How is Ocean Wind Measured?
Ocean wind is measured using either in situ (i.e., on site) or remote sensing (i.e., from a distance) instruments and techniques. In situ wind measurements may come from buoys, ships, or platforms. The most common instrument used for in situ wind measurements is the mechanical anemometer, which utilizes the wind’s resistance to propel a very small turbine to determine the wind speed; these anemometers also have a wind vane, which looks similar to the tail fin of an airplane, which helps the anemometer to always point into the direction of the wind, thus allowing the anemometer to measure both wind speed and direction.
Wind can also be measured remotely using both ground-based and airborne instruments. Ground-based Doppler radar can measure ocean wind using the inbound and outbound radial velocities of hydro meteors from storms within close proximity to the radar station; the range is typically limited to several hundred kilometers due to signal attenuation.
Airborne ocean wind measurements can take place using both active and passive microwave instruments; the microwave frequency band is preferred due to its ability to penetrate through clouds and precipitation and its sensitivity to the ocean surface roughness. The ocean surface responds quickly to the motion of the air above, which provides a distinct roughness pattern depending on the relative speed and direction of the wind with respect to the ocean surface. The roughness of the ocean surface provides a specific “brightness” which can only be here observed using passive microwave radiometers; with the right combination of specific microwave wavelengths and processing algorithms, the brightness of the ocean surface can be accurately translated to a near-surface wind speed.
Specific microwave wavelengths are sensitive to a feature known as Bragg scattering, which is a characteristic of centimeter-scale ocean surface waves known as capillary waves. Capillary waves are directly influenced by changes in near-surface winds, which enable specially tuned airborne radars to observe these changes. These airborne radars transmit microwave pulses of energy to the ocean surface, which immediately scatters a portion of the reflected energy back to the radar. Once the radar cross section is normalized, the near-surface wind speed can be computed as a function of the backscattered energy. In contrast to passive microwave radiometers, the active radar system can combine measurements from different azimuth angles to derive the approximate direction of the wind. Due to the dependence on the principal of Bragg scattering, these types of radars are specifically categorized as scatterometers.