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 U.S. 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 §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:
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 m. The 1946 scale was based on the empirical U10 = 0.836 B3/2, where B = Beaufort Number and U10 is the wind speed in meters per second at a height of 10 m (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 4.1.
Observers on ships usually report weather observations, including Beaufort force, four times per day, at midnight, 6:00 AM, noon, and 6:00 PM Greenwich Mean Time (0000Z, 0600Z, 1200Z, and 1800Z). 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 4.5). Overall, the accuracy is around 10%.
The scatterometers on ERS-1 and ERS-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 Quickscat 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°. 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, or once every two days, the data must be used with numerical weather models to obtain 6-hourly wind maps required by some studies.
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° in direction over the range of 5–25 m/s.
Special Sensor Microwave SSM/I
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° (Atlas, Hoffman, and Bloom, 1993). Global, gridded data are available since July 1987 on a 2.5° longitude by 2.0° latitude grid every 6 hours (Atlas et al., 1996). 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
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 4.5). 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 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° for wind direction (Beardsley et al., 1997).
Calculation of Wind
Satellites, ships, and buoys measure winds at various locations and times of the day. If you wish to use the observations to calculate monthly averaged winds over the sea, then the observations can be averaged and gridded. If you wish to use wind data in numerical models of the ocean's currents, then the data will be less useful. You are faced with a very common problem: How to take all observations made in a six-hour period and determine the winds over the ocean on a fixed grid?
One source of gridded winds over the ocean is the surface analysis calculated by numerical weather models. The strategy used to produce the six-hourly gridded winds is called sequential estimation techniques or data assimilation. "Measurements are used to prepare initial conditions for the model, which is then integrated forward in time until further measurements are available. The model is thereupon re-initialized'' (Bennett, 1992: 67). The initial condition is called the analysis.
Usually, all available measurements are used in the analysis, including observations from weather stations on land, pressure and temperature reported by ships and buoys, winds from scatterometers in space, and data from meteorological satellites. The model interpolates the measurements to produce an analysis consistent with previous and present observations. Daley (1991) describes the techniques in considerable detail.
Surface Analysis from Numerical Weather Models
ECMWF calculations of winds have relatively good accuracy. Freilich and Dunbar (1999) estimated that the accuracy for wind speed at 10 meters is ± 1.5 m/s, and ± 18° for direction.
Accuracy in the southern hemisphere is probably as good as in the northern hemisphere because continents do not disrupt the flow as much as in the northern hemisphere, and because scatterometers give accurate positions of storms and fronts over the ocean.
The NOAA National Centers for Environmental Prediction and the US Navy also produces global analyses and forecasts every six hours.
Reanalyzed Output from Numerical Weather Models
The reanalyzed data are used to study oceanic and atmospheric processes in the past. Surface analyses issued every six hours from weather agencies are used only for problems that require up-to-date information. For example, if you are designing an offshore structure, you will probably use decades of reanalyzed data. If you are operating an offshore structure, you will watch the surface analysis and forecasts put out every six hours by meteorological agencies.
Sources of Reanalyzed Data Analyzed surface flux data are available from national meteorological centers operating numerical weather prediction models.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, email@example.com
All contents copyright © 2005 Robert H. Stewart,
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Updated on September 3, 2008