Chapter 4 - Atmospheric Influences

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:

Beaufort Scale
By far the most common source of wind data have been reports of speed based on the Beaufort scale. Even in 1990, 60% of winds reported from the North Atlantic used the Beaufort scale. The scale is based on features, such as foam coverage and wave shape, seen by an observer on a ship (Table 4.1).

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 m. The 1946 scale was based on the empirical U₁₀ = 0.836 B^3/2, 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%.

Figure 4.5 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 instruments on satellites. 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 depends 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 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.

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° to 55° 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° in direction over the range of 5–25 m/s.

Special Sensor Microwave SSM/I
Another satellite instrument that is widely used for measuring 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° from the vertical. The emission 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.

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
Satellite observations are supplemented by winds reported to meteorological agencies by observers reading anemometers on ships. The output of 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 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 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 14.14) 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° 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
Perhaps the most widely used weather model is that run by the European Centre for Medium-range Weather Forecasts ECMWF. It calculates a surface analysis including surface winds and heat fluxes (see Chapter 5) every six hours on a 1° × 1° grid from an explicit boundary-layer model. Calculated values are archived on a 2.5° grid. Thus the wind maps from the numerical weather models lack the detail seen in maps from scatterometer data, which have a 1/4° grid.

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
Surface analyses of weather over some regions have been produced for more than a hundred years, and over the whole earth since about 1950. Surface analyses calculated by numerical models of the atmospheric circulation have been available for decades. Throughout this period, the methods for calculating surface analyses have constantly changed as meteorologists worked to make ever more accurate forecasts. Fluxes calculated from the analyses are therefore not consistent in time. The changes can be larger than the interannual variability of the fluxes (White, 1996). To minimize this problem, meteorological agencies have taken all archived weather data and reanalyzed them using the best numerical models to produce a uniform, internally-consistent, surface analysis.

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.

1. The U.S. National Centers for Environmental Predictions, working with the National Center for Atmospheric Research have produced the NCEP/NCAR reanalysis based on 51 years of weather data from 1948 to 2005 using the 25 January 1995 version of their forecast model. The reanalysis period is being extended forward to include all date up to the present with about a three-day delay in producing data sets. The reanalysis uses surface and ship observations plus sounder data from satellites. Reanalysis products are available every six hours on a T62 grid having 192 × 94 grid points with a spatial resolution of 209 km and with 28 vertical levels. Important subsets of the reanalysis, including surface fluxes, are available on CD-ROM (Kalnay et al. 1996; Kistler et al. 2000).
2. The European Centre for Medium-range Weather Forecasts ECMWF has reanalyzed 45 years of weather data from September 1957 to August 2002 ERA-40 using their forecast model of 2001 (Uppala et al. 2005). The reanalysis uses mostly the same surface and ship data used by the NCEP/NCAR reanalysis plus data from the ERS-1 and ERS-2 satellites and SSM/I. The ERA-40 full-resolution products are available every six hours on a N80 grid having 160 × 320 grid points with a spatial resolution of 1.125° and with 60 vertical levels. The ERA-40 basic-resolution products are available every six hours with a spatial resolution of 2.5° and with 23 vertical levels. The reanalysis includes an ocean-wave model that calculates ocean wave heights and wave spectra every six hours on a 1.5° grid.

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