Validation of SeaWinds on QuikSCAT Winds with Research Vessel Observations
Wind observations from automated weather stations on research vessels were used to validate winds determined from observations of the SeaWinds Scatterometer on the QuikSCAT satellite. This validation was performed on the winds determined from the preiliminary geophysical model function. SeaWinds on QuikSCAT winds are shown to be tremendously accurate. The research vessel observations are clearly a practical source of the high quality observations required for such calibration/validation efforts.
One advantage of using observations from several research vessels is the sampling of a wide range of winds and sea states. Winds are sampled in many regions of the globe, insuring that the calibration of the remotely sensed winds is applicable to winds over any ocean. The results from each R/V were similar, indicating that there was no significant regional bias in the open ocean. The ship tracks and locations of collocations (ship and satellite observations) are shown for each of the research vessels used in this study:
R/V Atlantis: plot 1,
R/V Meteor,
R/V Oceanus: plot 1,
R/V Polarstern: plot 1, and
R/V Ron Brown: plot 1.
The ship data is quality controlled to insure the proper calculation of true winds, and to remove data that is likely to be in error due to flow distortion, erroneous data transmission, and other problems (Smith et al. 1999). The NSCAT and ship observations were found to be sufficiently accurate so that times when the ship was accelerating (which, due to the averaging technique commonly used, leads to errors in ship-based winds) were found to have statistically significant increases in the differences between ship and satellite winds. Therefore, the ship observations are screened for excessive acceleration.
Both the wind speed and the wind direction are validated for several SeaWinds on QuikSCAT products. The 'egg' product is the preliminary 25 km product, and egg(DIRTH) is a veriation with a differenct ambiguity removal scheme. The 'slice' product is the high-resolution product (made up of slices of the egg product).
The graphics in the following table are scatterplots of wind speed and direction from SeaWinds on QuikSCAT versus collocated winds from research vessels. The statistics on the directional graphics apply only to the directions for correctly selected ambiguities. RMS differences are calculated for each ship. These values are upper limits on SeaWinds accuracy. They include random variablity in the SeaWinds products as well as random variability in the research vessel observations, somewhat systematic errors due to flow distortion, and variability due to the displacement in space and time. The 'uncertainty' statistic listed on each figure is an attempt to remove some of these uncertainties. It assumes that total variability is partitioned evenly between R/V observations and SeaWinds observations. This uncertainty corresponds to one standard deviation in QuikSCAT values.
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| R/V Atlantis |
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| RSV Aurora Australis |
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| R/V Knorr |
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| R/V Melville |
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| R/V Meteor |
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| R/V Oceanus |
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| R/V Polarstern |
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| R/V Roger Revelle |
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| R/V Ron Brown |
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| R/V Thomas G. Thompson |
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| All Research Vessels |
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Vector Correlations are determined from the fraction of variance explained by a linear relationship, assuming that the uncertainty in ship observations is equal to the uncertainty in SeaWinds observations. This fraction can be retrieved by squaring a correlation coefficient. The scale ranges from -1 to +1, where +1 indicates a perfect match and zero indicates no correlation. These correlations have are based only on vectors with correctly selected ambiguities.
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| R/V Atlantis |
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| RSV Aurora Australis |
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| R/V Knorr |
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| R/V Melville |
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| R/V Meteor |
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| R/V Oceanus |
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| R/V Polarstern |
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| R/V Roger Revelle |
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| R/V Ron Brown |
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| R/V Thomas G. Thompson |
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| All Research Vessels |
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The accuracy of NSCAT speeds and directions provided a remarkable increase in the accuracy and quantity of wind observations over earth's bodies of water. It has been shown that much of the uncertainty in wind speeds can be accounted for in better considerations of the surface currents (personal communication, Peter Cornillon, 1998). SeaWinds on QuikSCAT provides continued excellent accuracy with improved coverage.
Acknowledgments.
We thank Barrie Walden (WHOI) for providing observations from the R/V Atlantis and R/V Oceanus, Dr. Volker Wagner (Marine Data Service, Deutscher Wetterdienst Geschdftsfeld Seeschiffahrt), Dennis Shields and Jonathan Shannahoff (NOAA Corps Operations), and Gert Koenig-Langlo (Alfred Wegener Institute for Polar and Marine Research). We also thank Shawn Smith and Jiraporn Whalley for quality-controlling the ship data. Funding for this project is from the OSU (NASA JPL) SeaWinds Project. COAPS receives its base funding from ONR's Secretary of Navy Grant to Dr. James J. O'Brien.
References
Bourassa, M. A., M. H. Freilich, D. M. Legler, W. T. Liu, and J. J. O'Brien, 1997: Wind observations from new satellite and research vessels agree. EOS Trans. of Amer. Geophys. Union, 597 & 602.
Smith, R. S, M. A. Bourassa, and R. J. Sharp, 1998: Establishing more truth in true winds. J. Atmos. Oceanic Technol., in press.
Last Updated 11 November, 1999