15 research outputs found
Gaussian Anamorphosis for Ensemble Kalman Filter Analysis of SAR-Derived Wet Surface Ratio Observations
Flood simulation and forecast capability have been greatly improved thanks to
advances in data assimilation (DA) strategies incorporating various types of
observations; many are derived from spatial Earth Observation. This paper
focuses on the assimilation of 2D flood observations derived from Synthetic
Aperture Radar (SAR) images acquired during a flood event with a dual
state-parameter Ensemble Kalman Filter (EnKF). Binary wet/dry maps are here
expressed in terms of wet surface ratios (WSR) over a number of subdomains of
the floodplain. This ratio is further assimilated jointly with in-situ
water-level observations to improve the flow dynamics within the floodplain.
However, the non-Gaussianity of the observation errors associated with
SAR-derived measurements break a major hypothesis for the application of the
EnKF, thus jeopardizing the optimality of the filter analysis. The novelty of
this paper lies in the treatment of the non-Gaussianity of the SAR-derived WSR
observations with a Gaussian anamorphosis process (GA). This DA strategy was
validated and applied over the Garonne Marmandaise catchment (South-west of
France) represented with the TELEMAC-2D hydrodynamic model, first in a twin
experiment and then for a major flood event that occurred in January-February
2021. It was shown that assimilating SAR-derived WSR observations, in
complement to the in-situ water-level observations significantly improves the
representation of the flood dynamics. Also, the GA transformation brings
further improvement to the DA analysis, while not being a critical component in
the DA strategy. This study heralds a reliable solution for flood forecasting
over poorly gauged catchments thanks to available remote-sensing datasets.Comment: 19 pages, 13 figures. Submitted to the IEEE Transactions on
Geoscience and Remote Sensin
Enhancing Flood Forecasting with Dual State-Parameter Estimation and Ensemble-based SAR Data Assimilation
Hydrodynamic
Reducing Uncertainties of a Chained Hydrologic-hydraulic Model to Improve Flood Forecasting Using Multi-source Earth Observation Data
The challenges in operational flood forecasting lie in producing reliable
forecasts given constrained computational resources and within processing times
that are compatible with near-real-time forecasting. Flood hydrodynamic models
exploit observed data from gauge networks, e.g. water surface elevation (WSE)
and/or discharge that describe the forcing time-series at the upstream and
lateral boundary conditions of the model. A chained hydrologic-hydraulic model
is thus interesting to allow extended lead time forecasts and overcome the
limits of forecast when using only observed gauge measurements. This research
work focuses on comprehensively reducing the uncertainties in the model
parameters, hydraulic state and especially the forcing data in order to improve
the overall flood reanalysis and forecast performance. It aims at assimilating
two main complementary EO data sources, namely in-situ WSE and SAR-derived
flood extent observations.Comment: Copyright 2023 IEEE. Published in the IEEE 2023 International
Geoscience & Remote Sensing Symposium (IGARSS 2023), scheduled for July 16 -
21, 2023 in Pasadena, California, US
Surface Measurements for Oceanographic Satellites: the SUMOS in-situ and airborne campaign
International audienceAs a supporting contribution to the validation of observations by the China-France Oceanography Satellite, CFOSAT, launched on 29 October 2018, the SUMOS campaign was conducted in February and March 2021 by CNES and a number of research teams from the CNRS, Météo-France and Ifremer.This campaign was mainly oriented for SWIM product assessment. SWIM is an innovative wave scatterometer It is a Ku-band real aperture radar which illuminates the surface sequentially with 6 incidence angles: 0°, 2°, 4°, 6°, 8° and 10°, thanks to its rotating antenna (5.6 rpm), it acquires data in all azimuth orientations. From SWIM measurements are generated, among other: nadir significant wave height and wind speed, wave slope spectra given for 70*90 resolution cells.The campaign was carried out in the Bay of Biscay between the 15th February and 4th March 2021. It was designed to collect a set of co-located observations of wind, waves and associated parameters (turbulent air-sea fluxes) by both in-situ and remote sensing means. The resulting data will be used to:• improve CFOSAT/SWIM instrument data inversion methods,• better identify the performance and limits of the parameters provided by CFOSAT/SWIM,• advance mission objectives concerning the study of wave hydrodynamics and wind/wave/flux relationships in turbulent conditions.The campaign also took advantage of the SKIM mission for multiscale monitoring of sea surface kinematics to acquire Ka-band Doppler radar scatterometry data via KaRADOC (an instrument developed by the IETR) to support concepts for future satellite missions using this technique.Two national experimentation platforms were deployed in and over the Bay of Biscay: F-HTMO, the ATR 42 research aircraft operated by SAFIRE, the French facility for airborne research (Météo-France/CNRS/CNES) and the French Oceanographic research vessel L’Atalante operated by Genavir.Data were acquired using these two platforms in coordination with CFOSAT passes over the Bay of Biscay. The operation plan included:1. in-situ measurements of surface waves with 20 drifting buoys (SPOTTER) to measure directional wave spectra and 3 so-called “FLAME” drifting devices to measure wind, and the turbulent fluxes close to the surface; all these systems have been deployed by the L'Atalante ship.2. measurements taken on board the ship itself using in particular an imaging X-band radar operated by Germany’s Helmholtz-Zentrum from Geesthacht. These measurements are used to estimate the directional spectrum of the waves and surface currents. In addition, a stereo video camera and a polarimetric imaging system was deployed by LOPS and collected about 50 hours of measurements to characterize the short and intermediate waves and study their relation with breaking and with remotely sensed parameters3. airborne radar measurements using the Ku-band Radar for Observation of Surfaces (KuROS) developed and operated by LATMOS, were performed from the ATR 42 research aircraft so as to provide directional wave spectra using the same concept of measurement as used for SWIM.4. airborne measurements with the KaRADOC radar developed and operated by the IETR, also carried by the ATR 42. These measurements are used in particular to study the concept of acquiring surface current data by applying a pulse-pair method to a Doppler radar signal. KaRADOC was designed as part of the SKIM project proposed by LOPS.In all, the ATR 42 flew 17 times in perfect coordination with the in-situ measurements carried out on board the L'Atalante and the drifting buoys deployed from the vessel. Of these 17 flights, 13 were synchronized with CFOSAT satellite passes (SUMOS campaign) and the last 4 were used for the SKIM project.Each flight provided between 3 and 4 hours of data over long distances across the Bay of Biscay. The research vessel was positioned at different points during this period according to the satellite's passes and the constraints of launching and recovering the drifting buoys.A large set of high-quality data was acquired, and a wide range of weather conditions was observed, enabling campaign objectives to be reached.The first results of this campaign are very positive. They show very close agreement between measurements from SWIM, the drifting buoys and the MFWAM model. Analysis from the full collocated data set is in progress with in particular studies devoted to the experimental estimation or validation of the Modulation Transfer Function which is used to invert SWIM observations into directional wave spectra.The data set will be made available to the wider scientific community, including altimetry community, as soon as they become available.The scientific altimetry community will be able to take advantage of these observations to validate the different products on wave height (SWH) at nadir
Gaussian Anamorphosis for Ensemble Kalman Filter Analysis of SAR-Derived Wet Surface Ratio Observations
International audienceFlood simulation and forecast capability have been greatly improved thanks to advances in data assimilation (DA) strategies incorporating various types of observations; many are derived from Earth Observations from space. This paper focuses on the assimilation of 2D flood observations derived from Synthetic Aperture Radar (SAR) images acquired during a flood event with a dual state-parameter Ensemble Kalman Filter (EnKF). Resulting binary wet/dry maps are here expressed in terms of wet surface ratios (WSR) over a number of subdomains of the floodplain. This ratio is assimilated jointly with in-situ water-level observations to improve the flow dynamics within the floodplain. However, the non-Gaussianity of the observation errors associated with these SAR-derived measurements break a major hypothesis for the application of the EnKF, thus jeopardizing the optimality of the filter analysis. The novelty of this paper lies in the treatment of the non-Gaussianity of the SAR-derived WSR observations with a Gaussian anamorphosis (GA) process. This DA strategy was validated and applied over the Garonne Marmandaise catchment (South-west of France) represented with a TELEMAC-2D hydrodynamic model, first in a twin experiment and then for a major flood event that occurred in January-February 2021. It was shown that assimilating SAR-derived WSR observations, in complement to the in-situ water-level observations significantly improves the representation of the flood dynamics. The GA process brings further improvement to the DA analysis, while has also been demonstrated to be a non-essential element. This study heralds a reliable solution for flood forecasting over poorly gauged catchments thanks to available remote-sensing datasets
Swim now in orbit: a new horizon for wave observation
International audienceThe CFOSAT (China France Oceanography SATellite) mission is a CNES-CNSA space mission for ocean surface wind and wave observation. The satellite has been successfully launched on October, 29th, 2018 from China. The satellite behaves perfectly well until now with nice scientific acquisitions. For the very first time, wind and waves are observed at the same time and place. These simultaneous observations open new research fields on climate monitoring, sea-air interactions understanding and sea state forecasting.CFOSAT embarks two payloads: a wave scatterometer, SWIM (French contribution) and a wind scatterometer, SCAT (Chinese contribution). Both are Ku-band real aperture radar taking advantage of different backscattering properties. At high incidence angles (SCAT), the radar signal is sensitive to Bragg scattering, i.e. small surface roughness induced by the wind. At low incidence angles (SWIM), the radar backscattering depends only on large slopes, i.e. the waves.In this paper, we will present the status on the SWIM instrument and the SWIM products produced by the French ground segment. This status will be provided after 7 months in orbit, at the end of the commissioning phase. At the time of the writing of the abstract (few days after the launch), we are just checking the health of the satellite and the instruments as well as the generation of the first products. We are not thus able to provide quantitative results to the reviewers at this stage. During the commissioning phase, the internal calibration modes of SWIM are tested (internal impulse response, antenna calibration law, reception noise level). The different acquisition modes are also tested (sequence of the six beams and on-board processing options).The CFOSAT system is composed of the satellite with SWIM and SCAT on-board and two ground segments, one in China (CHOGS) and one in France (FROGS). Both mission centers process all the SWIM and the SCAT data. This paper is relative to the processing performed at FROGS for SWIM only. The CFOSAT data are processed in near real time in the French mission center up to level 2 (CNES) and in differed time from level 2 to level 4 (IFREMER). The products are available on-line on the AVISO+ website.The main scientific products of SWIM are the 2D wave spectrum at a spatial scale of 70x90 km², the backscattering coefficient profiles per 0.5° step in incidence and 15° step in azimuth and the significant wave height and wind speed from the nadir beam.The paper will detail the performance of the instrument relative to SNR and backscattering coefficient accuracy, pointing knowledge accuracy. The overall availability of the data will also be discussed (coverage over ocean, distance to coasts, coverage of the continents and ice sheets).We will also present the results on level 2 products
CFOSAT: Latest Improvements in the Swim Products and Contributions in Oceanography
International audienceFor the first time, co-located wind vectors and wave spectral characteristics are available thanks to the French/Chinese CFOSAT mission, which includes a wind scatterometer SCAT and a wave scatterometer SWIM. Three years after its launch, CFOSAT data is thoroughly qualified and various scientific work has been undertaken. This paper focuses on CFOSAT SWIM data, its performance and scientific contribution to oceanography, coastal and sea ice study
CAL/VAL Phase for the Swim Instrument Onboard cFOSAT
International audienceThe Chinese-French oceanography satellite, CFOSAT, was launched on October 2018. Two Ku-band scatterometers are on-board: SCAT for the wind observation and SWIM for the wave observation. This paper presents the most recent results on the SWIM data quality analysis a few months after the end of the CAL/VAL phase