Research On The Theory And Application Of Spaceborne Synthetic Aperture Radar Ocean Current Remote Sensin

Ocean current is one of the important movement forms of the ocean,which has a direct impact on global climate change.Accurate acquisition of ocean surface current is a research hotspot in the field of marine science.In recent years,with the development of satellite remote sensing technology,obtaining sea surface current information from Synthetic Aperture Radar(SAR)has become a new trend.In this study,a semi-empirical model that can calculate the velocity of sea surface waves is newly developed,and the model is applied to the actual ocean current retrieval combined with SAR observations.The main content of this paper can be roughly summarized as follows(1)Synthetic Aperture Radar Ocean Current Imaging MechanismObtain polarization ratios,polarization differences,Bragg scattering,and wave breaking in coand cross-polarization by decomposing quad-polarization radar images of different atmospheric and oceanic phenomena(atmospheric gravity waves,oceanic internal waves,temperature fronts,and ocean currents).The study found that the control mechanism of the ocean current feature imaging in the SAR image is the modulation of the breaking wave by the ocean current,while the Bragg scattering of the sea surface has no relationship with the radar imaging of the ocean current.At the same time,it is also found that the modulation of the breaking wave by the ocean current in the copolarization is 2 ～ 3 times that of the cross-polarization.Based on 1696 RADARSAT-2 quadpolarization radar images which are matched with the buoy wind field,a wave-breaking microwave scattering model is developed.In this model,the wave breaking radar signals in co-polarization and cross-polarization can be simulated as long as the sea surface wind speed,wind direction and radar incident angle are input.The study found that for VV polarization,the contribution of wave breaking to radar signal decreases with the increase of incidence angle,from 70% to 20%;for HH and VH polarization,the contribution of wave breaking to radar signal can reach 60%.(2)Synthetic Aperture Radar Ocean Current Remote Sensing MethodBased on the newly developed wave breaking microwave scattering model and the geophysical model function for calculating radar backscattered signals(C-SARMOD2),a semi-theoretical and semi-empirical model(Dual Co-Pol Doppler model,DPDop)is developed.The input parameters of the model are sea surface wind field,two-dimensional wave spectrum and radar parameters.Compared with the traditional empirical model,the establishment of the semi-theoretical and semiempirical DPDop model does not depend on the Doppler frequency shift observed by radar,which avoids the systematic error.In order to expand the application range of the model,the DPDop model is further simplified in this study,so that the input of the original two-dimensional wave spectrum becomes the input of wave characteristic parameters(significant wave height,mean wave number and mean wave direction).The simulations of the original and simplified DPDop models are agreed with SAR observations,and the root mean square errors(RMSE)of original and simplified DPDop are 5.55 Hz and 6.11 Hz,respectively.At the same time,based on the SAR observation,the DPDop model can be used to retrieve the reliable radial current velocity of the sea surface.Compared with the observation of the high-frequency radar,the bias and the RMSE are-0.04 m/s and 0.15 m/s,respectively.(3)Synthetic Aperture Radar Ocean Current Remote Sensing VerificationTo further validate the DPDop model,this study collected Doppler velocity product from SAR observations of three hurricanes and the Agulhas Current,and geophysical Doppler velocities are obtained through scallop signals and land corrections.The wave field simulated by the model is input into DPDop to calculate and remove the wave contribution from the geophysical Doppler velocity to obtain the sea surface radial current velocity,which is compared with the current velocity observed by HF radar,underwater glider and drifting buoy.The error is about 0.15 m/s,which verifies the accuracy of DPDop’s calculation in moderate and low sea conditions and peripheral regions of the hurricane.(4)Synergetic Observations of Ocean Current and Wind FieldIt is challenging to extract the surface current field under high sea conditions.This study uses the sea surface wind field observed by multi-source satellites and the current field measured by drifting buoys to study the relationship between the velocity and wind speed on the left and right sides of the tropical cyclone moving path,and the relationship between the current field and the wind field.The study found that: under high wind speed,the current velocity increases approximately linearly with the wind speed,and the wind stress on the right side and the near-inertial current resonance cause the velocity of the mixed layer on the right side to be stronger than that on the left side;the drift ratio on the left side of the moving path is 2%,and the right side from 2% to4%,the current velocity on the right side is more responsive to the wind speed;the stronger wind drive on the right side of the path makes the drift angle(difference between the wind direction and the current direction)smaller,while the weak near-inertial current on the left side causes a larger drift angle.In summary,the semi-empirical model DPDop developed based on the wave breaking microwave model can be used for retrieval of SAR sea surface radial current velocity under low,moderate and high sea conditions,and more accurate velocity information can be obtained.At the same time,the ratio of the current velocity to the wind speed on the left and right sides of the tropical cyclone moving path is helpful for the retrieval of the ocean current under high wind speed in the next step.