Applications Of A Few Signal Decomposition Methods In The Calculation Of Air-Sea Flux By Eddy Covariance Method

The air-sea flux is an important parameter in the air-sea interaction and play an important role in ocean and climate studies.Eddy covariance method(ECM)is the most accurate air-sea flux calculation method,and its results are often used to test the accuracy of other flux algorithms.Therefore,ensuring its own scientificity and accuracy is of great significance.And the determination of the Cutoff Time Scale(CTS)in the calculation of air-sea flux by ECM is a top priority.This study is centered on the time scale.Based on 38-day observation data from a tower and 15-day observation data from a buoy,three signal processing methods which are Multi-Resolution Decomposition(MRD),Empirical Mode Decomposition(EMD)and Wavelet Decomposition(WD),are applied to the calculation of air-sea flux by ECM,respectively.In this paper,we compares MRD,EMD and WD,firstly.The results show that the MRD method is simple in operation but has the lowest degree of signal separation.EMD method has the highest autonomy and its signal decomposition ability is also higher than that of MRD.WD method has the highest degree of signal separation,but the prerequisite is to set the appropriate base wavelet and decomposition layer number.The value of CTS is also estimated in this paper based on different methods,with mean values of 78.2 s(based on MRD estimation),57.5 s in the longitudinal direction and 68.8 s in the lateral direction(based on EMD estimation),84.3 s in the longitudinal direction and 83.1 s in the lateral direction(based on WD estimation).Although the results differ from each other,they are far smaller than the value of CTS(10 minutes)which is used currently.In addition,this paper proposes that calculation of flux should consider the gap scale of two variables at the same time,which further filters the flux contribution of the non-turbulence part of the data.The results show that the relative deviations between the momentum flux obtained by the traditional flux calculation method(The value of CTS is fixed at 10 minutes)and the flux obtained by the new calculation methods which are applied MRD,EMD,and WD are 43.5%,76.5%and 60%,respectively.The calculation accuracy of air-sea flux has been improved,significantly.Finally,in order to study the influencing factors of CTS,a series of turbulence characteristics are calculated and the relationship between them and the CTS is analyzed separately.Using multiple linear regression,the relationship(Equation 5.14)between the root mean square(RMS)and standard deviation of wind speed data and CTS is obtained and validated with the buoy data.The results show that this formula has a certain overestimation of the CTS value,but it can reflect the trend of CTS,effectively.