Mesoscale Mapping Capabilities of Multiple-Satellite Altimeter Missions

Abstract

The purpose of this paper is to quantify the contribution of merging multiple-satellite altimeter missions to the mesoscale mapping of sea level anomaly (H), and zonal (U) and meridional (V) geostrophic velocities. A space/time suboptimal interpolation method is used to estimate the mean and standard deviation of the H, U, and V mapping errors (as a percentage of signal variance) for different orbit configurations. Only existing or planned orbits [TOPEX/Poseidon (T/P), Jason-1, ERS-1/2?ENVISAT, Geosat?GFO] are analyzed. Jason-1 and T/P orbits are assumed to be interleaved. A large number of simulations are performed, including studies of sensitivity to a priori space scales and timescales, noise, and latitude. In all simulations, the Geosat orbit provides the best sea level and velocity mapping for the single-satellite case. In most simulations, the Jason-1?T/P orbit provides the best two-satellite mapping. However, the gain from an optimized two-satellite configuration (Jason-1 + T/P) compared to a nonoptimized configuration (T/P + ERS or T/P + Geosat) is small. There is a large improvement when going from one satellite to two satellites. Compared to T/P, the combination of T/P and ERS, for example, reduces the H mean mapping error by a factor of 4 and the standard deviation by a factor of 5. Compared to ERS or even Geosat, the reduction is smaller but still by a factor of more than 2. The H mapping improvement is not as significant when going from two to three or three to four satellites. Compared to the Geosat, ERS, and T/P mean mapping errors, the Jason-1 + T/P mean mapping error is, respectively, reduced by 5%, 9%, and 17% of the signal variance. The reduction in mean mapping error by going from two to three and from three to four satellites is, however, only 1.5% and 0.7% of the signal variance, respectively. These results differ from Greenslade et al. mainly because of the definition of resolution adopted in their study. The velocity field mapping is also more demanding in terms of sampling. The U and V mean mapping errors are two to four times larger than the H mapping error. Only a combination of three satellites can actually provide a velocity field mean mapping error below 10% of the signal variance. The mapping of V is also less accurate than the mapping of U but by only 10%?20%, even at low latitudes. These results are confirmed using model data from the Parallel Ocean Climate Model (POCM). POCM H, U, and V are thus very well reconstructed from along-track altimeter data when at least two satellites are used. The study also shows that the Jason-1?T/P orbit tandem scenario has to be optimized taking into account the other satellites (GFO and ENVISAT). It also confirms the usually agreed upon main requirement for future altimeter missions: at least two (and preferably three) missions (with one very precise long-term altimeter system to provide a reference for the other missions) are needed.