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MET 61 Introduction to Meteorology - Lecture 10

MET 61 Introduction to Meteorology - Lecture 10. Atmospheric Dynamics Dr. Eugene Cordero Ahrens: Chapter 9 W&H: Chapter 7, pg 271-296 Class Outline: Principle forces in the atmosphere Pressure gradient Coriolis Geostrophic wind. Atmospheric forces.

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MET 61 Introduction to Meteorology - Lecture 10

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  1. MET 61 Introduction to Meteorology - Lecture 10 Atmospheric Dynamics Dr. Eugene Cordero Ahrens: Chapter 9 W&H: Chapter 7, pg 271-296 Class Outline: • Principle forces in the atmosphere • Pressure gradient • Coriolis • Geostrophic wind MET 61 Introduction to Meteorology

  2. Atmospheric forces • Fundamental Forces in the atmosphere • Pressure Gradient Force • Gravity • Rotation of the Earth • Friction MET 61 Introduction to Meteorology

  3. Pressure • Ultimately responsible for our weather MET 61 Introduction to Meteorology

  4. Determines the direction and speed of winds: Predominate force in atmospheric flows Can help explain general circulation of atmosphere. Horizontal Pressure Changes MET 61 Introduction to Meteorology

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  8. Pressure gradient: Dependent on spacing between isobars Dense or tight clustering of isobars - strong or large pressure gradient Weak clustering of isobars - weak pressure gradient Pressure Gradient Force Pressure gradient directed from high to low pressure MET 61 Introduction to Meteorology

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  10. Orient yourself (location, date and time) Identify what you are looking at Determine the interval of the field Reading a weather map MET 61 Introduction to Meteorology

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  13. C • A • B • At what local time is this map valid? • What fields are we looking at? • Indicate the direction of the pressure gradient force at points A-C. MET 61 Introduction to Meteorology

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  15. Atmospheric Thickness MET 61 Introduction to Meteorology

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  17. Hypsometric Equation • Combination of ideal gas law with hydrostatic balance. • Relates atmospheric thickness with average temperature. • Thickness of atmosphere relates to difference between two atmospheric layers; zt (m) = thickness between two pressure levels MET 61 Introduction to Meteorology

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  20. knots MET 61 Introduction to Meteorology

  21. The rotation of the Earth • Rockets, migrating birds, and large scale weather systems are all deflected due to the rotation of the Earth. • The Earth’s rotation causes both • Translational movement • Rotational movement • The Coriolis Force is the name of this rotational force that deflects motion. MET 61 Introduction to Meteorology

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  25. Coriolis Force • Affects direction, not speed of object • Maximum at the poles • Zero at the equator (only translational movement) • Fc=2W v sinj • W - omega - • Earth’s rotational rate • W =360 degrees/24 hours or • v - wind speed • j - latitude Calculate Coriolis force for wind moving at 10m/s 2 radians/86400 seconds=7.27x10-5 s-1 2(7.27x10-5 s-1)(10m/s)(sin37)=8.8e-4 m/s2 MET 61 Introduction to Meteorology

  26. Take home message: N. Hem - deflects air to the right S. Hem - deflects air to the left Relatively small acceleration, thus requires long periods of time to influence motion. Coriolis Force MET 61 Introduction to Meteorology

  27. Geostrophic balance • Geostrophic balance is balance between: Pressure gradient force and Coriolis force • Result: flow of air is parallel to isobars • friction is assumed to be zero MET 61 Introduction to Meteorology

  28. Geostrophic Wind example Northern Hemisphere L Pressure Gradient Force 1000 mb 1004 mb Geostrophic Wind 1008 mb Coriolis Force H MET 61 Introduction to Meteorology

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  33. A • B • C • Indicate with arrows the pressure gradient and Coriolis force at A, B and C. • Indicate the direction of the wind at each point. • Which point do you think the wind will be stronger?

  34. Geostrophic Wind • Assume friction is zero • Flow is parallel to isobars • Balance between pressure gradient and Coriolis force  - density, f - Coriolis parameter (=2 W sin j) Vg - geostrophic wind speed • d – distance between isobars •  p – pressure difference MET 61 Introduction to Meteorology

  35. http://www.met.sjsu.edu/weather/avn.html MET 61 Introduction to Meteorology

  36. Estimate the geostrophic wind speed for this situation MET 61 Introduction to Meteorology

  37. http://www.met.sjsu.edu/weather/avn.html MET 61 Introduction to Meteorology

  38. Geostrophic Wind with Friction Northern Hemisphere Friction decreases speed of wind, thus Coriolis force is weaker. L Pressure Gradient Force 1000 mb 1004 mb Geostrophic Wind Friction 1008 mb Coriolis Force H MET 61 Introduction to Meteorology

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  42. B • A MET 61 Introduction to Meteorology

  43. B • A MET 61 Introduction to Meteorology

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