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.
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.
The satellite remote sensing of wind speed and direction (i.e., vectors) over the ocean began with the first satellite microwave scatterometer aboard SKYLAB (May 25, 1973 – June 22, 1973), which measured the reflected backscattered energy from the ocean surface.
These unprecedented measurements from space provided the foundation for scientists to develop a more practical understanding of the relationship between ocean surface backscatter, brightness temperatures, and near-surface wind speed. The success of SKYLAB led to the development of many more missions, including: Seasat, Nimbus-7, SSM/I, ERS (ESA), NSCAT, SeaWinds on QuikSCAT, SeaWinds on ADEOS-2, and ASCAT (ESA/EUMETSAT). During the 3 month operation of Seasat (July 7, 1978 – October 10, 1978), ocean-based measurements from ships, buoys, and platforms were utilized to help create the first geophysical model function (GMF) to convert both active and passive microwave measurements into ocean surface wind observations.
Decades of recalibration and tuning from the ever increasing number of in situ and remotely sensed wind observations have resulted in continuous improvements in GMFs. Consequently, there are many GMFs to choose from, and each GMF is tuned as a function of the specific measurement characteristics of each scatterometer or radiometer instrument.
PO.DAAC currently distributes ocean surface wind data for many of the previously noted missions in multiple data formats. QuikSCAT-derived products are among the most popular in terms of unique users. QuikSCAT is also NASA’s only remaining operational scatterometer mission that continues to provide daily ocean surface wind vector data over the globe.
The QuikSCAT mission, originally intended as a “quick recovery” 3-year mission, has celebrated its 10 year anniversary as of June 19, 2009. Unfortunately, the QuikSCAT antenna motor reached the end of its life on November 22, 2009. Consequently, no wind vector data is available beyond November 21, 2009.
