Measurement of Waves
Because waves influence so many processes and operations at sea, many techniques have been invented for measuring waves. Here are a few of the more commonly used. Stewart (1980) gives a more complete description of wave measurement techniques, including methods for measuring the directional distribution of waves.
Sea State Estimated by Observers at Sea
This is perhaps the most common observation included in early tabulations of wave-heights. These are the significant wave-heights summarized in the U.S. Navy's Marine Climatological Atlas and other such reports printed before the age of satellites.
Accelerometer Mounted on Meteorological or Other Buoy
This is a less common measurement, although it is often used for measuring waves during short experiments at sea. For example, accelerometers on weather ships measured wave-height used by Pierson & Moskowitz and the waves shown in Figure 16.2. The most accurate measurements are made using an accelerometer stabilized by a gyro so the axis of the accelerometer is always vertical.
Double integration of vertical acceleration gives displacement. The double integration, however, amplifies low-frequency noise, leading to the low frequency signals seen in Figures 16.4 and 16.5. In addition, the buoy's heave is not sensitive to wavelengths less than the buoy's diameter, and buoys measure only waves having wavelengths greater than the diameter of the buoy. Overall, careful measurements are accurate to ?10% or better.
Gauges may be mounted on platforms or on the seafloor in shallow water. Many different types of sensors are used to measure the height of the wave or subsurface pressure which is related to wave-height. Sound, infrared beams, and radio waves can be used to determine the distance from the sensor to the sea surface provided the sensor can be mounted on a stable platform that does not interfere with the waves. Pressure gauges described in ?.8 can be used to measure the depth from the sea surface to the gauge. Arrays of bottom-mounted pressure gauges are useful for determining wave directions. Thus arrays are widely used just offshore of the surf zone to determine offshore wave directions.
Pressure gauge must be located within a quarter of a wavelength of the surface because wave-induced pressure fluctuations decrease exponentially with depth. Thus, both gauges and pressure sensors are restricted to shallow water or to large platforms on the continental shelf. Again, accuracy is ?10% or better.
The satellite altimeters used to measure surface geostrophic currents also measure wave-height. Altimeters were flown on Seasat in 1978, Geosat from 1985 to 1988, ERS-1 & 2 from 1991, Topex/Poseidon from 1992, and Jason from 2001. Altimeter data have been used to produce monthly mean maps of wave-heights and the variability of wave energy density in time and space. The next step, just begun, is to use altimeter observation with wave forecasting programs, to increase the accuracy of wave forecasts.
The altimeter technique works as follows. Radio pulse from a satellite altimeter reflect first from the wave crests, later from the wave troughs. The reflection stretches the altimeter pulse in time, and the stretching is measured and used to calculate wave-height (Figure 16.12). Accuracy is ?10%.
16.11 Shape of radio pulse received by the Seasat altimeter, showing the influence of ocean waves. The shape of the pulse is used to calculate significant wave-height. From Stewart (1985).
Synthetic Aperture Radars on Satellites These radars map the radar reflectivity of the sea surface with spatial resolution of 6-25m. Maps of reflectivity often show wave-like features related to the real waves on the sea surface. I say "wave-like" because there is not an exact one-to-one relationship between wave-height and image density. Some waves are clearly mapped, others less so. The maps, however, can be used to obtain additional information about waves, especially the spatial distribution of wave directions in shallow water (Vesecky and Stewart, 1982). Because the directional information can be calculated directly from the radar data without the need to calculate an image (Hasselmann, 1991), data from radars and altimeters on ERS-1 & 2 are being used to determine if the radar and altimeter observations can be used directly in wave forecast programs.