Background Error Correlation Structure Of Statistical Analysis With The Envisat Asar Wave Spectra Data Assimilation Research

Ocean wave data assimilation can improve the simulation and forecast level of the realistic wave field. The key factors influencing wave data assimilation effects are the choice of observation data and the presentation of background error covariance matrix (or background error correlation function). One new method to construct the background errors was given. The statistical analysis and parameteration of the correlation structure of significant wave height (SWH) background error were studied. The optimal interpolation (OI) assimilation of Envisat ASAR ocean wave data was carried out. And two different anisotropic background error correlation functions were presented. Finally, several groups of assimilation experiments under different settings were run to check their assimilation effects.Firstly, based on the third-generation wave model named LAGFD-WAM, the difference between the SWH forecasts with 24 hours interval is considered as the approximation of SWH background errors. The time series of SWH background errors were constructed from the time series of the wave model forecasts. And the background error covariance matrices and the background error correlation function matrices were also constructed. Then the correlation structure of SWH background errors was statistically analyzed. The results indicated that the SWH background error is anisotropic and inhomogeneous. Hence, four different ellipse-type anisotropic analytic forms of background error correlation functions were presented. The three parameters of error ellipses were fitted. And the influence of the forecast interval on the background error structure statistic was discussed too.Secondly, Envisat ASAR wave mode ocean wave spectra products from year 2003 to 2008 which had been filtered and quality-controlled were compared with buoy observation data in Pacific in order to assess the wave spectra data. The results show that: (1) ASAR one-dimensional frequency spectra agree well with buoy observations in spectral shapes and bad in spectral values. The agreement in the low frequency domain is better than that in high frequency domain. (2) ASAR SWH is a little lower than SWH of buoy observations. The mean bias of SWH is -0.05m and the root mean square (RMS) error is 0.62m. (3) The mean wave period of ASAR is higher than that of buoy observations. The mean bias is 0.97s and RMS error is 1.42s.Finally, several ocean wave assimilation experiments under different situations and settings were run. The main conclusions are that (1) the difference of assimilation effects among traditional four isotropic background error correlation functions is not obvious and the key is still the choice of the correlation length scale. Different background error correlation functions under the same background error correlation length scale can be approximated by the one background error correlation function under different correlation length scales. (2) The optimal background error correlation length scale exists. For the auto-regressive background error correlation function, the assimilation effect is found to be best when the correlation length scale is assumed to be from 400km to 500km. And the modeled SWH RMS error reduced relatively 26% than that with no data assimilation in the best case. (3) The assimilation effect of anisotropic background error correlation functions is better than that of isotropic background error correlation functions. The effect of the experiment with the original anisotropic background error correlation function from wave model output statistics is the best, followed by the experiment with the ellipse-type anisotropic background error correlation function, and the last is the experiment with the traditional isotropic background error correlation function. The SWH RMS errors reduced relatively 26.4%, 24.5% and 23.4% respectively than that with no data assimilation in these three cases. (4) The assimilation experiment here also showed that assimilation effect of the wave spectra data OI was better than the only SWH OI.