1 / 23

AOS 101

AOS 101. Atmospheric Forces. April 8/10. Vector. Vectors have a magnitude and a direction (e.g. winds) Can be broken down into components: |a| = magnitude of vector a. v. u. Vectors can be added:. A + B = C. B. Resultant vector. Components can also be added: A = 3 i + 1 j

zavad
Download Presentation

AOS 101

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. AOS 101 Atmospheric Forces April 8/10

  2. Vector • Vectors have a magnitude and a direction (e.g. winds) • Can be broken down into components: • |a| = magnitude of vector a v u

  3. Vectors can be added: A+B=C B Resultant vector Components can also be added: A = 3i + 1j B = 3i + 4j C = A + B = (3+3)i + (1+4)j = 6i +5j A

  4. Newton’s Second Law • Forces and accelerations are also vectors having directions and magnitudes. • Newton’s second law: • An object will accelerate in the same direction as the force acts on it

  5. However, several forces will typically act on an object simultaneously • Hence, an object will accelerate in the direction of the net force where Fnet = F1 + F2 + F3 + F4 + ..., the sum of all force vectors.

  6. Zero Net Force • Sometimes all forces will completely cancel out and Fnet = 0 • In this situation, the forces are said to be balanced • If there is zero net force, there is zero acceleration meaning: • If the object is at rest, it will stay at rest • If the object is moving, it will continue to move at the same velocity.

  7. BOOK Free Body Diagram • Draw vectors representing each force mg mg Fr BOOK mg mg

  8. Four Forces in the Atmosphere • Pressure Gradient Force • Coriolis Force • Friction (surface) • Centrifugal (highly curved)

  9. 1. Pressure Gradient (PG) Force • Always points from high to low • Always perpendicular to pressure contours (isobars) LOW FPG P1 = 1000 hPa P2 = 1004 hPa HIGH

  10. ρa = density of air ~ 1 kg/m3 • |P2 – P1| = difference in pressure (abs. value) • D = distance between observations • Larger pressure difference, shorter distance (i.e. isobars closer together) results in larger force

  11. 2. Coriolis (cor) Force • Apparent force due to earth’s rotation • Always perpendicular and to the right (in the N.H.) of the wind Fcor

  12. |V| = magnitude of wind (in m/s) • f = 1.4 x 10-4 * sin(latitude) • Midlatitudes: f ~ 1.0 x 10-4 • An increase in wind speed results in a stronger force • In the absence of friction, will oppose PG force

  13. Geostrophic Balence • PG and Coriolis forces balance: • How does this happen? • Wind will increase until magnitude of Coriolis force (fv) is the same as PG force.

  14. Thus, balanced wind speed can be found by: • When geostrophically balenced, winds will be parallel to isobars, with low pressure to the left LOW FPG P1 = 1000 hPa P2 = 1004 hPa HIGH Fcor

  15. 3. Frictional (Fr) Force • Due to roughness of the earth’s surface • Only important within 1-2 km of the ground (planetary boundary layer) • Negligible in free atmosphere (above 2 km) • Always in the opposite direction of the wind FFr

  16. LOW FPG P1 = 1000 hPa • At the ground, winds will not be parallel to isobars • Winds will cross isobars towards lower pressure FFr P2 = 1004 hPa Fcor HIGH

  17. 4. Centrifugal (cen) Force • Outward “force” due to inertia while curving • Always perpendicular to wind vector, outward from circle L Fcen

  18. |V| = magnitude of wind (in m/s) • R = radius of circulation • Only important for strong winds (e.g. the jet stream) that are highly curved.

  19. Two possibilities Fcen FPG Fcen Fcor Fcor FPG H L

  20. Summary • Pressure Gradient • ALWAYS perpendicular to isobars • ALWAYS points from high to low pressure • Coriolis • ALWAYS perpendicular to wind vector • ALWAYS (in the NH) to the right of wind vector • Friction • ALWAYS in opposite direction of wind vector • Only important with 1-2 km of the ground • Centrifugal • ALWAYS points outward from circle, perpendicular to wind vector • Only important for strong winds and highly curved flows

  21. Application to Weather Maps • Weather maps will typically be on an isobaric surface (constant pressure sfc.) • Height of the surface plotted instead of pressure • “height” features are same as “pressure” features • High height area is same as high pressure area • Low height area is same as low pressure area • PGF acts from high heights to low heights

More Related