Applications Of Surface Ocean Currents Measured By High Frequency Surface Wave Radar

High frequency surface wave radar (HFSWR), a new remote sensing tool with more than40a development, is capable of all-weather remote sensing of large area ocean surface hydrodynamic parameters with relatively high precision. It's considered to be one of the most efficient and economic tools for monitoring the ocean environment in the Exclusive Economic Zone. Nowadays, many countries around the world have been dedicated to the study of HFSWR. The currents measured by HFSWR is an average currents of a certain depth, the precision of ocean surface current is examined by many experiments and has been considered satisfying the requirements for routine marine measurements. The applications of surface currents measured by HFSWR play an important role in various national departments.The domestic study of HFSWR for environment monitoring began at1987. With the supports of the National High Technology Research and Development Program of China ('863Program') from Ninth Five-Year Plan to Eleventh Five-Year Plan, the Oceanography Laboratory of Wuhan University has developed a series of matured products called OSMAR (Ocean State Monitoring and Analyzing Radar). The research of this paper is supported by the Twelfth Five-Year Plan of'863Program' under contract No.2012AA091701. Surface ocean currents measured in the open sea area to the east of Zhoushan Islands and the southwestern area of Taiwan Straits are used to carry out the research. The details of the research are presented below:1) Based on the quasi-harmonic analysis of11days'vector ocean currents obtained from two high frequency surface wave radars located at Zhujiajian Island and Shengshan Island, the spatial distribution characteristics of surface tidal currents in the open sea area to the east of the Zhoushan Islands of Zhejiang Province, China are studied. The following conclusions are drawn from the analysis:the tidal current pattern in the open sea area to the east of Zhoushan Islands is primarily regular semidiurnal, which is significantly affected by the shallow water constituents. The directions of the major axes of tidal current ellipses of M2lie approximately in the NW-SE direction. With the increasing of distance away from the coast, the directions of the tidal current ellipses gradually shift toward the E-W direction. The tidal currents are mainly reversing currents. The spatial distribution of probable maximum current velocities decreases gradually from northeast to southwest which is basically in accordance with the spatial distribution of measured maximum current velocities. The residual currents near the coast are larger than those far away from the coast. The directions of the residual currents are basically north by east, and the angle to the due north increases gradually with the increasing distance away from the coast. The topography shows a certain impact on the spatial distribution of shallow water constituents, the rotation of tidal currents, the probable maximum currents and the residual currents.2) The spatial distribution characteristics of surface tidal currents in the southwestern Taiwan Straits is studied by applying the Quasi-Harmonic analysis to the3days'vector currents obtained by two high frequency surface wave radar (HFSWR) systems located at the Fujian province. The analysis shows that the tidal current pattern at the southwestern Taiwan Straits is primarily regular semi-diurnal and the shallow water constituents show a significant impact on this. The spatial distribution of tidal current ellipses of M2is probably affected by the interaction of two different tide waves, one from the northern mouth of Taiwan Straits and another from the Bashi Channel. The directions of the major axes of the tidal current ellipses of M2coincide roughly with the axis of the Taiwan Straits. The spatial distribution of the magnitude of the probable maximum current velocity (PMCV) increases gradually from northeast to southwest which is in accordance with the spatial distribution of the magnitude of the measured maximum current velocity (MMCV). The directions of the residual currents are in accordance with the direction of the prevailing monsoon wind at the Taiwan Straits and the direction of Taiwan warm current during the summer. The bathymetry also shows a significant effect on the spatial distribution characteristics of the tidal currents.3) The spatial distribution characteristics of surface tidal currents in the southwestern Taiwan Straits is studied by applying the Harmonic analysis to the2months vector currents obtained by two high frequency surface wave radar (HFSWR) systems located at the Fujian province. The characters of tidal currents at three selected points?the spatial distribution of tidal current ellipses of M2tidal constituent?the spatial distribution of maximum current speed and the spatial distribution of residual currents are given respectively. The analysis show that tidal current pattern at the southwestern Taiwan Straits is primarily regular semi-diurnal. The directions of tidal current ellipse of M2tidal constituent lie approximately in the NE-SW direction and the directions of the major axes of the tidal current ellipses of M2coincide roughly with the axis of the Taiwan Straits. With the increasing of distance away from the coast, the directions of the tidal current ellipses gradually shift toward the N-S direction. The tidal currents are mainly reversing currents. The spatial distribution of maximum current speed is greatly affected by the northeast monsoons, and the range of variations are larger, especially at the Taiwan Bank, where the maximum current speed reaching150cm/s. The residual currents are basically heading for the S-W direction, which is also affected by the northeast monsoons.4) The response of ocean surface currents to the severe tropical storm Lionrock is investigated using six days' currents data measured by high frequency surface wave radar (HFSWR) in the southwestern Taiwan Straits. Analysis shows that significant changes of ocean surface currents are captured by HFSWR during the typhoon period. A larger expansion of the first order spectra is observed during the typhoon period, and the SNR of the first order spectra increases about2-10dB. The spatial distribution of maximum currents shows that an increment up to about100cm/s is induced by the Lionrock at the southern and northeastern area, and the increment is nearly symmetrical about the track of Lionrock. The increments of the maximum currents are larger at the right of the track of Lionrock than that of the left. Nearly doubled vorticity and divergence values are induced by Lionrock. Strong anti-clockwise rotated vorticity values are observed during the typhoon period. And greatly strengthened positive divergence is also captured at the central observation area of the typhoon period.5) The complex Empirical Orthogonal Function (complex EOF) analysis is performed to three days'ocean surface currents measured by two High Frequency Surface Wave Radar (HFSWR) systems located at Fujian Province. The results show that the first three modes explain91%,4%and1.4%of the variance, respectively. The cumulative variance reaches96%, which are the main modes. The spatial and temporal distribution characteristics of mode1show that mode1mainly contains the information of semidiurnal tidal currents and reflects the information of water depth to a certain extent. Mode2mainly reveals the information of wind-driven current and shows the rotary characteristics of ocean surface currents. Mode3mainly reflects the information of upwelling currents.6) The complex EOF analysis of ocean surface currents to the Lionrock is investigated using six days'currents data measured by OSMAR in the southwestern Taiwan Straits. The analysis shows that the first three EOF modes account for89.51%of the variance and are judged to contain the only statistically significant results, and are the main modes. The analysis has also provided insight into the dynamics and spatial structure of the ocean surface currents. Mode1, which alone accounts for81.01%of the variance, shows a clear correlation to the tidal currents. Mode2, which accounts for6.27%of the variance, mainly reflects the divergence information of surface ocean currents, and the diverging flows are approximately symmetrical about the track of Lionrock and the magnitude of the temporal coefficients at the typhoon period is much larger than that at pre-typhoon and post-typhoon periods. Mode3, which accounts only2.22%of the variance, mainly reflects the vorticity information induced by Lionrock.