| Ocean surface currents,which play a key role in the Earth’s climate,affect almost any process that takes place in the oceans and at the same time directly affect many important socio-economic activities,such as global maritime trade and shipping,or issues such as marine pollution and safety.Therefore,the measurement of current is particularly important.In situ current sensors are used to obtain coastal currents,such as buoy moorings with Eulerian techniques and drifters of floating with the moving water with Lagrangian techniques.However,it is difficult to meet the needs of practical application because of its high cost and limited observation range.There are many ways to retrieve ocean current by satellite remote sensing,such as sea surface heights from altimetry and wind field from scatterometer can use to estimate current,but the defect is that the period is long,and the inshore current data can’t be obtained.The shore-based radar can obtain the current from the Prague scattering principle,but the observation range is limited to near the coast.Synthetic Aperture Radar(SAR)can only obtain the velocity of the current projecting to the radar line of sight,and there are many problems to be solved.Retrieved current with sequential satellite image has many advantages,such as lots of data resource,wide range of observations and low cost,which makes it possible to retrieved current of globality and systematicness.The Himawari-8 meteorological satellite has the advantages of high temporal resolution and spatial resolution,multi-band setting,high positioning accuracy and strong data service capability,which is a relatively advanced AHI sensor in the international geostationary orbit imager.At present,this satellite data is hardly used to retrieve the ocean current at home and abroad.In this paper,we use the Himawari-8 satellite data for five days from Joint Remote Sensing Satellite Receiving Station,Xiamen University,April 13,April 28,April 30,May 18,May 27,2017,to retrieve the coastal current in Hangzhou Bay.The maximum cross-correlation(MCC)methodology and the Generalized Hough transformation(GHT)methodology are used to retrieve the current respectively.The results are analyzed and verified by the Taiwan Strait NowcastJForecast System(TFOR).The results show that the two methods can retrieve the current well.In this paper,we use a satellite image of five days every 10 minutes to retrieve the current of 181 pairs of available image pairs at hourly interval.Firstly,the inversion results of the two methods are verified and analyzed.A total of 78212 current data points were retrieved by MCC method based on gray scale matching.Compared with the corresponding numerical model data,the average angle error(AAE)was 31.04°,the average relative error(AME)was 0.93,the mean absolute error(MAE)was 24.02cm/s and the root mean square error(RMSE)was 30.48 cm/s,respectively.The spearman correlation coefficient in u direction(Rc-u)was 0.79,and the spearman correlation coefficient in v direction(Rc-v)was 0.65.The estimate of the current is better.A total of 68047 current data points are obtained by the GHT method based on feature matching.Compared with the corresponding numerical model data,the average angular error(AAE)was 36.35°,the average relative error(AME)was 1.02,and the mean absolute error(MAE)was 26.36cm/s.The root mean square error(RMSE)was 33.60 cm/s,the spearman correlation coefficient in a direction(Rc-u)was 0.75,and the spearman correlation coefficient in v direction(Rc-v)was 0.60.Compared with the MCC method,it is slightly worse,but the estimating results are still better.Secondly,the overlapping current data points obtained by MCC and GHT estimates are compared and verified from two aspects of time scale and spatial scale,taking the numerical model data as the reference standard.The results show that the estimates results of the two methods are in good agreement.The assessment error of GHT method in Changjiang Estuary and offshore area with strong turbidity characteristic front is smaller. |
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