4.4: Measurement of Wind

Wind at sea has been measured for centuries. Maury (1855) was the first to systematically collect and map wind reports. Recently, the US National Atmospheric and Oceanic Administration (NOAA) has collected, edited, and digitized millions of observations going back over a century. The resulting International Comprehensive Ocean, Atmosphere Data Set (ICOADS), discussed in Section 5.5, is widely used for studying atmospheric forcing of the ocean.

Our knowledge of winds at the sea surface come from many sources. Here are the more important, listed in a crude order of relative importance:

Beaufort Scale

By far the most common source of wind data up to 1991 have been reports of speed based on the Beaufort scale. The scale is based on features, such as foam coverage and wave shape, seen by an observer on a ship (table \(\PageIndex{1}\)).

The scale was originally proposed by Admiral Sir F. Beaufort in 1806 to give the force of the wind on a ship’s sails. It was adopted by the British Admiralty in 1838 and it soon came into general use. The International Meteorological Committee adopted the force scale for international use in 1874. In 1926 they adopted a revised scale giving the wind speed at a height of 6 meters corresponding to the Beaufort Number. The scale was revised again in 1946 to extend the scale to higher wind speeds and to give the equivalent wind speed at a height of 10 meters. The 1946 scale was based on the equation \(U_{10} = 0.836 B^{3/2}\), where \(B\) = Beaufort Number and \(U_{10}\) is the wind speed in meters per second at a height of 10 meters (List, 1966). More recently, various groups have revised the Beaufort scale by comparing Beaufort force with ship measurements of winds. Kent and Taylor (1997) compared the various revisions of the scale with winds measured by ships having anemometers at known heights. Their recommended values are given in table \(\PageIndex{1}\).

Table \(\PageIndex{1}\). Beaufort Wind Scale and State of the Sea.

Beaufort Number	Descriptive term	m/s	Appearance of the Sea
0	Calm	0	Sea like a mirror.
1	Light air	1.2	Ripples with appearance of scales; no foam crests.
2	Light breeze	2.8	Small wavelets; crests of glassy appearance, not breaking.
3	Gentle breeze	4.9	Large wavelets; crests begin to break; scattered whitecaps.
4	Moderate breeze	7.7	Small waves, becoming longer; numerous whitecaps.
5	Fresh breeze	10.5	Moderate waves, taking longer to form; many whitecaps; some spray.
6	Strong breeze	13.1	Large waves forming; whitecaps everywhere; more spray.
7	Near gale	15.8	Sea heaps up; white foam from breaking waves begins to be blown into streaks.
8	Gale	18.8	Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown in well-marked streaks.
9	Strong gale	22.1	High waves; sea begins to roll; dense streaks of foam; spray may reduce visibility.
10	Storm	25.9	Very high waves with overhanging crests; sea takes white appearance as foam is blown in very dense streaks; rolling is heavy and visibility reduced.
11	Violent storm	30.2	Exceptionally high waves; sea covered with white foam patches; visibility still more reduced.
12	Hurricane	35.2	Air is filled with foam; sea completely white with driving spray; visibility greatly reduced.

Observers on ships everywhere in the world usually report weather observations, including Beaufort force, at the same four times every day. The times are at 0000Z, 0600Z, 1200Z and 1800Z, where Z indicates Greenwich Mean Time. The reports are coded and reported by radio to national meteorological agencies. The biggest error in the reports is the sampling error. Ships are unevenly distributed over the ocean. They tend to avoid high latitudes in winter and hurricanes in summer, and few ships cross the southern hemisphere (figure \(\PageIndex{1}\)). Overall, the accuracy is around 10%.

Figure \(\PageIndex{1}\): Location of surface observations made from volunteer observing ships and reported to national meteorological agencies. From NOAA, National Ocean Service.

Scatterometers

Observations of winds at sea now come mostly from scatterometers on satellites (Liu, 2002). The scatterometer is a instrument very much like a radar that measures the scatter of centimeter-wavelength radio waves from small, centimeter-wavelength waves on the sea surface. The area of the sea covered by small waves, their amplitude, and their orientation depend on wind speed and direction. The scatterometer measures scatter from 2–4 directions, from which wind speed and direction are calculated.

The scatterometers on ERS-1 and -2 have made global measurements of winds from space since 1991. The NASA scatterometer on ADEOS measured winds for a six-month period beginning November 1996 and ending with the premature failure of the satellite. It was replaced by another scatterometer on QuikScat, launched on 19 June 1999. Quikscat views 93% of the ocean every 24 hr with a resolution of 25 km.

Freilich and Dunbar (1999) report that, overall, the NASA scatterometer on ADEOS measured wind speed with an accuracy of ±1.3 m/s. The error in wind direction was ±17\(^{\circ}\). Spatial resolution was 25 km. Data from QuikScat has an accuracy of ±1 m/s.

Because scatterometers view a specific oceanic area only once a day, the data must be used with numerical weather models to obtain 6-hourly wind maps required for some studies.

Windsat

Windsat is an experimental, polarimetric, microwave radiometer developed by the US Navy that measures the amount and polarization of microwave radiation emitted from the sea at angles between 50\(^{\circ}\) to 55\(^{\circ}\) relative to the vertical and at five radio frequencies. It was launched on 6 January 2003 on the Coriolis satellite. The received radio signal is a function of wind speed, sea-surface temperature, water vapor in the atmosphere, rain rate, and the amount of water in cloud drops. By observing several frequencies simultaneously, data from the instrument are used for calculating the surface wind speed and direction, sea-surface temperature, total precipitable water, integrated cloud liquid water, and rain rate over the ocean regardless of time of day or cloudiness.

Winds are calculated over most of the ocean on a 25-km grid once a day. Winds measured by Windsat have an accuracy of ±2 m/s in speed and ±20\(^{\circ}\) in direction over the range of 5–25 m/s.

Special Sensor Microwave SSM/I

Another satellite instrument that is used to measure wind speed is the Special-Sensor Microwave/Imager (SSM/I) carried since 1987 on the satellites of the U.S. Defense Meteorological Satellite Program in orbits similar to the noaa polar-orbiting meteorological satellites. The instrument measures the microwave radiation emitted from the sea at an angle near 60\(^{\circ}\) from the vertical. The radio signal is a function of wind speed, water vapor in the atmosphere, and the amount of water in cloud drops. By observing several frequencies simultaneously, data from the instrument are used for calculating the surface wind speed, water vapor, cloud water, and rain rate.

Winds measured by SSM/I have an accuracy of ±2 m/s in speed. When combined with ECMWF 1000 mb wind analyses, wind direction can be calculated with an accuracy of ±22\(^{\circ}\) (Atlas, Hoffman, and Bloom, 1993). Global, gridded data are available since July 1987 on a 0.25\(^{\circ}\) grid every 6 hours. But remember, the instrument views a specific oceanic area only once a day, and the gridded, 6-hourly maps have big gaps.

Anemometers on Ships

Satellite observations are supplemented by winds reported to meteorological agencies by observers reading anemometers on ships. The anemometer is read four times a day at the standard Greenwich times and reported via radio to meteorological agencies.

Again, the biggest error is the sampling error. Very few ships carry calibrated anemometers. Those that do tend to be commercial ships participating in the Volunteer Observing Ship program (figure \(\PageIndex{1}\)). These ships are met in port by scientists who check the instruments and replace them if necessary, and who collect the data measured at sea. The accuracy of wind measurements from these ships is about ±2 m/s.

Calibrated Anemometers on Weather Buoys

The most accurate measurements of winds at sea are made by calibrated anemometers on moored weather buoys. Unfortunately there are few such buoys, perhaps only a hundred scattered around the world. Some, such as Tropical Atmosphere Ocean (TAO) array in the tropical Pacific (figure \(14.4.1\)) provide data from remote areas rarely visited by ships, but most tend to be located just offshore of coastal areas. NOAA operates buoys offshore of the United States and the TAO array in the Pacific. Data from the coastal buoys are averaged for eight minutes before the hour, and the observations are transmitted to shore via satellite links.

The best accuracy of anemometers on buoys operated by the us National Data Buoy Center is the greater of ±1 m/s or 10% for wind speed and ±10\(^{\circ}\) for wind direction (Beardsley et al. 1997).