Conference Agenda

GF-3 (GF stands for GaoFen, which means High Resolution in Chinese) is the China's first C band multi-polarization high resolution microwave remote sensing satellite. It was successfully launched on Aug. 10, 2016 in Taiyuan satellite launch center. The synthetic aperture radar (SAR) on board GF-3 works at incidence angles ranging from 20 to 50 degree with several polarization modes including single-polarization, dual-polarization and quad-polarization. GF-3 SAR is also the world’s most imaging modes SAR satellite, with 12 imaging modes consisting of some traditional ones like stripmap and scanSAR modes and some new ones like spotlight, wave and global modes. GF-3 SAR is thus a multi-functional satellite for both land and ocean observation by switching the different imaging modes.

TG-2 (TG stands for TianGong, which means Heavenly Palace in Chinese) is a Chinese space laboratory which was launched on 15 Sep. 2016 from Jiuquan Satellite Launch Centre aboard a Long March 2F rocket. The onboard Interferometric Imaging Radar Altimeter (InIRA) is a new generation radar altimeter developed by China and also the first on orbit wide swath imaging radar altimeter, which integrates interferometry, synthetic aperture, and height tracking techniques at small incidence angles and a swath of 30 km. The InIRA was switch on to acquire data during this mission on 22 September.

This paper gives some preliminary results for the quantitative remote sensing of ocean winds and waves from the GF-3 SAR and the TG-2 InIRA. Comparisons to the ECMWF ERA-Interim reanalysis data show good agreements but more valuable details.

Sentinel-1 wave mode operates in novel ‘leap frog’ acquisitionmode. A vignette is acquired every 100 km at two alternate incidence angles (23° and 36.5° respectively ), withtwo images at the same incidence 200km apart. This mode enables us not only to conduct global analysis for each incidence angle, but also to investigatethe dependence of geophysical parameters on incidence angles.

The SAR image cross-spectrum between a pair of sublooks separated in time has been widely used to help remove 180° ambiguity of detected ocean swell systems. Ocean swells move along the waves' propagation direction during this offset time of the order of SAR integration time (~0.4s for Sentinel-1), therefore, co- and cross-spectra can help to consistently estimate velocity characteristics of the randomly moving sea surface scatterers related to scales larger than SAR spatial resolution. The short radial waves longer than SAR spatial resolution but shorter than ocean swells are, to first order, driven by surface winds. Thus, a new parameter, averaged complex cross-spectra over short radial waves domain, is proposed to manifest impacts of wind speed and direction on these moving scatterers.

Large data sets have been constituted systematically by co-locating Sentinel-1A/Bwave mode acquisitions and ECMWF forecast winds. Efficiently, distinctive features are revealedfor two incidence angles as well as dual-polarizations.

Specifically, to first order,there exhibits a linear relationship between imaginary cross-spectra and radial winds. More encouragingly, it reaches the maximum at downwind (wind blowing toward to the antenna), drops to zero at crosswind (wind blowing along azimuth direction) and continuously decreases to the minimum at upwind. Its sensitivity to wind direction is potential to wind retrieval applications.

At 23° incidence, the spectral parameter does not exhibit up-to-downwind difference of spectral. At variance, a strong asymmetry with respect to radial wind at 36.5° incidence is found. This asymmetry is much more pronounced than up-to-downwind asymmetry for the Normalized Radar Cross Section (NRCS).

Taking advantage of exceptionally continuous HH acquisitions by Sentinel-1B wave mode, we also present preliminary results for HH polarization as well as dual-polarizations' comparisons. In general, HH shows similar features to VV at each incidence angle. However, both spectral magnitude and imaginary part are larger for HH than those for VV for given incidence and radial wind, which could be explained by larger tilt modulation for HH polarization.

As a signed quantity, the imaginary cross-spectra of short radial waves can be considered to be a parameter strongly correlated with the Doppler Centroid Anomaly (DCA). Indeed, both parameters are directly linked to the time evolution of the detected sea surface scatters. As such, both parameters shall closely trace the wind direction within a single SAR imagette. Yet, Sentinel-1 DCA is, today, unfortunately unreliable, with large latitudinal variations for given surface winds. Free of geometrical configuration, the imaginary cross-spectra can thus serve to replace the DCA estimate to help constrain the retrieval of wind field at moderate to high spatial resolution.
In addition, similarly to the residual DCA, a residual parameter can be derived for the residual imaginary cross-spectral parameter, i.e. the difference between the expected parameter for a given wind speed and direction and the local measurement. As anticipated, this residual spectral parameter shall complement observations of surface wind-wave-current interactions.

Conventional VV-polarized NRCS is saturated as wind speed approaching typhoon intensity. In view of this, high wind speeds are underestimated by CMOD5.N especially in the typhoon eyewall regions. Compared to VV-polarization, NRCS in VH polarization is more appropriate to retrieve high wind speeds with cross-polarized retrieval model such as C-2PO or C-2POD. However, cross-polarization wind speed retrieval models overestimate low and moderate winds. Typhoon has capacity of inducing strong dynamical and thermal impacts on the ocean. During the passage of a typhoon, surface wind stress generates intense turbulence and currents in the upper ocean. In general, CSFR data are used as a wind forcing to simulate ocean surface and subsurface response to the typhoon, while the coarse resolution of CSFR can not accurately depict fine structure characteristc of typhoon. Furthermore, there are obvious difference between typhoon intensity and eye center location from CSFR data and satellite observations.

In order to obtain reasonable wind speeds in typhoon eye and eyewall regions as well as in periphery areas, we build a cost function both involving VV and VH-polarization SAR observation. Co- and cross-polarization wind speed retrieval models, CMOD5.N and C-2POD, can yield simulated NRCS in VV and VH poalrizations. The initial first guess wind field is not dervied from numerical weather prediction model, but VH-polarized SAR image itself. According to wind steak features appearing on the typhoon SAR images, we first extracted wind directions by using local gradient method and two-diemensional interpolation technique.The retrieved wind directions are compared with HWRF simulations and WindSat measurements. Subquently, we use C-2POD model and VH-polarized NRCS to retrieve wind speeds. As a result, the initial first guess u and v components can be estimated by using retrieved wind speeds and wind directions. The optimum wind vector estimate for any given wind cell will therefore correspond to a minimum in the cost function.

We use the Regional Oceanic Modeling System (ROMS) to simulate characteristics of upper ocean response to the typhoon. The SAR-derived wind fields are used to force the ROMS. The model outputs reveal the sea surface temperature (SST) cooling and sea surface salinity (SSS) augment on the righ side of the typhoon track. The decrease of SST and increase of SSS are caused by the fact that the entrainment of cold and high salt water breaks through the thermocline and gets into the mixed layer and surface. We compare SST and SSS simulations with microwave Optimally interpolated SST products and SMAP SSS data. Moreover, we also use ROMS simulate the mixed layer depth and upwelling velocity during the passage of the typhoon. Results show the deepening of the mixed layer and increase of the upwelling velocity due to the typhoon-induced vertical mixing. We analyze NPP VIIRS-derived chlorophyll concentration before and after the passage of typhoon, and catch sight of obvious chlorophyll bloom. The increase of clorophyll concentration provides us with evidence for the cold water uplifting. The typhoon pumped cold water from the lower layer with rich nutrients which enhanced the surface biological production.

Secchi disk depth (SDD, m^-1), which can reflect the clarity and turbidity of coastal seawater, is an important parameter to describe the optical properties of water. In this work, an empirical model for SDD retrieval was established on the basis of the MODIS Rrs data and the in-situ SDD data which were collected in the Yellow Sea and the East China Sea. The SDD model was further applied to retrieve the monthly SDD during the period of 2002-2016 for the Bohai Sea, the Yellow Sea, and the East China Sea; and the variations in SDD was first analyzed with the river discharges and the eutrophication indexed by Chl-a. Over the western Yellow Sea which is characterized by "green tides" of floating Ulva spp blooms in the summer since 2007, the variations in SDD were specially analyzed to investigate the effects of "green tides" on the optical environment.

On August 10, 2016, carrying the first Chinese C-band multi-polarization SAR, Gaofen-3 (GF-3) satellite was successfully launched into a polar sun-synchronous orbit of 755 km altitude with an inclination of 98° and 26 days repeat cycle. Following several months of on-orbit commissioning phase, GF-3 SAR images have been in operation since January, 2017. As one of the primary users, the State Oceanic Administration (SOA) is operationally conducting GF-3 SAR ocean wind retrieval and plan to officially release the near real time SAR wind products soon. In this paper, we present the first results of GF-3 SAR derived winds and preliminary assessment using the buoy measurements.

In principle, the GF-3 SAR wind inversion methodology is to combine SAR observed NRCS at co-polarized channel with a priori wind information from ECMWF, taking into account that both NRCS observations and models may contain errors. In order to extract the wind speed and direction, the cost function is minimized with the help of look-up table computed from geophysical model function (GMF), making the inversion scheme more efficient.

The coastal winds were estimated from the GF-3 SAR at 1 km resolution in four imaging mode, including Standard Strip, Quad‐Polarization Strip I, Quad‐Polarization Strip II and Narrow Scan imaging mode, and the retrievals are presented. One case of the coastal katabatic wind off U.S. west coast captured by GF-3 is discussed.

The preliminary accuracy assessment of GF-3 SAR wind speed retrievals is carried out against in situ measurements from NDBC buoys. Only the buoys located inside the GF-3 SAR wind cell (1 km) were considered as co-located in space, while the time interval between observations of SAR and buoy was limited to less the 30 min. This criteria yield 38 co-locations during the period from January to April 2017, showing the RMSE of 2.59 m/s. Different performances due to GMF and Polarization Ratio (PR) are discussed. The preliminary results present that GF-3 wind retrievals are encouraging for operational implementation.