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ATOC 4720 class33

ATOC 4720 class33. 1. The geostrophic wind: scale analysis 2. Effects of friction 3. The gradient wind. 1. The geostrophic winds: scale analysis. For unit mass, HORIZONTAL equation of motion:. Scale analysis: estimate the order of magnitude for each term, using the observed values.

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ATOC 4720 class33

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  1. ATOC 4720 class33 1. The geostrophic wind: scale analysis 2. Effects of friction 3. The gradient wind

  2. 1. The geostrophic winds: scale analysis For unit mass, HORIZONTAL equation of motion: Scale analysis: estimate the order of magnitude for each term, using the observed values. Very useful tool to identify the most important processes

  3. Observed scales: Velocity V: 10-30 m/s; Time t (1 day): S; /S (Mid Latitude) Above from atmospheric Planetary boundary layer (PBL), friction is small (a=0).

  4. Acceleration term: Coriolis term: The Coriolis term is one order of magnitude larger Than the acceleration term. In free atmosphere, friction is negligible. Therefore, to the lowest order: 0 0

  5. Geostrophic balance: PGF is balanced by CF:

  6. Horizontal equations of motion in component form In the above, we discussed the Vector form: In real situation, we often use component form:

  7. z y x (for simplicity, we look at local Cartesian coordinate:) Horizontal equation of motion in “x” direction, with a Unit vector “i”: Similarly, In “y”:

  8. Exercise: Perform scale analysis for each equation, to obtain geostrophy for each velocity component, using the observed scales provided earlier. Geostrophic winds: (P-vertical coordinate) (Z-vertical coordinate) Gestrophic winds:

  9. Northern Hemisphere (NH): y L H CF PGF P1 P2 P3 P4 P5 x

  10. V In PBL, friction is important. V cross the isobars! Frictional convergence toward the low pressure! Important!

  11. 3. The gradient wind

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