EVALUATION OF ENVISAT ASAR WAVE MODE RETRIEVAL ALGORITHMS FORSEA-STATE FORECASTING AND WAVE CLIMATE ASSESSMENT

摘要

The Envisat mission will offer new ocean wave products from its ASAR Synthetic Aperture Radar: an imagette crossspectrumrnderived from two consecutive looks (level 1b) and an ocean wave spectrum product (level 2), developed for ESArnby NORUT IT and IFREMER. The wave validation team evaluates these data products and compares the level 2 product tornwave spectra retrieved with alternative processing methods. Three aspects were investigated:rn The potential benefits of the level 1b product over the earlier ERS SAR wave mode spectra.rn The quality of the level 2 ocean wave retrieval algorithm.rn The performance of the Envisat ASAR wave mode products.rnFirst two aspects were carried out before the launch on "simulated" ASAR wave mode data, obtained by re-processing ofrnERS-2 SAR. The comparison of level 1b with ERS SAR was based on the SPRA scheme of ARGOSS. Reference data wererndata from a wave forecasting model.rnBased on the simulated data set it was found that:rn Using the ASAR level 1b product can reduce the error in wave height by at least 10-15%.rn The level 2 product and the forecasted wave spectra match rather well over a wide range of ocean wavelengths,rnand also overall wave heights and periods agree closely to the reference data. The outcomes underline thernimportance of providing guidance for the use of the level 2 product, as wind-sea outside the linearly mapped wavernnumber region is not included and adding this information is left to the user. If this is not done, overall parametersrnlike the significant wave height will lose their validity at high wind sea conditions.rn The inversion scheme relies on the assumption that the azimuthal bandwidth and the mapping relations derivedrnfrom a wind sea estimate are closely related. Moreover it assumes that the short wave energy can be parameterised.rnFor certain ocean wave spectra (high sea states, short period swell) these approximations might result in arndeterioration of the performance of the level 2 product.rn The 180-degree ambiguity is solved very well with the ASAR cross spectra.rnAlthough the number of Envisat acquisitions was limited, some interesting results were found already:rn Initial problems with the wave mode products are solved already.rn The retrieved wind speed and the normalised inverse wave age (together they define the energy contribution of thernwind-sea) do correlate and the applicability of both parameters to estimate the wind sea energy is questionable.rn For current data set the phase velocity of the wind sea peak is approximately constant.