1 Sampling the Mesoscale Ocean Surface Currents with Various Satellite Altimeter Configurations by W.J. Emery, D. Baldwin and D. Matthews Aerospace Eng. Sci Univ of Colorado, Boulder, CO, 80309 Abstract Ten-day composites of Maximum Cross Correlation (MCC) ocean surface current vectors from 1 km spatial resolution Advanced Very High Resolution Radiometer (AVHRR) 11-micron thermal infrared images are used to simulate the ocean surface current retrieval capabilities of three satellite altimeter configurations over a large California coastal region. Ground track positions of the nadir sampling TOPEX/Poseidon (TP; now the Jason-1) satellite altimeters are used to compute the cross track velocity components from the corresponding optimally interpolated MCC vectors for a ten-day period. Next the Jason-1 only and the TP plus Jason-1 "tandem mission" sampling are simulated as well as the combination of all available satellite altimeters including ERS-2 and Geosat Follow-On (GFO). Finally we simulate surface current retrievals from the proposed Wide Swath Ocean Altimeter (WSOA), which will have both along and cross track velocity components over a spatial swath. Comparisons of vector current fields, their differences and wavenumber spectra from Optimally Interpolated (OI) maps of the “simulated altimetry velocities” with the corresponding MCC field indicates that the 1) combined coverage from Jason-1 plus TP as well as the combination of all available satellite altimeters results in a better representation of the currents than that of Jason-1 alone and 2) the retrieved currents from the WSOA altimeter provide an even greater improvement over the tandem mapping and multiple satellites. It will be possible in the future to regularly map the mesoscale surface currents of the ocean with wide swath ocean altimeters. Introduction The determination of ocean currents from satellite borne altimeters has been the topic of considerable research ever since the Repeat Track Mission of the GEOSAT satellite in the late 1980's [1], [2], [3], [4]. Ocean currents derived from nadir sampling altimeters are limited representations of the actual surface flow since the along track only measurements of sea surface heights estimates only the cross-track component of the surface geosgtrophiic velocity. This limits the application of such single satellite altimeter currents to the study of very large-scale ocean currents due to the large distances between successive tracks. As a result single altimeter missions have not been able to resolve the mesoscale and shorter time and space scales in the ocean. This paper examines the effects of altimeter sampling by comparing simulated flows using surface currents computed with the Maximum Cross Correlation [5], [6], [7] method applied to 1 km AVHRR thermal infrared imagery for different altimeter configurations. An approximately ten-degree latitude/longitude region off of the coast of Southern California was chosen as the area of study. The simulated altimeter retrievals are accomplished by using the altimeter measurement locations to appropriately sample the MCC composite vector field for the cross track velocity vectors. The sampled fields can then be compared to the Optimum Interpolation of the original 10-day MCC composite as a measure of how well the altimeter retrievals represent the actual flow as estimated by the MCC method. The effects of cross track only velocities and sparse coverage are studied further by comparing MCC velocities with simulated current retrievals for the proposed Wide Swath Ocean Altimeter (WSOA) instrument [8]. The WSOA will use an interferometric antenna system to measure sea surface heights over a 200 km wide swath [9] thus enabling computations of both along- and cross-track components of the velocity as well as providing much denser coverage than cross-track only instruments. For all configurations (Jason-1, TP + Jason-1, Jason-1+TP+ERS- 2+GFO, and WSOA), comparisons are made using fields which have been enhanced using an Optimal Interpolation (OI) algorithm [10]. Finally computing wavenumber spectra from each sampling system compares the OI vector fields. Method The basic reference surface current map was computed using the MCC method [7] applied to high- resolution (~ 1 km spatial resolution) thermal infrared brightness temperature images in the study region. Approximately 6 - 8 images were available each day giving from 5 to 7 daily image pairs to compute the MCC surface currents. A very conservative “threshold” cloud filter was applied to mask out covered regions of each image and surface currents were computed only for those image segments that were "cloud-free" in both the first and second images. For the ten-day composite a total of 70 images were used to compute coastal surface currents (Fig. 1a).