AOSS 401, Fall 2013 Lecture 2 Forces September 5 , 2013

1. AOSS 401, Fall 2013Lecture 2ForcesSeptember 5, 2013 Richard B. Rood (Room 2525, SRB) rbrood@umich.edu 734-647-3530 Cell: 301-526-8572

2. Class News • Ctools site (AOSS 401 001 F13) • Syllabus • Lectures • Homework (and solutions) • New Postings to site • Homework 1: Due September 12 • Read Greene et al. paper: Discussion Tuesday

3. Weather • National Weather Service • Model forecasts: • Weather Underground • Model forecasts: • NCAR Research Applications Program

4. Outline • Conservation of Momentum • Forces • Pressure gradient force • Viscous force • Gravitational Force • Coriolis Force • Centrifugal Force Should be review. So we are going fast. You have the power to slow me down.

5. Back to Basics:Newton’s Laws of Motion • Law 1: Bodies in motion remain in motion with the same velocity, and bodies at rest remain at rest, unless acted upon by unbalanced forces. • Law 2: The rate of change of momentum of a body with time is equal to the vector sum of all forces acting upon the body and is the same direction. • Law 3: For every action (force) there is and equal and opposite reaction.

6. Newton’s Law of Motion F = ma Force = mass x acceleration In general we will work with force per unit mass; hence, a = F/m And with the definition of acceleration Boldwill represent vectors.

7. Newton’s Law of Motion Which is the vector form of the momentum equation. (Conservation of momentum)

8. What are the forces? • Pressure gradient force • Gravitational force • Viscous force • Apparent forces • Can you think of other classical forces and would they be important in the Earth’s atmosphere? • Total Force is the sum of all of these forces.

9. Newton’s Law of Motion Where i represents the different types of forces.

10. How do we express the forces? • In general, we assume the existence of an idealized parcel or “particle” of fluid. • We calculate the forces on this idealized parcel. • We take the limit of this parcel being infinitesimally small. • This yields a continuous, as opposed to discrete, expression of the force. • Use the concept of the continuum to extend this notion to the entire fluid domain.

11. An intrinsic assumption • There is an equation of state that describes the thermodynamic properties of the fluid, the air.

12. Surface forces • Pressure gradient force and the viscous force are examples of a surface force. • Surface forces are proportional to the area of the surface of our particle of atmosphere. • Surface forces are independent of the mass of the particle of atmosphere. • They depend on characteristics of the particle of atmosphere; characteristics of the flow.

13. z k y j x i A particle of atmosphere r≡ density = mass per unit volume (DV) DV = DxDyDz m = rDxDyDz ------------------------------------- p ≡ pressure = force per unit area acting on the particle of atmosphere Dz Dy Dx

14. Outline • Conservation of Momentum • Forces • Pressure gradient force • Viscous force • Gravitational Force • Coriolis Force • Centrifugal Force Should be review. So we are going fast. You have the power to slow me down.

15. Pressure gradient force (1) (x0, y0, z0) p0 = pressure at (x0, y0, z0) Dz . p = p0 + (∂p/∂x)Dx/2 + higher order terms Dy Dx x axis

16. Taylor series approximation. This will be used over and over. Pressure gradient force (2) p = p0 - (∂p/∂x)Dx/2 + higher order terms Dz . p = p0 + (∂p/∂x)Dx/2 + higher order terms Dy Dx x axis

17. Pressure gradient force (3)(ignore higher order terms) FBx = (p0 - (∂p/∂x)Dx/2) (DyDz) Dz . FAx = - (p0 + (∂p/∂x)Dx/2) (DyDz) A B Dy Dx x axis

18. Pressure gradient force (4) Total x force Fx = FBx + FAx = (p0 - (∂p/∂x)Dx/2) (DyDz) - (p0 + (∂p/∂x)Dx/2) (DyDz) = - (∂p/∂x)(DxDyDz) We want force per unit mass Fx/m = - 1/r (∂p/∂x)

19. z k y j x i Vector pressure gradient force

20. Outline • Conservation of Momentum • Forces • Pressure gradient force • Viscous force • Gravitational Force • Coriolis Force • Centrifugal Force Should be review. So we are going fast. You have the power to slow me down.

21. Viscous force (1) • There is in a fluid friction, which resists the flow. It is dissipative, and if the fluid is not otherwise forced, it will slow the fluid and bring it to rest. Away from boundaries in the atmosphere this frictional force is often small, and it is often ignored. (We will revisit this as we learn more.) • Close to the boundaries, however, we have to consider friction.

22. Viscous force (2) velocity ≡ u m/sec Ocean Land Biosphere velocity must be 0 at surface

23. Viscous force (3) velocity ≡ u m/sec Ocean Land Biosphere Velocity is zero at the surface; hence, there is some velocity profile.

24. Viscous force (4)(How do we think about this?) The drag on the moving plate is the same as the force required to keep the plate moving. It is proportional to the area (A), proportional to the velocity of the plate, and inversely proportional to the distance between the plates; hence, u(h) = u0 F = μAu0/h h u(z) u(0) = 0 Proportional usually means we assume linear relationship. This is a model based on observation, and it is an approximation. This is often said to be “Newtonian.” The constant of proportionality assumes some physical units. What are they?

25. Viscous force (5) n ≡ m/r = kinematic viscosity coefficient Where Laplacian is operating on velocity vector≡ u = (u, v, w)

26. Surface forces • Pressure gradient force and the viscous force are examples of a surface force. • Surface forces are proportional to the area of the surface of our particle of atmosphere. • Surface forces are independent of the mass of the particle of atmosphere. • They depend on characteristics of the particle of atmosphere; characteristics of the flow.

27. Highs and Lows Motion initiated by pressure gradient Opposed by viscosity

28. Pressure gradients • What causes pressure gradients?

29. Outline • Conservation of Momentum • Forces • Pressure gradient force • Viscous force • Gravitational Force • Coriolis Force • Centrifugal Force Should be review. So we are going fast. You have the power to slow me down.

30. Body forces • Body forces act on the center of mass of the parcel of fluid. • Magnitude of the force is proportional to the mass of the parcel. • The body force of interest to dynamic meteorology is gravity.

31. Newton’s Law of Gravitation Newton’s Law of Gravitation: The force between any two particles having masses m1 and m2 separated by distance r is an attraction acting along the line joining the particles and has the magnitude proportional to G, the universal gravitation constant.

32. Gravitational Force

33. Gravitational force for dynamic meteorology Newton’s Law of Gravitation: M = mass of Earth m = mass of air parcel r = distance from center (of mass) of Earth directed down, towards Earth, hence - sign

34. a mg0 a2 =g0 a Gravity for Earth

35. Adaptation to dynamical meteorology a is radius of the Earth z is height above the Earth’s surface Can we ignore z, the height above the surface? How would you make that argument?

36. Some basics of the atmosphere Troposphere ------------------ ~ 2 Mountain Troposphere ------------------ ~ 1.6 x 10-3 Earth radius Troposphere: depth ~ 1.0 x 104 m This scale analysis tells us that the troposphere is thin relative to the size of the Earth and that mountains extend half way through the troposphere.

37. Gravitational force per unit mass

38. Our momentum equation + other forces Now using the text’s convention that the velocity isu= (u, v, w).

39. Outline • Conservation of Momentum • Forces • Pressure gradient force • Viscous force • Gravitational Force • Coriolis Force • Centrifugal Force Should be review. So we are going fast. You have the power to slow me down.

40. Apparent forces

41. Back to Basics:Newton’s Laws of Motion • Law 1: Bodies in motion remain in motion with the same velocity, and bodies at rest remain at rest, unless acted upon by unbalanced forces. • Law 2: The rate of change of momentum of a body with time is equal to the vector sum of all forces acting upon the body and is the same direction. • Law 3: For every action (force) there is and equal and opposite reaction.

42. Back to basics:A couple of definitions • Newton’s laws assume we have an “inertial” coordinate system; that is, and absolute frame of reference – fixed, absolutely, in space. • Velocity is the change in position of a particle (or parcel). It is a vector and can vary either by a change in magnitude (speed) or direction.

43. Apparent forces:A mathematical approach • Non-inertial, non-absolute coordinate system

44. z y x z’ y’ x’ Two coordinate systems Can describe the velocity and forces (acceleration) in either coordinate system.

45. One coordinate system related to another by:

46. Velocity (x’ direction) So we have the velocity relative to the coordinate system and the velocity of one coordinate system relative to the other. This velocity of one coordinate system relative to the other leads to apparent forces. They are real, observable forces to the observer in the moving coordinate system.

47. Two coordinate systems z’ axis is the same as z, and there is rotation of the x’ and y’ axis z’ z y’ y x x’

48. One coordinate system related to another by: T is time needed to complete rotation.

49. Acceleration (force) in rotating coordinate system The apparent forces that are proportional to rotation and the velocities in the inertial system (x, y, z) are called the Coriolis forces. The apparent forces that are proportional to the square of the rotation and position are called centrifugal forces.

50. Apparent forces:A physical approach • Coriolis Force • http://climateknowledge.org/figures/AOSS401_coriolis.mov