Conference Agenda

It is presented the recent progresses of ocean surface winds and typhoons remote sensing including (1) GF-3 SAR ocean wind retrieval: the first view and preliminary assessment; (2) Preliminary analysis of Chinese GF-3 SAR quad-polarization measurements to extract winds in each polarization; (3) Assessments of ocean wind retrieval schemes and geophysical model functions used for Chinese GF-3 SAR data at each polarization; (4) Combined co- and cross-polarized SAR measurements under extreme wind conditions; (5) Sea Surface Wind Speed Retrieval and Validation of the Interferometric Imaging Radar Altimeter Aboard the Chinese Tiangong-2 Space Laboratory; (6) Top cloud motion field of Typhoon Megi–2016 revealed by GF-4 Images.

Thanks to the Satellite Hurricane Observation Campaign (SHOC) initiative, the ESA Sentinel-1 mission planning team allows acquisitions over Tropical Cyclone since 2016. This data collection yielded to a catalogue of about 100 hits over Tropical Cyclone (TC) eyes. In parallel, the hurricane watch program from CSA also organises acquisitions over TC eyes. This study co-analyses data from the two missions and presents performances of our algorithm for ocean surface wind field retrieval at high resolution. As a first step the quality of the Normalized Radar Cross Section (NRCS) for both polarization and sensors is compared and found to be very consistent. The relationship between NRCS, wind speed and direction is analyzed for extreme cases. Then, the wind speed performances are compared to other satellite remote sensing data, airplane measurements and analysis from experts in TC centers (tracks). The impact of rain on the ocean wind measurement is discussed. Finally, to complement the NRCS, other radar parameters such as Doppler Centroid and the energy of the MeAn Cross-Spectra (MACS) high frequency part are also analyzed. In particular, we show how MACS could be used to constrain the wind retrieval.

A new space-borne dataset of global ocean swell, called Environment Monitoring Services (CMEMS) “fireworks” Level-3 product, derived from Sentinel-1A/B Level-2 ocean swell spectra is presented and its performances are assessed. The Level-2 swells inverted from synthetic aperture radars (SAR) images are retro-propagated along the great circle and refocused at their remote origins (coinciding strong storms), producing a higher level product to describe the swell temporal and spatial evolution from origin until land across the oceans. The Level-3 “fireworks” are now operationally produced by Copernicus CMEMS.

Here, we assess their performances using sentinel-1A/B wave mode data for the period from July 2016 to Nov. 2018, based on the “virtual” buoy concept. Reference data are in situ directional wave measurements from two different buoy networks:National Data Buoy Centre (NDBC) and Coastal Data Information Program (CDIP). Comparison results show a good agreement between Sentinel-1 Level-3 swells and buoy measurements, with root mean square error of 48 cm, 45.66 m and 21.21° for swell height, peak wavelength and direction, respectively. Influence of buoy network on the validation results are also examined, revealing better wave directional measurement accuracy for Waverider buoys used in CDIP than in NDBC network.

Sentinel-1 A & B SAR constellation is collecting data in Wave Mode (WV) and ESA is routinely producing and delivering Level-2 Ocean products with ocean surface wind, waves and radial velocities. In particular, the wave component of this product contains the image cross spectra (real and imaginary parts) and the 2D ocean spectrum with associated waves partitions. Here, we focus on the wave measurements originating from Tropical Cyclones. As a first step, we developed a method to filter out the wave partitions with low quality. This quality control procedure is performed for each acquired track and relies on the expected swell consistency between successive acquisitions along any given track. This method is evaluated against model outputs (statistical analysis) and buoys (case study). Then, we analyze the waves properties (wavelength and wave propagation direction) with respect to the Tropical Cyclones properties. The impact of storm size, translation speed and intensity on the extended fetch and waves escaping from the storm source is illustrated and discussed.

Swells from strong storms can spread over very long distances. Ocean currents alter this propagation, with the possible formation of constructive or destructive interference. This effect, still neglected in current models of atmospheric, oceanic, and even wave prediction, is often traced in current measurements, altimetry or even scatterometry / radiometry at medium and high resolution.

Large-scale currents are indirectly measured by satellite. Since the small-scale currents are generally unknown, we propose to consider them as random in wave dynamics simulations. Specifically, the statistical spatial structure of these currents is inferred from large-scale currents through self-similar assumptions. The temporal correlations of the small-scale currents is neglected due to the short-time wave-current interaction. The dispersion ratio is modified and becomes stochastic. From there, we can derive and simulate the random dynamics of wave group the rays. Analytic and semi-analytic solutions have also been derived for simple – though realistic – cases.

Our results not only improve wave simulation capabilities, but also bring new insights about the large wave’s developments at small scales and the wave-current effects on satellite measurements. At longer term, those type of random dynamics will bring new data assimilation procedures for joint wave-current estimations from space.