Is the Cross Sea Dangerous?
Xiaoming Li(1), Susanne Lehner(1) and Thomas Bruns(2)
(1) German Aerospace Center (DLR), Oberpfaffenhofen, 82234, Wessling, Germany
(2) German Weather Service (DWD), Bernhard-Nocht-Straße 76, 20359 ,Hamburg, Germany
A sea state with two wave systems traveling at oblique angles is called cross sea. The crossing sea state has become of particular interest in the community of ocean waves. The conditions are quite common in the ocean and occur when a windsea and a swell, or two swell systems coexists. A study carried out by Toffoli et al. , based on data collected from 1995 to 1999 by Lloyd’s Marine Information service, has revealed that a large percentage of ship accidents occurred in crossing sea states. As the unique sensor SAR can provide two-dimensional sea surface information, independent of cloud and sunlight conditions, SAR is particularly powerful for cross sea detection, e.g. the cross sea case in the Draupner wave [Lehner et al., 2005] and cross sea state generated by two distant storms [Li et al., 2009].
In the present study, ERS-2 SAR, ENVISAT ASAR wave mode data acquired globally are used for cross sea detection. A cross sea that occurred in the South Pacific is analyzed for case study using ERS-2 SAR wave mode data and the swell dissipation is discussed as well. Many ship accidents happened when fast temporal variations of sea state parameters were observed close to the accident time. Also, for most of the cases, accidents occur during crossing sea conditions, or just after crossing sea conditions, when the swell and the wind sea are just “almost” aligned. During the scientific journeys of German research vessel “Polarstern”, a heavy rolling event caused by the cross sea state is recorded. The cross sea situation was observed by the shipboard instruments, as well as the ASAR wave mode data. In this case, observation of ASAR shows the natural variability of sea state, as well as the situation that windsea and swell are aligned which causes the heavy rolling of the R/V [Bruns et al., 2009]. Based on the rolling event of the ship with rolling angle up to 20°, we investigate the global distribution of cross sea using ASAR wave mode data. The distribution considers the number of wave systems; angle between different wave systems derived from ASAR wave mode data, and jointly with the significant wave height, mean wave period (and/or wavelength) retrieved using the CWAVE_ENV algorithm.
Cross seas also often occur in the coastal regions, e.g. generated by wave refraction and diffraction. In the present study, a cross sea case is analyzed using high resolution TerraSAR-X data acquired in the Terceira island situated in the North Atlantic. The methodology of spectral analysis is used for detecting spatial variations of coastal wave. Less angle spreading is observed in the cross sea state. Based on the developed XWAVE geophysical model function, SWH for different wave system is calculated to examine the cross sea state.