Assessing high-wavenumber spectral slopes (and effective resolution) in new altimeter products
Event: 2018 Ocean Surface Topography Science Team Meeting
Session: Instrument Processing: Measurement and Retracking
Presentation type: Type Oral
New altimeter products offer the possibility of high-resolution along-track nadir altimetry. We compare these products in two test regions: the California Current and the tropical Pacific. In both regions, we also have available for reference Acoustic Doppler Current Profile (ADCP) and 1 to 2 km resolution output from the MITgcm ocean model. Our analysis cross-compares Jason 1 and 2 data processed with the Adaptive Leading Edge Subwaveform (ALES) retracker, AltiKa spectra, and Sentinel 3 data. The different altimeters produce consistent red spectral slopes for length-scales longer than about 50-70 km. At shorter scales, spectra are white (or nearly white), and energy levels differ depending on the data source. Sentinel 3 data in principle offer high resolution, but have the disadvantage of being collected on a relatively new orbit, meaning that high wavenumber spectral slopes appear to be dominated by the spectral slope of the geoid. (Cryosat-2, which operates on a non-repeating orbit, also poses challenges for geoid contamination.) The ALES Jason-1/2 and AltiKa products both reduce the classic “spectral bump” seen in nadir altimetry and should be able to resolve higher wavenumber processes, but they are not in agreement leading to uncertainty about the “true” structure of the high-wavenumber spectral slope. While spectra are difficult to interpret with confidence for scales shorter than about 50 km, the significant improvements in high wavenumber spectra in recent years suggest that new altimeters (perhaps with further advances to data quality control) will be able to resolve higher wavenumbers.