Researches On Wind Field Inversion And Wave-Current Interaction Effects For Compact High Frequency Radar

The monitoring of ocean surface parameters is of great help to the exploitation and utilization of ocean resources,navigation and fishery activities on the sea surface,and the design and construction of onean engineering.As an important remote sensing tool for ocean surface,high frequency(HF)sea-state radar can monitor large-area,all-weather and real-time wind,wave and current parameters of ocean surface.Due to the advantages of small area,good economy,easy erection and easy maintenance,compact HF radars are installed more widely in the world.At present,the detection of ocean surface current parameters based on the compact HF radar has been relatively mature,while the detection of ocean surface wind parameters still has many problems.These problems mainly include:(1)Uncertainty of wind direction factor.The wind direction factor is the key to the wind direction inversion of high frequency radar.The commonly used half-angle cosine wind direction distribution cannot well explain the observed doppler spectrum intensity distribution of HF radar,so it affects the accurate estimation of wind direction by HF radar.(2)The difficulty of wind speed estimation due to the uncertainty of wind-wave relationship in different sea areas.(3)The challenge of wind speed field estimation due to the broad beam and low spatial resolution of compact HF radar.(4)Since the wind parameters of HF radar are derived indirectly from the state of the sea surface,the wave-current interaction on the sea surface will affect the radar echo and lead to errors in the estimation of wind parameters.For the above problems,this paper investigates relevant researches as following:(1)Extracting of wind spreading factor distribution from the compact HF radar echo data by two methods.Based on the results of the first method,we modified the existing wind spreading factor by fitting the first-order peak ratio under different sea conditions.For the second method,we finds that the bimodal wind direction distribution can better fit the data by obtaining the parameter distribution of the half-angle cosine model in different directions.Finally,two wind spreading factors are used to estimate the wind direction and obtain a satisfactory wind direction map.This study proves that the compact HF radar has the ability to extract wind spreading factor and the extracted wind spreading factors can improve the performance of wind direction inversion.(2)Analysing different wind-wave models in HF radar wind speed inversion.Radar data in infinite fetch indicate that the simple PM relationship can obtain more stable wind speed estimation than the SMB relationship and JONSWAP relationship.The neural network,which is trained with one–year buoy data,can learn more complex wind-wave relationship and performs better than the traditional experiential wind-wave relationship when the state of sea surface is more complex.(3)Proposing a spatial recursion method to extract wind vector field with first order peak intensity to solve the problem of disability for the traditional wind-wave relationship method to invert the wind velocity field because of the broad beam of the compact HF radar.The semi-empirical model of first-order peak intensity and wind speed,wind spreading factor and wind speed were established by using buoy data.The theoretical model of echo energy attenuation and wind speed and wind direction was comparied with actual radar attenuation and used as the final attenuation model.The wind speed obtained from the previous range cell is used as an approximation to compensate the first-order peak of the next range cell to extract the final wind vector field.Finally,the wind vector field results verify the effectiveness of the spatial recursion method.This method solves the problem of wind field extraction of compact HF radar and it also can be used to increase the coverage of wind speed inversion of the narrow-beam HF radar.(4)Proposing a method to extract the effect of wave-current interaction by combing the frequency shift and power of HF radar first-order data and compensating it in the process of wind parameter inversion.The theoretical analysis of the one-dimensional and two-dimensional wave-current interaction demonstrates that the first-order spectrum intensity changes in the deep water area are similar to the theoretical results of the two-dimensional oblique incidence: there is an obvious inverted “V” shaped distribution.The method makes the HF radar can be used to explore the wave-current interaction relationship in different sea areas.The influence of current on Bragg wave amplitude will lead to the corresponding error of wind parameter estimation based on the first-order spectrum power of HF radar.When the effect of the current on Bragg waves is extracted,the power of the first-order spectrum can be compensated according to the radar-measured current vector and so that the first-order spectrum based wind parameter estimation of HF radar has higher accuracy.The comparison results show that the current based compensation to the first-order peak can improve the inversion results of the wind field.