Abstract
In this study, we proposed an empirical algorithm for significant wave height (SWH) retrieval from TerraSAR-X/TanDEM (TS-X/TD-X) X-band synthetic aperture radar (SAR) co-polarization (vertical-vertical (VV) and horizontal-horizontal (HH)) images. As the existing empirical algorithm at X-band, i.e., XWAVE, is applied for wave retrieval from HH-polarization TS-X/TD-X image, polarization ratio (PR) has to be used for inverting wind speed, which is treated as an input in XWAVE. Wind speed encounters saturation in tropical cyclone. In our work, wind speed is replaced by normalized radar cross section (NRCS) to avoiding using SAR-derived wind speed, which does not work in high winds, and the empirical algorithm can be conveniently implemented without converting NRCS in HH-polarization to NRCS in VV-polarization by using X-band PR. A total of 120 TS-X/TD-X images, 60 in VV-polarization and 60 in HH-polarization, with homogenous wave patterns, and the coincide significant wave height data from European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis field at a 0.125° grid were collected as a dataset for tuning the algorithm. The range of SWH is from 0 to 7 m. We then applied the algorithm to 24 VV and 21 HH additional SAR images to extract SWH at locations of 30 National Oceanic and Atmospheric Administration (NOAA) National Data Buoy Center (NDBC) buoys. It is found that the algorithm performs well with a SWH stander deviation (STD) of about 0.5 m for both VV and HH polarization TS-X/TD-X images. For large wave validation (SWH 6–7 m), we applied the empirical algorithm to a tropical cyclone Sandy TD-X image acquired in 2012, and obtained good result with a SWH STD of 0.3 m. We concluded that the proposed empirical algorithm works for wave retrieval from TS-X/TD-X image in co-polarization without external sea surface wind information.
Highlights
It is well known that space-borne synthetic aperture radar (SAR) is an efficiently instrument for wind and wave observation in a large coverage with high spatial resolution at seas
CMOD5 [33] that was derived from ERS-1 SAR images and European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis wind data, XMOD2 in [3] by retuning the coefficients in the C-band GMF CMOD-IFR2 [34] with the VV-polarization has been exploited in [4] by using collocated VV-polarization TS-X/TD-X images and National Data
SAR-derived wind speed from VV-polarization and HH-polarization TS-X/TD-X image has known accuracy at within 2 m/s Root-Mean-Square Error (RMSE) of wind speed [4,5], XWAVE is restrictedly used during operational application, due to prior wind direction is necessary in the process of wind retrieval by using XMODs
Summary
It is well known that space-borne synthetic aperture radar (SAR) is an efficiently instrument for wind and wave observation in a large coverage with high spatial resolution at seas. Algorithm SPRA employs wind speed from scatterometer to produce the first-guess wind-sea spectrum by using empirical parametric wave function and information on swell is regarded as the difference between the retrieval results mapping spectrum and the original SAR spectrum. SAR images and collocated NOAA in situ buoys dataset in the tuning dataset and the algorithm performs well under tropical cyclone condition. SAR images and collocated NOAA in situ buoys dataset are retrieval is presented and the coefficients of the proposed empirical function are tuned by the dataset. A case study for wave retrieval using two images acquired during in tropical comparison of SWH retrieved from SAR imagery and those measured by buoys is shown in Section cyclone in 2012 is presented.
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