Atmospheric Influences on Physical Oceanography

1. Chapter 4 Atmospheric Influences Physical oceanography Instructor: Dr. Cheng-Chien Liu Department of Earth Sciences National Cheng Kung University Last updated: 24September 2003

2. Driving forces • Sun • Sunlight • Evaporation • Infrared emissions • Sensible heating of the sea by warm or cold winds • Occur at constant moisture content with dry-bulb temperature of air  • Geothermal heating  barely influence • Atmosphere • Uneven distribution of heat  winds • Evaporation  transfer heat  carry heat poleward • A coupled dynamic system (air-sea)

3. The Earth in space • The Earth’s orbit (Fig 4.1) • Near circular with small eccentricity 0.0168 • Aphelion • Perihelion • Inclination of rotating axis: 23.450 • The vernal equinox (21 March) • The autumnal equinox (21 September) • The tropic of Cancer • The tropic of Capricorn • Seasons • Averaged solar insolation  maximum in early January • If the insolation were rapidly and efficiently redistributed over Earth

4. Atmospheric wind systems • Fig 4.2 • Annual wind speed and sea level pressure for 1989 • Along the Equator: weak  • Tropics: trade winds  • Subtropics near 300: weak winds • 400 – 500: roaring forties  • 500 – 600: strong wind  • Reasons of the strength and direction of winds • Uneven distribution of solar heating • Uneven distribution of land masses • Circulation of winds in a vertical plane • The mean value of winds over the ocean: U10 = 7.4 m/s

5. Atmospheric wind systems (cont.) • Fig 4.3 • Distribution of winds in the atmosphere • The meridional cells in the atmosphere and the influence of Earth’s rotation on the winds • Cross section • Seasonal influence • Fig 4.4 • Asian monsoon • Winter  • Summer 

6. The planetary boundary layer • Atmospheric boundary layer • Definition • Thickness Zi • Few tens of meters  weak wind, cold water • A kilometer  strong wind, warm water • Exchange • Momentum • Heat • Lowest part ( 0.1 Zi) • Constant vertical fluxes of heat and momentum • Heat and momentum  log h • ∴measure U10

7. Measurement of wind • First: Maury (1847) • COADS • NOAA  combined data back over a century • For studying atmospheric forcing of the ocean

8. Measurement of wind (cont.) • Beaufort Scale • Admiral Sir F. Beaufort (1806) • Most common source of wind data (1990, 60%) • Based on features (foam coverage, wave shape) • Revision (1946): U10 = 0.836B3/2 • Revision (1997): Table 4.1 • Source of errors • Uneven distribution of ships (Fig 4.5) • Careless observers • Error coding • Accuracy > 10%

9. Measurement of wind (cont.) • Scatterometers • Principles • Measure the scatter of centimeter-wavelength radio waves from small, centimeter-wavelength waves on the sea surface • Small wave amplitudes = fn(wind speed, wind direction) • Limitation • Ambiguous direction • Can be removed by use of a few surface observations or numerical models • Spaceborne platform • ERS-1, ERS-2 (1991-) • ADEOS (six months from 1996) • Error • Accuracy: 1.3 m/s • For V > 6 m/s, fewer than 3% has ambiguity error • Spatial resolution: 25km

10. Measurement of wind (cont.) • Special Sensor Microwave/Imager SSM/I • US Defense Meteorological Satellite program (1987 –) • Principles • Microwave radiation emission = fn(wind speed, water vapor, water mass in cloud drops) • Limitation: ambiguous direction • Accuracy: • Speed: 2 m/s • Direction:  220 (when combined with ECMWF 1000 mb wind analyses) • Available data: • Global grid data: 2.50 longitude  2.00 latitude • Since July 1987, every six hours

11. Measurement of wind (cont.) • Anemometers on ships • Data • Read four times a day at GMT and report via radio • Error • Sparse in time and space • May never be calibrated • Instantaneous rather than averaged values • Coding error

12. Measurement of wind (cont.) • Calibrated anemometer on ships • Few ships • Volunteer Observation Ship program • Best accuracy:  2 m/s • Calibrated anemometers on weather buoys • Few buoys ( 100) • Tropical Atmosphere Ocean (TAO) array • Satellite links • Accuracy • Speed:  1 m/s or 10% • Direction: 100

13. Measurement of wind (cont.) • Surface analysis from Numerical General Models • Various observations  monthly averages (OK) • Various observations  numerical model (problem) • Data assimilation (sequential estimation techniques) • 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 reinitialized • ECMWF data • European Centre for Medium-range Weather Forecasts • Surface fluxes, wind stress, heat flux, … • Every six hours on a 10 10 grid • Accuracy • Wind speed: 1.5 m/s • Direction: 180

14. Measurement of wind (cont.) • Reanalyzed output from numerical general circulation models • Numerical models  continuous improved  calculated fluxes varied > inter-annual variability • Use the best numerical models available  reanalyze data  uniform , internally consistent, surface analysis • An example: offshore structure • Design  reanalyzed data • Operation  surface analysis and forecasts for every six hours

15. Measurement of wind (cont.) • Sources of reanalyzed data • NCEP/NCAR • ECMWF • NASA

16. Sampling problem in Scatterometry • Sampling error • Monthly maps  bands parallel to the satellite track • Uneven distribution of samples • Example: Fig 4.6 • A weak storm • Catch in A but miss in B  6 m/s difference • Sampling error 1 – 2 m/s in mid-latitudes

17. Wind stress • Wind stress • The vertical transfer of horizontal momentum • Our real interests • T = rCDU102 • r = 1.3 kg /m3: the density of air • CD: the drag coefficient • Fig 4.7: Correlation between T and U102 CD • Dissipation method • 1000CD = 0.29 + 3.1/U10 + 7.7/U102 (3  U10  6 m/s)1000CD = 0.60 + 0.070U10 (6  U10  26 m/s)

18. Important concepts • Driving source of energy  sunlight • Boundary layer • Speed  height • Fluxes of heat and momentum  constant • Measurements of wind • Output from atmospheric circulation models  the most useful source of global wind velocity • Calculation of T