given on the distributed geophysical data record (GDR). Two inverted barometer correction procedures were considered for the data reduction. One used a constant atmospheric pressure for all data while the one adopted for use, for most computations, introduced a cycle average pressure. The maximum difference between the two estimates was 3.0 cm with a clear annual signal. With the modified correction the TOPEX sea surface was compared to The Ohio State University (OSU) mean sea surface, given on the GDR, to estimate three translations (Δx = −2.3 cm; Δy = 25.0 cm; Δz = −0.3 cm) and a bias (43.3 cm) between the two surfaces. The only significant translation is Δy which indicates the reference frame of the TOPEX system differs from that used in the OSU mean sea surface system. The bias between the TOPEX mean sea surface and the OSU mean sea surface was used to estimate an equatorial radius of 6,378,136.55 m based on an 18‐cm bias estimate of the TOPEX altimeter. Examination of the average difference, by cycle, between the TOPEX sea surface and the OSU mean sea surface suggested a bias change of 3.1±2.2 mm/yr with a positive sign indicating the average ocean surface is rising or the altimeter measured distance is decreasing. Models were implemented that solved directly for a bias, bias rate, annual/semiannual, and tide correction terms. These latter two terms were represented by a degree 8 spherical harmonic expansion with the tide corrections being to the components of the Cartwright and Ray tide model given on the GDR. The most complete solution gave a bias rate of 3.6±0.6 mm/yr when the OSU mean sea surface was used and 3.2±0.4 mm/yr when the TOPEX data were used to define a mean sea surface. When the bias and bias rate terms were included in the estimation process, the annual/semiannual terms and the tide terms estimate changed slightly. The greatest change (±1 mm) took place in the annual term. The computations indicated that a simultaneous solution for bias, bias rate, and annual/semiannual terms gave the most accurate results. Nonsimultaneous solutions led to slightly different bias rate values. The root mean square difference between the TOPEX sea surface and the OSU mean sea surface, after translation and bias correction, was ±17 cm for a typical cycle. However, some locations were identified where the difference could reach 2.3 m and were repeated over several cycles indicating errors in the mean sea surface. Most of the large differences occur in regions lacking altimeter data prior to the TOPEX/POSEIDON mission and/or areas of significant bathymétrie signature. Geoid gradients are needed for the reduction of individual track data to a reference track. The accuracy of the determination of such gradients was determined through the comparison of predicted (from a mean sea surface) along‐track gradients to the observed gradients. Among four mean sea surfaces tested the best agreement (±0.9 cm/km) was found with the OSU mean sea surface placed on the TOPEX geophysical data record.