Atmospheric Winds

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Atmospheric Winds

Due to horizontal differences in air pressure, airflows from areas of high pressure to areas of low pressure. Horizontal movement of the air is called wind. The three types of winds are given below :

Planetary Winds : The winds blowing throughout the year from one latitude to another in response to latitudinal differences in air pressure are called planetary or prevailing winds. Trade winds, Westerlies and Polar winds are planetary winds.

Periodic Winds : Periodic winds change their direction periodically with the change in season, e.g. Monsoons, Land and Sea Breeze etc.

Local Wind : Local winds develop as a result of local differences in temperature and pressure. Loo is an example of local winds.

Hurricane : This is also known as tropical cyclone or tropical storm. This is a disturbance of about 650 km across, spinning around a central area of very low pressure, with 10 over 140 km per hour.

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Important Concepts of Winds

  1. Sunlight is the primary energy source driving the atmosphere and oceans.
  2. There is a boundary layer at the bottom of the atmosphere where wind speed decreases with height, and in which fluxes of heat and momentum are constant in the lower 10m – 20m.
  3. Wind is measured many different ways. The most common are from observations made at sea of the Beaufort force of the wind. Wind is measured from space using scatterometers and microwave radiometers. The output from atmospheric circulation models is perhaps the most useful source of global wind velocity.
  4. The flux of momentum from the atmosphere to the ocean, the wind stress, is calculated from wind speed using a drag coefficient.

Measurement of Winds

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. 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.

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.

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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.

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 ).

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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