ABSTRACT

Since 1986, nine years of satellite-derived wave data havebeen accumulated and this data base will expand dramatically in the next two years as two more satellites are dded. Several researchers have begun using this data to develop extreme criteria. However, one potential problem with satellite data is their rather poor space-time resolution-satellites only revisit a site once every 10 to 30 days, and their tracks are separated by 100-200 km. With this coarse sampling, the satellite may miss storms since they have characteristic length and time scales as short as a few hours and tens of km. The purpose of this paper is to explore the impact of this under sampling on the calculated 100-yr wave height. We accomplish this by running Monte Carlo simulations of simplified but realistic storms sampled by a simulated satellite, site, and hind cast model. We study the sensitivity of the calculated 100-yr wave to variations in storm type, radius, and forward speed, number of satellites, satellite track, and satellite sampling region. We find the coefficient of variation (COV) of the 100-yr wave based on 20 yrs of satellite data is less than 10% in regions like the North Sea that are dominated by extra tropical storms, provided the satellite data is averaged over a 200-300 km region. This is roughly equivalent to the cove of agood hind cast model. For regions dominated by tropical storms like the Gulf of Mexico the cove for satellite or site-derived extremes is greater than 10%, although the 300-km averaged satellite data is probably acceptable for preliminary design. The situation improves in regions like the South China Sea where storm frequency is roughly four times the Gulf, In any case, some caution must be exercised in using satellite data near the coast where the satellite data itself may be less reliable and simple averaging may remove real spatial gradients.

INTRODUCTION

The feasibility of measuring ocean wave heights from a satellite altimeter was demonstrated by SEASAT in 1978, but it has only been in the last few years that enough altimeter wave data has been collected to enable the compilation of climatologists. Data from GEOSAT (1986-1990), TOPEX (1992-present) and ERS-1 (1991- present) are now available. Combined, these represent about 9 satellite-years of global coverage. The rate of data accumulation is going to increase substantially inthe near future with the launch in 1995 of ERS-2 followed in 1996 by GEOSAT 2. Potentially, this will give four concurrent satellites in operation in the next few years.

The ability of the satellite altimeter to accurately measure wave heights has been demonstrated by several previous studies. Dobson et al. (1987) and Glazman and Pilorz (1990) both compared GEOSAT wave heights to measurements made by the NOAA National Data Buoy Center and found the GEOSAT estimates biased O.4 m lower than the buoy measurements. Carter et al, (1992) m-analyzed this data and concluded that the buoy waves were 13% larger than the altimeterwaves.

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