What is Ocean Wind?

Ocean wind is defined as the motion of the atmosphere relative to the surface of the ocean. Typically ocean winds are measured very close to the ocean surface by buoys, platforms, and ships. The most common reference height for near-surface ocean wind measurements is 10 meters above sea level. More recently, the advancement of satellite remote sensing has enabled high-resolution near-surface ocean wind measurements from space using both passive and active instruments. Today, the combination of all available satellite wind measurements can provide global coverage over the ice-free oceans at multiple times per day.

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.

What are the Benefits of Measuring Ocean Wind?

Satellites have provided the unprecedented opportunity of observing the near-surface ocean wind on a near-global scale (i.e., with exceptions over ice and land). This has played a crucial role in improving climate and weather forecasts as well as helping to advance our understanding of the physics and dynamics of our oceans, which has ultimately improved our ability to assess the societal and economic impacts brought about by climate change and severe weather. More specifically, there have been a number of unforeseen benefits, including: marine biological/fisheries management, NAVY search and rescue operations, shipping route planning and preparation, tropical/extra tropical cyclone forecasting, and estimating wind energy potential to be harnessed by future ocean wind turbine farms. In addition, the applications of microwave radiometry and scatterometry have expanded into many intriguing areas, such as: monitoring unintentional oil spills, measuring sea-surface salinity concentration, sea-ice monitoring, soil moisture monitoring, and the monitoring of freeze/thaw cycles over land.