This paper addresses two questions critical for the successful real world application of the Cooperative Research on Extreme Seas and their Impact (CresT) Joint Industry Project (JIP) design methodology in harsh operating environments: (1) how accurately may very extreme sea states (VESS) be specified by modern numerical spectral wave models? About 20 storms in which VESS (defined as with significant wave height (HS) > 14 m) have been measured by various in-situ and satellite-mounted altimeters are hindcast and it is shown that when the meteorological forcing is accurately specified, a proven 3rd generation (3G) wave model provides skillful and unbiased specification of peak HS and by implication of the associated spectral properties. The second question addressed is: how do current 3G models behave when applied to even more extreme meteorological forcing than observed in the real storms studied? The same hindcast methodology is, therefore, applied to a population of synthetic hurricanes whose combinations of intensity and scale are predicted by deductive modeling studies of Gulf of Mexico hurricanes carried out following Hurricane Katrina (2004). The model results suggests that for a tropical cyclone to generate say peak HS > 20 m would require the peak wind intensity of a major hurricane (Category 3 or greater) combined with a larger size and faster translation speed than may be maintained by a tropical cyclone in tropical or subtropical settings. Large scale cyclonic and relatively rapidly translating storms with major hurricane force peak wind speeds indeed exist as a class of mid-latitude extratropical cyclones, dubbed “winter hurricanes”. Hundreds of such storms have been detected in global satellite altimeter data in virtually all major ocean basins. The peak sea states in the most extreme examples are also found to be simulated quite skillfully with the hindcast technology applied. The hindcast results are explored to infer the upper limit to the naturally occurring dynamic range of sea states in tropical and winter hurricanes.

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