Jungang Yang

The validation of HY-2 altimeter measurements of a significant wave height based on buoy data

HY-2 has been launched by China on August 16, 2011 which assembles multi-microwave remote sensing payloads in a body and has the ability of monitoring ocean dynamic environments. The HY-2 satellite data need to be calibrated and validated before being put into use. Based on the in-situ buoys from the National Data Buoy Center (NDBC), Ku-band significant wave heights (SWH, hs) of HY-2 altimeter are validated. Eleven months of HY-2 altimeter Level 2 products data are chose from October 1, 2011 to August 29, 2012. Using NDBC 60 buoys yield 902 collocations for HY-2 by adopting collocation criteria of 30 min for temporal window and 50 km for a spatial window. An overall RMS difference of the SWH between HY-2 and buoy data is 0.297 m. A correlation coefficient between these is 0.964. An ordinary least squares (OLS) regression is performed with the buoy data as an independent variable and the altimeter data as a dependent variable. The regression equation of hs is hs(HY-2)=0.891×hs(NDBC)+0.022. In addition, 2016 collocations are matched with temporal window of 30min at the crossing points of HY-2 and Jason-2 orbits. RMS difference of Ku-band SWH between the two data sets is 0.452 m.

Numerical simulation and preliminary analysis on ocean waves during Typhoon Nesat in South China Sea and adjacent areas

Using the wave model WAVEWATCH III (WW3), we simulated the generation and propagation of typhoon waves in the South China Sea and adjacent areas during the passage of typhoon Nesat (2011). In the domain 100°–145°E and 0°–35°N, the model was forced by the cross-calibrated multi-platform (CCMP) wind fields of September 15 to October 5, 2011. We then validated the simulation results against wave radar data observed from an oil platform and altimeter data from the Jason-2 satellite. The simulated waves were characterized by five points along track using the Spectrum Integration Method (SIM) and the Spectrum Partitioning Method (SPM), by which wind sea and swell components of the 1D and 2D wave spectra are separated. There was reasonable agreement between the model results and observations, although the WW3 wave model may underestimate swell wave height. Significant wave heights are large along the typhoon track and are noticeably greater on the right of the track than on the left. Swells from the east are largely unable to enter the South China Sea because of the obstruction due to the Philippine Islands. During the initial stage and later period of the typhoon, swells at the five points were generated by the propagation of waves that were created by typhoons Haitang and Nalgae. Of the two methods, the 2D SPM method is more accurate than the 1D SIM which overestimates the separation frequency under low winds, but the SIM method is more convenient because it does not require wind speed and wave direction. When the typhoon left the area, the wind sea fractions decreased rapidly. Under similar wind conditions, the points located in the South China Sea are affected less than those points situated in the open sea because of the influence of the complex internal topography of the South China Sea. The results reveal the characteristic wind sea and swell features of the South China Sea and adjacent areas in response to typhoon Nesat, and provide a reference for swell forecasting and offshore structural designs.

An evaluation of input/dissipation terms in WAVEWATCH III using in situ and satellite significant wave height data in the South China Sea

A WAVEWATCH III version 3.14 (WW3) wave model is used to evaluate input/dissipation source term packages WAM3, WAM4 and TC96 considering the effect of atmospheric instability. The comparisons of a significant wave height acquired from the model with different packages have been performed based on wave observation radar and HY-2 altimetry significant wave height data through five experiments in the South China Sea domain spanning latitudes of 0°–35°N and longitudes of 100°–135°E. The sensitivity of the wind speed correction parameter in the TC96 package also has been analyzed. From the results, the model is unable to dissipate the wave energy efficiently during a swell propagation with either source packages. It is found that TC96 formulation with the “effective wind speed” strategy performs better than WAM3 and WAM4 formulations. The wind speed correction parameter in the TC96 source package is very sensitive and needs to be calibrated and selected before the WW3 model can be applied to a specific region.

SAR imagery in studying internal waves in the northern South China Sea

Synthetic Aperture Radar is a most useful instrument for internal wave observation. The northern of South China Sea (SCS) is one of interested sea areas to researchers, and the large amplitude and long-crested internal solitary waves in this area are often encountered in SAR images. The waves are generated by the interaction between the tidal current and shallow bottom topography in the Luzon Strait. The spatio-temporal distribution of internal solitary waves in the northern of SCS is studied by analyzing the ERS SAR images from 1995 to 2001. In the South China Sea internal wave occurs frequently from May to August, and infrequently from November to next March. The frequent occurring area of internal wave is located in north of coral reef Dongsha. Using an ERS-2 SAR image the amplitudes of internal waves in Xijiang oil field in Northern of South China Sea are estimated by one-dimensional model with good result compared with data in situ.

An evaluation of sea surface height assimilation using along-track and gridded products based on the Regional Ocean Modeling System (ROMS) and the four-dimensional variational data assimilation

Remote sensing products are significant in the data assimilation of an ocean model. Considering the resolution and space coverage of different remote sensing data, two types of sea surface height (SSH) product are employed in the assimilation, including the gridded products from AVISO and the original along-track observations used in the generation. To explore their impact on the assimilation results, an experiment focus on the South China Sea (SCS) is conducted based on the Regional Ocean Modeling System (ROMS) and the four-dimensional variational data assimilation (4DVAR) technology. The comparison with EN4 data set and Argo profile indicates that, the along-track SSH assimilation result presents to be more accurate than the gridded SSH assimilation, because some noises may have been introduced in the merging process. Moreover, the mesoscale eddy detection capability of the assimilation results is analyzed by a vector geometry–based algorithm. It is verified that, the assimilation of the gridded SSH shows superiority in describing the eddy’s characteristics, since the complete structure of the ocean surface has been reconstructed by the original data merging.

A statistical analysis of mesoscale eddies in the Bay of Bengal from 22–year altimetry data

Eddy properties in the Bay of Bengal are studied from 22 a archiving, validation and interpretation of satellite oceanographic (AVISO) data using a sea level anomaly (SLA)-based eddy identification. A geographical distribution and an eddy polarity, an eddy lifetime and propagation distances, eddy origins and terminations, eddy propagation directions and trajectories, eddy kinetic properties, the evolution of eddy properties, seasonal and interannual variabilities of eddy activities are analyzed in this area. Eddies exist principally in the western Bay of Bengal and most of them propagate westward. The polarity distribution of eddies shows cyclones prefer to occur in the northwest and south of the Bay of Bengal, while anticyclones mainly occur in the east of the bay. Five hundred and sixty-five cyclones and 389 anticyclones with the lifetime that exceeds 30 d are detected during the 22 a period, and there is a preference for the cyclones for all lifetime and propagation distances. The kinetic properties of all observed eddies show the average amplitude of the cyclones is larger than that of the anticyclones, whereas that is opposite for average radius, and their average velocities are basically the same. Moreover, the evolution of eddies properties reveals that the eddies with a long lifetime that exceeds 90 d have a significant double-stage feature of the former 50 d growth period and the dying period after 50 d. For the seasonal variability of the eddies, the cyclones occur more often in spring while the anticyclones occur more often in summer. The analysis of long-lived eddy seasonal distributions shows that there is the obvious seasonal variation of the eddy activities in the Bay of Bengal. The interannual variability of an eddy number shows an obvious negative correlation with the EKE variation.

An efficient parallel algorithm for ocean circulation numerical model based on irregular rectangle decomposition scheme

A parallel algorithm of circulation numerical model based on message passing interface (MPI) is developed using serialization and an irregular rectangle decomposition scheme. Neighboring point exchange strategy (NPES) is adopted to further enhance the computational efficiency. Two experiments are conducted on HP C7000 Blade System, the numerical results show that the parallel version with NPES (PVN) produces higher efficiency than the original parallel version (PV). The PVN achieves parallel efficiency in excess of 0.9 in the second experiment when the number of processors increases to 100, while the efficiency of PV decreases to 0.39 rapidly. The PVN of ocean circulation model is used in a fine-resolution regional simulation, which produces better results. The capability of universal implementation of this algorithm makes it applicable in many other ocean models potentially.

Comparison of water depth detection results with SAR and TM images in Taiwan Shoal

Water depth of the shallow water area can be reflected in SAR (Synthetic Aperture Radar) imagery and TM imagery. The water depth can be detected by SAR and by TM. The water depth SAR detection is based on the mathematical model describing the movement of tidal current and the interaction between the Radars microwave and the micro-scale waves in the sea surface. The water depth TM detection is based on the relation between the water depth and the gray value of TM image. In order to testify the capability of the water depth detection by SAR and by TM, one area in Taiwan Shoal, which locates in 118.55°~118.75°E, 23.0°~23.3°N, is taken as the study area for the water depth detection by SAR and by TM. The detection results shows that SAR image and TM image also can be used in the water depth detection, but the detection results by SAR is better than by TM and the largest water depth detected by SAR is larger than by TM. The water depth TM detection result based on several typical real water depths includes the large-scale information of underwater topography, and this can be used in the water depth SAR detection as the initial water depth. A new water depth detection method combing SAR image, TM image and several typical real water depths is concluded.

Field truthing experiment of ship detection by using SAR imagery

A ship field truthing experiment is designed and conducted in coast off Qingdao sea area. In the experiment a working boat equipped with Differential Global Position System receivers and anemometer navigating in experiment area to acquire vessels location, length, width, type and sea surface wind speed. The ship detector based on constant false alarm rate (CFAR) is applied in the simultaneously acquired ASAR image. Ship length is retrieved from ASAR image and compared with field data. Ships SAR imaging characteristic is analyzed based on field data and ASAR image.