Velocity Mapping Capabilities of Present and Future Altimeter Missions: The Role of High-Frequency Signals

Abstract

A detailed analysis of the velocity field mapping capabilities from existing and future multiple altimeter missions is carried out using the Los Alamos North Atlantic high-resolution model. The velocity mapping errors on the instantaneous fields and on 10-day averaged fields are systematically computed for all analyzed configurations. The T/P+ERS (Jason-1+ENVISAT) mapping error on the velocity remains acceptable (20%?30%) relative to the ocean signal. Mapping errors of 10-day averaged fields are twice as small, which shows that this configuration has a good potential for mapping lower frequencies of the velocity field. Compared to T/P+ERS, T/P+Jason-1 has a smaller error by about 20%?30% mainly because it is less sensitive to the aliasing of high-frequency signals. The mapping errors are twice as small with a three interleaved Jason-1 configuration. One of the main findings of this study is the role of high-frequency signals that strongly limit the velocity mapping accuracy. The high-wavenumber high-frequency signals contribute to the total velocity variance by up to 20% in high eddy energy regions. This explains why the velocity mapping errors remain larger than about 15%?20% of the signal variance even for the four satellite configurations. This also explains why they do not decrease with the number of satellites as rapidly as expected. The aliasing of high-frequency signals is also a very serious issue. The high-frequency signals can induce large erroneous or inconsistent gradients between neighboring or crossing tracks. This strongly impacts the velocity estimation and explains why the meridional velocity mapping errors are larger than the zonal velocity mapping errors for the T/P+ERS configuration. However, it is shown that these aliasing problems can be partly reduced if they are properly taken into account in the mapping procedure.