Inversion Of Upper Ocean Vector Flow Field Based On SAR Doppler Informatio

Ocean current is one of the most important movements of sea water,and it carries on important impact on the weather and climate over the ocean,as well as considered volume transport of biogeochemical elements through the oceans,so that it influences the formulation of ship shipping routes,the monitoring and prediction of marine environment as well as maritime rescue.In addition,ocean current also has great scientific research value.With the development of oceanography in China,research on the upper ocean dynamics has risen great attention recently.Ocean current data also become more important for studying many of the upper ocean processes.However,nowadays observations can not meet the plenty needs of researches,so it is quiet urgent to develop new remote sensing methods for ocean current observation.Microwave remote sensing is a relatively efficient and low-cost method to obtain the dynamic information from sea surface.In recent decades,Doppler centriod has become an important parameter of sea surface remote sensing,but still few research public is reported on the retrieval of current direction from Doppler information at present.In this study we explore the technologies of retrieving the upper ocean vector field information from spaceborne Synthetic Aperture Radar(SAR)Doppler centriod,as well as its application in the sea surface large and meso-scale processes,so as to provide reference for subsequent research.For the first,by highly making use of the key point that the radial velocity carries the real sea surface direction information,we constructed the initial guess field by numerical model simulation,and combined with the algorithm of sea surface direction retrieval we developed,including aspect-angle method,iterative method and iterative differential approximation method,and finally we succeeded to fulfill the direction retrieval in the inner area of the Gulf Stream.Aspect-angle method is an empirical algorithm.Iterative method took the numerical simulation data as the initial field,well the Doppler image as the approximation field,and through iterative approximation to achieve the optimal solution of the real field.Iterative differential approximation algorithm(IDAA)combines iterative method and image differentiation with optimization so that the iterative approximation process can effectively converge to the true value.For the Gulf Stream direction retrieval,conducting IDAA to approximate Doppler radial velocity image,the retrieval results are compared with numerical simulation to verify that to be reasonable.Two-dimensional vector field of arbitrary water depth within a specific water depth range is also retrieved by confirming the unique flow function of the vertical dynamic profile for numeric Gulf Stream simulation.Through proving the effectiveness of the IDAA,the retrieval of Ekman current is completely realized in this study by combined with the improved strategy.The unique structure of Ekman current solution contains the flow direction information from the two-dimensional scalar field.From this point we make up the conceptual deficiencie of the IDAA to improve and accomplish the retrieval of three-dimensional Ekman current.Validation made by comparison with the empirical model constructed from in-site buoy data shows that at both 0 m and 15 m depth the effectiveness and applicability of retrieved Ekman current field are very satisfactory.Ekman current retrieval is the core work of this study.Since Ekman current retrieval requires combining of both retrieved wind field with current field,this study also compares and analyzes the capability of wind-current joint retrieval from different microwave remote sensing equipment in order to continue to expand Ekman current results from SAR to these equipment.Through two examples,the interaction between Gulf Stream extension and strong storm and the observation of mesoscale eddy,the incomparable advantage of SAR over other equipment on sea surface current retrieval is strongly demonstrated.In summary,we construct three methods for current direction retrieval in this paper,which partially solves the problem of real-time sea surface current direction retrieval after Dopper centriod technology proposed for 16 years since 2005.We tend to think it will actively promote the application of Doppler information into the processes of air-sea interface dynamics and the new generation of ocean exploration technology.The advantages of Doppler technology in microwave remote sensing will also reveal a new viewpoint on the oceanographic dynamics in the future.