Water Level Anomaly using Cryosat-2 and Saral over the Brahmaputra river
The proposed studies are in line with the work performed for many years by the "Instrument and Physical Measurement" team in CLS with the past and current altimetry missions: TOPEX/Poseidon, Jason-1, Jason-2, ENVISAT/RA-2, HY-2, Cryosat-2, Saral and Sentinel-3. These studies will contribute to the various objectives of the Joint Research Announcement “Ocean Surface Topography Science Team” through the assessment and the improvement of the altimeter parameter estimation :
- Development and assessment of new retrieval algorithms and/or models/corrections likely to improve the quality of the data over open ocean and particular zones: coastal oceans, inland waters, ice surfaces. For open ocean, attention will be specifically drawn to the phenomena that depart from the "nominal" oceanic conditions : rain cells, sigma blooms, internal waves etc ... Both conventional and new delay doppler altimeters (Cryosat-2, Sentinel-3 and Sentinel-6) will be considered.
- Scientific application studies: mesoscale variability, ocean currents, coastal altimetry, continental water monitoring, polar oceans observations, continental and sea-ice studies, …
- Operational applications studies (example: iceberg detection, a service that is already operated but also sea ice coverage and thickness determination)
The proposed work is organized around 5 themes all dealing with a state-of-the-art analysis of the altimeter waveforms and relative parameters derived by the retracking algorithm, and all taking advantages of the high-level expertise of our team:
- Theme 1: Development of optimal algorithms (retracking/corrections) depending on instrument/missions characteristics and over-flown surfaces
- Theme 2: Computation of mission performances, comparison between missions, and recommendation for future missions
- Theme 3: Derivation/assessment of the Sea State Bias (SSB): non parametric methods and the new methods currently developed at CLS.
- Theme 4: Analysis of the instrument behavior along the orbit and ageing with time and impact on the estimation of the long term climate trends.
- Theme 5: Icebergs detection in all altimeter echoes including stacked SAR echoes
For many years, our group is investigating processing solutions over open-ocean but in the recent years, particular efforts have been made to improve the waveform processing over all surfaces like ice sheet, sea ice, coastal, inland water areas and also over ocean areas affected by rain cells, sigma blooms or internal waves. The originality of our work is to keep the same processing philosophy for conventional and delay doppler altimetry, which consists in privileging a physical (not empirical) numerical approach for the retracker without any approximation. This approach is moreover crucial to guarantee a smooth continuity between the different surfaces without jumps at the interfaces. It insures as well continuity between missions (Ku, Ka, conventional, delay doppler). In terms of performances, huge improvements have been made recently and efforts will be pursued in the future. The consistency between missions/frequencies/modes allows us anticipating the computation of higher level merged products for all surfaces, as it is done for many years over ocean with the DUACS system. The above figure gives an illustration of a level 4 altimetry product over a hydrological zone, here over the Brahmaputra river basin.