| Summary: | During the Antarctic summer season 2008/2009 the wave radar system WaMoS® II was installed onboard of the German research vessel „Polarstern”. The experiment was part of the German project DeMarine-Security, which is financed by the Minister of Economy and supervised by DLRAgency. It contributes to the ESA/EU-Initiative Global Monitoring for Environment and Security (GMES). Purpose was to collect quasi-in-situ data for the comparison with satellite-borne SAR and altimeter instruments (Envisat, TerraSAR-X, Jason). On March 7th in the central South Atlantic Ocean “Polarstern” was heading towards Punta Arenas against a rough cross sea. In the night, a sudden event of heavy rolling hit the vessel and lasted for a few minutes. Using WaMoS® II data, as well as ENVISAT and wave model data, we investigate the conditions under which the event occurred. It is shown that the rolling was caused by a “parametric” resonance when the period of encounter came close to one half of the vessels’s Eigen-period. We conclude that an onboard wave radar can be helpful in diagnosing and forecasting critical conditions
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