Sea Technology

NOV 2013

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priate sources of ancillary data on the same time and space scales as the ASCAT winds. ASCAT, NARR, AVHRR-OI The ASCAT product used provided daily estimates of wind speed on a 25-kilometer spatial grid starting in January 2008. After exploring possible sources for surface sea and air temperature, relative humidity and barometric pressure, we found that the North American Regional Reanalysis (NARR) product from the National Centers for Environmental Prediction was largely compatible in spatial resolution with the ASCAT winds. We then evaluated the NARR felds. For this, note we limit the time frame of interest to 2009. We frst examined the NARR wind feld through a comparison with the buoy-based winds and found the annually averaged wind speed to be biased low relative to buoy-based wind observations by 1 to 2 meters per second. This 20 percent bias in speed can lead to a 50 percent error in wind power, given the cubic relationship between the two quantities. Because the NARR winds fail to correctly capture the 10-meter wind feld, we evaluate the NARR representation of the ancillary felds needed for the stability-based extrapolation scheme of the ASCAT winds. The annual mean difference between NARR and buoymeasured SST in 2009 for stations on the shelf was typically more than 2° C and exhibits a strong seasonal pattern of large differences (up to 10° C) in winter and small differences (less than 1° C) in summer, with NARR SST always greater. It is clear that the SST in NARR fails to capture low SST nearshore and north of Cape Hatteras during cooler months. A substitute SST product was sought. A number of daily products are now available that utilize optimal interpolation (OI) and a blend of infrared and microwave radiometers and insitu observations. All are global products but at varying spatial resolution. The 25-kilometer resolution National Climate Data Center optimally interpolated Advanced Very High Resolution Radiometer (AVHRR-OI) was found to best represent SST along the North Carolina coast relative to buoy-based observations. The other products, though available at fner spatial resolution, often fail to accurately capture the cooler nearshore waters in winter and north of Cape Hatteras. The annual mean difference between NARR and buoymeasured SAT in 2009 for stations on the shelf was typically about 1° C and exhibits a seasonal pattern of larger differences (up to 5° C) in winter and negligible differences in summer, with NARR SAT typically greater. The largest annual mean difference (2° C) was observed on the shelf north of Cape Hatteras at the same location as the largest mean difference in SST (3.4° C). The sea-air temperature difference (as SST-SAT) is a critical component of the MOS extrapolation scheme. A comparison of the difference using NARR and using buoy observations reveals that NARR overestimates the difference such that unstable conditions persist over the entire area of interest, even in winter. The buoy-based difference indicates stable conditions on the shelf north of Cape Hatteras and in the nearshore waters south of Cape Hatteras during winter and early spring. When the AHVRR-OI SST product is used in combination with the NARR SAT, the difference exhibits periods of stable conditions in the same locations as the buoy observations but is biased toward greater stability and weaker unstable conditions. November 2013 / st 37

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