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Doppler scatterometry is a promising new technique for the simultaneous measurement of ocean surface currents and winds. These measurements have been recommended by the recent US NRC Decadal Review for NASA as being priority variables for the coming decade of Earth observations. In addition, currents and winds are useful for many applications, including assessing the operating conditions for oil platforms or tracking the dispersal of plastic or oil by surface currents and winds. While promising, Doppler scatterometry is relatively new and understanding the measurement characteristics is an important area of research. To this end, Chevron sponsored the deployment of DopplerScatt, a NASA/JPL Ka-band Doppler scatterometer, over instrumented sites located at the edge of a Gulf of Mexico Loop Current Eddy (LCE). In addition to in situ measurements, coincident synoptic maps of surface currents were collected by the Areté ROCIS instrument, an optical current measurement system. Here we report on the results of this experiment for both surface currents and winds. Surface current comparisons show that the Ka-band Current Geophysical Model Function (CGMF) needs to include wind drift currents, which could not be estimated with prior data sets. Once the CGMF is updated, ROCIS and DopplerScatt show good agreement for surface current speeds, but, at times, direction differences on the order of 10° can occur. Remote sensing optical and radar data agree better among themselves than with ADCP currents measured at 5 m depth, showing that remote sensing is sensitive to the the currents in top 1 m of the ocean. The LCE data provided a unique opportunity to study the effects of surface currents and stability conditions on scatterometer winds. We show that, like Ku-band, Ka-band estimates of winds are related to neutral winds (and wind stress) and are referenced relative to the moving frame provided by the current. This is useful for the study of air-sea interactions, but must be accounted for when using scatterometer winds for weather prediction.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractHurricane Ivan (September 2004) generated the highest waves ever measured or hindcast in the Gulf of Mexico. The implications of Ivan on deepwater Gulf of Mexico metocean design criteria are examined from three aspects: (1) whether there is an obvious deficiency in the ODGP wind and wave hindcast model, (2) whether Ivan generated "freak" conditions that break the NTS/API gust factor relationships and wind spectra characterizations or the Forristall wave height and crest height distributions, and (3) whether Ivan is, from a statistical standpoint, truly an "unexpected" event. To address the first two issues, wind and wave measurements collected on two industry platforms near Ivan's track are examined. Excellent comparisons are found between the industry measurements and the ODGP hindcast model, which further corroborate the findings of [2]. Analysis of the industry wind measurements suggests NTS/API gust factors are valid for Ivan, while the NTS/API spectra are found to agree reasonably well with the measured spectra except perhaps for a tendency of API to overestimate low-frequency energy. Measured wave heights and wave crests are found to fit the Forristall distributions quite well, providing no support for "freak" waves during Ivan. To address the last issue, the Gulf of Mexico hindcast record is examined. The maximum wave heights generated by Ivan correspond to roughly a 2,500-year local return period, assuming the extremal distribution before Ivan was correct. Adding Ivan to the existing hindcast database is found to raise 100-year H s values close to the storm track by 1.0 m or more. Through consideration of hurricane encounters in a finer spatial sense (160 km zones) rather than simply considering the entire Gulf as a single encounter zone, a strong argument can be made that the conditions generated by Ivan could reasonably be expected at a local level within the context of 105 years of hurricane experience throughout the Gulf. However, while Ivan may be described as a rare but not unexpected event at the local level, prudence dictates that winds and waves generated by Ivan be included in the hindcast database used for future Gulf of Mexico designs, as opposed to treating them as local outliers to existing extremal distributions and neglecting them.