Comment on essd-2022-292

Abstract

This paper describes a high-resolution Level-3 (L3) altimeter Data Unification and Altimeter Combination System (DUACS) experimental regional product developed with support from the French space agency (CNES). Deduced from full rate (20 Hz to 40 Hz) Level-2 (L2) altimeter measurements, this product delivers Sea Level Anomalies (SLA) and other essential physical variables with a nearly 1-Km sampling over the North Atlantic 0cean. It allows us to resolve wavelengths from ~35 km to ~55 Km depending on the altimeter considered. This was made possible by recent advances in radar altimeter processing for both Synthetic aperture Radar (SAR) and Low-Resolution Mode (LRM) measurements, and improvements made to different stages of the DUACS processing chain. Firstly, the new adaptive and LR-RMC (Low Resolution with Range Migration Correction) processing techniques were considered for Jason and Sentinel-3 (S3A) respectively. They significantly reduce errors at short wavelengths and, for the adaptive processing, possible land contamination near the coast. Next, up-to-date geophysical and environmental corrections were selected for this production. This includes specific corrections intended to reduce the measurement noise on LRM measurement and thus enhance the observability at short wavelengths. Compared to the 1 Hz product, the observable wavelength reached with the experimental high-resolution product are reduced by up to one third, or up to half in the North-East Atlantic region. The residual noises were optimally filtered from full rate measurements taking into consideration the different observing capabilities of the altimeters processed. A specific data recovery strategy was applied, significantly optimizing the data availability both in the coastal and open ocean. This experimental L3 product is thus better resolved than the conventional 1 Hz product, especially near the coast where it is defined up to ~5 km against ~10 km for the 1 Hz version. The multi-mission cross-calibration processing was also optimized with an improved Long Wavelengths Error (LWE) correction leading to a better consistency between tracks with an 9–15 % reduction of SLA variance at cross-over. The experimental L3 product improves the overall consistency with tide gauge measurements with a reduction of the variance of the SLA differences by 5 and 17 % compared to the 1 Hz product from S3A and Jason-3 (J3) measurements respectively. Primarily intended for regional applications, this product can significantly contribute to improving high resolution numerical model outputs via data assimilation. It also opens new perspectives for a better understanding of regional sea surface dynamics, with an improved representation of the coastal currents and a refined spectral content that reveals the unbalanced signal.