1 / 62

Physics of the Atmosphere Physik der Atmosphäre

Physics of the Atmosphere Physik der Atmosphäre. SS 2010 Ulrich Platt Institut f. Umweltphysik R. 424 Ulrich.Platt@iup.uni-heidelberg.de. Last Week. Simple energy balance calculations reveal a lot about our climate: presence of natural greenhouse effect

bandele
Download Presentation

Physics of the Atmosphere Physik der Atmosphäre

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. Physics of the Atmosphere Physik der Atmosphäre SS 2010 Ulrich Platt Institut f. Umweltphysik R. 424 Ulrich.Platt@iup.uni-heidelberg.de

  2. Last Week • Simple energy balance calculations reveal a lot about our climate: • presence of natural greenhouse effect • latitudinal and vertical structure of T and energy • radiative-convective equilibrium • Feedbacks are important and can only be addressed with numerical models • The global energy budget is in delicate balance, small changes have large effects

  3. Contents

  4. Outline for Today • Intro. to Atmospheric Dynamics • Global Circulation • The Intertropical Convergence Zone • Origin of High- and low pressure systems • Interhemispheric exchange

  5. Global Tropospheric Circulation Real circulation on a rotating Earth: Three convection cells between Equator and Pole Hypothetical circulation on a non-rotating Earth: Just one convection cell between Equator and Pole

  6. The Concept of Air Masses The term „Air Mass“ denotes an extended volume of air with unique properties (e.g. temperature, humidity, PV, vertical stability). Definition of an Air-Mass: - horizontal extent > 500 km - vertical extent > 1 km - horizontal temperature gradient < 1K / 100km Air Masses are generated if constant conditions prevail for sufficiently long times. Prerequisites are small pressure gradients and thus only slow motion of the air mass. Owing to these conditions, homogeneous air masses predominantly form in the tropics and in polar regions. In mid-latitudes temperature- and pressure gradients are usuallytoo strong to allow formation of distinct air masses.

  7. Air Mass Designation Origin, Path into Europa Polar Air T Island, northern ocean, northern Europe Sub Polar air PS Iceland-Greenland, North-east and Eastern Europe Moderate Zone air X Mid Atlantic, Central- and Eastern Europe Subtropical Air TS Azors, Mediterranean, Sout-Eastern Europe Tropical Air T Characterisation of Air Masses According to the airmass criteria X denotes not an air mass originating from mid-latitudes, but rather an air mass modified (heated, humidified) at mid-latitudes. According to their origin over land or sea, the air mass designators will be added an index c or m, respectively. In the mid-latitude region (sub-) Tropical air borders to (sub-) polar air, thus there are large horizontal temperature gradients, which can lead to strong therma winds.

  8. Mean Distribution of Air Mass Types a Summerb Winter From Malberg, 1994 Geh 1971

  9. Some Fluid Mechanics(see Lecture: Environmental Physics, MKEP4)

  10. The subject of hydrodynamics is the investigation and description of the motion of fluids, (i.e. liquids and gases). • We distinguish between two fundamentally different types of representation: • 1.) Description of flow as the temporal development of the local velocity field in a given point in space •  Eulerian Representation • 2.) Description of flow by the temporal development of the position of a particular element of mass of the fluid in space. •  Lagrangian Representation • In the Lagrangian Representation the velocity is then obtained from the temporal derivative: The Motion of the Atmosphere Leonhard Euler, 1707-1783 Joseph Louis Lagrange,1736-1813

  11. Point Mechanics vs. Continuum Mechanics Point mechanics Continuum mechanics small, but finit volume Point mass zero extension Lagrangian representation: Eulerianrepresentation: Temporal evolution of velocity at fixed points in space ?

  12. Conservative Quantities in Space and Time c=„concentration“ of some quantity

  13. The Navier – Stokes – Equations (Conservation of Momentum) The Navier-Stokes-Equations (Euler Equ.) describe the motion of a volume of fluid under the influence of the various (volume)-forces acting on it. 1) The Inertial Force The velocity of a fluid element can change for two reasons: 1) The velocity of the entire flow changes. 2) the element is transported to a different location in the velocity field where the velocity is different. In case 2. we obtain for e.g. the x-component: In 3 dimensions:

  14. The Lagrangian Derivative (Material Derivative) The Lagrangian derivative or Material derivative describes the acceleration of a fluid element due to change of position in the flow field, it is a consequence of the Eulerian standpoint. Comment 1: The expression should be interpreted as the component-wise application of the scalar operator on , this expression is known as ‘vector gradient’. Comment 2: The Material Derivative contains a non-linear term (proportional to v v), it is the dominant reason for the difficulties in solving the Navier – Stokes equations and for the unstable solutions that can arise.

  15. Pressure in the fluid at F(x) F(x+x) A x The Pressure Gradient Force Force on the left and right surface, respectively, of a (cubic) fluid volume: Net force on the fluid volume: Force per volume: In three dimensions:

  16. The Gravity Force Usually it is expressed in terms of the Geopotential: Thus the gravitational force density becomes:

  17. Inertial Forces due to Earth‘s Rotation Vector of angular velocity in the local system: Zentrifugal force: Maximum value of FZ at Equator: W2r = 0.034 m s-2, W2r/g ≈ 0.0035 << 1 Þ is taken up in g (or F)

  18. 4. The Coriolis force Coriolis-parameter f = 2  sin ,  = geographical latitude f > 0 on the northern hemisphere f < 0 on the southern hemisphere, i.e. points into earth In the local coordinate system we have:

  19. The Coriolis Force (2) vz = 0, x = y = 0 The flow is parallel to the earth’s surface  vz = 0 In practice only the component z =  sin  of perpendicular to the surface is of importance. With the “Coriolis Parameter” f = 2  sin  = 2 z the Coriolis force becomes:

  20. The Consequence of the Coriolis Force on Earth

  21. z vx dynamic kinematic viscosity 5. The Friction force „Shear stress-gradient force" Shear stress, due to velocity gradients in fluids leads to friction. Shear stress in the xy-plane xz is e.g. caused by velocity gradient of vx in z-direction with xz = shear stress. Note:  has the dimension of a pressure (force per unit area, e.g. N/m2) but in contrast to pressure the Force vector lies in the surface, thus .

  22. A vx(z) z FRx(z+dz) (z+dz) z+dz (z) y z FRx(z) x Friction Force cont. In a homogenous shearing stress field (linear velocity gradient) the forces on a volume element cancle. Only gradients in shearing stress give rise to a net force on a fluid volume element. Resulting force in e.g. x-direction due to gradient in xz in z-direction: In 3-D:

  23. The Navier-Stokes Equation Summing all terms we obtain the Navier-Stokes equation: Note: here the Navier-Stokes Eq. Is given in terms of force densities, sometimes accelerations are given:

  24. Magnitude of Terms in the Navier-Stokes Equation Very rough assumptions of the atmospheric spatial scales, etc.: Horizontal/vertical scale Lh 106 m Lv 104 m Horizontal/vertical velocity vx vy 10 m/s vz 0.1 m/s Horizontal/vertical pressure gradient (p)x 10-3 Pa/m (p)z 10 Pa/m Air density  1 kg/m3 Coriolis parameter f  10-4s-1 Lagrangian derivative force: Horizontal pressure gradient force Vertical pressure gradient force Gravitational force Coriolis force (molecular) Friction force See Pressure altitdue relationship

  25. The Reynolds Number: • Comparison of terms of the Navier-Stokes eq. (forces or accelerations) yields dimensionless „numbers“, e.g.: • Reynold number: Re > Rec ~ 1000: turbulent flow, Flow in the atmosphere is usually turbulent Example: v  1 m/s, L  1000 m, n10-5 m2/s ( 0.1 cm2/s) Re  108 >> Rec Osborne Reynolds, 1842-1912

  26. Examples for the Application of the Navier-Stokes Equation: 1) Acceleration of an air parcel by a horizontal pressure gradient. We neglect friction force, coriolis force, gravity and Lagrange acceleration: Force on a volume element dV: We retain: Its acceleration: Example: Horizontal pressure gradient: With the air density (at sea level)  1.29 Kg/m3 we obtain the acceleration: This is equivalent to about 14 m/s per hour. In other words the typical wind speed in the atmosphere would be reached within about one hour.

  27. Sample Weather Map(Europe, July 27, 2002)

  28. 1010 R 1020 1010 2) The Life time of high (low) pressure systems (simplifications as in example 1): Question: How long will it take an air-mass to cover a distance equivalent to the ‘Radius’ R of a high pressure system (see weather map)? Covered distance s at constant acceleration a: Witha from Example 1: Numerical example: R = 500 Km dp/dx = 510-3 N/m3 (see example 1) corresponding to a  410-3 m/s2 In about 4 hours the air would move from the centre of the high pressure system to the rim.

  29. In practice the lifetime of the system would be even shorter, since the ‚overpressure’ in the high pressure system would already be released, when the area A of the system had increased by: The corresponding change in radius of the high pressure system would be: i.e. about 25 Km Since t  s1/2 this would correspond to about 1 hour lifetime. However high pressure systems tend to live days or weeks!? Note: For low pressure systems entirely analogous considerations hold, just the direction of the pressure gradient (and thus the direction of the geo­strophic flow) has to be reversed.

  30. 3. Friction free motion of an air-parcel under the Influence of Coriolis and Pressure Gradient Force We consider a stationary flow (settling time from example 1) i.e. dv/dt=0: x – Component of v: With y = 0 (only the horizontal component of the coriolis force and thus z are considered) and f = 2 sin (Coriolis Parameter) we obtain: or y – Component of v: With x = 0 we obtain: is perpendicular to the horizontal pressure gradient. The friction free flow under the influence of pressure gradient force and Coriolis force is called Geostrophic Wind. or  The vector of the resulting wind

  31. 4. Purely inertial motion of an air-parcel We neglect friction force, gravity, pressure gradient forces, and Lagrangian derivative: Centrifugal force: Thus we obtain the radius of an inertial circle as: Numerical Example: At an wind speed of v0 = 5 m/s the inertial circle radii become:  = 90o: 35 Km  = 45o: 45 Km  = 15o: 130 Km  =  0o: infinite (at the Equator) The time for one rotation is: Example: 12 hours for  = 90o

  32. B C A 5. The Ekman – Spiral Close to the surface the influence of friction reduces the wind speed to levels well below the geostrophic speed vg. Since (Fc  v) the influence of the Coriolis force is reduced. Since the direction of the friction force is opposite to the direction of the wind the, close to the ground the wind direction will turn into the direction of the pressure gradient. A) Close to the ground the friction force is relatively large, v points approximately in the direction of pressure gradient force. B) In intermediate altitudes there is already a considerable angle between FP and v. C) In the geostrophic case (at several 100 m altitude) the friction force can be neglected and FC is anti parallel to FP. The air parcel moves at right angle to the pressure gradient force.

  33. The Ekman – Spiral (2) Schematic view of the orientation of the wind vector at different altitudes (Zg 1500 m) [Guyot, Physics of the Environment and Climate, WILEY, 1998]

  34. Wind direction free atmosphere Wind direction boundary layer Descending air "Leakage" only close to the ground because of the Ekman - Spiral H R 1020 hPa 1010 hPa 6. Life time of a high pressure system including Ekman – spiral, or: Why is there fair weather under high pressure? High pressure system, top view The descending air is heated due to adiabatic compression  Usually clouds dissolve  fair weather High pressure system, side view

  35. 7. Lifting of an air-parcel An air-parcel (volume: V) with a potential temperature  which deviates by  from the temperature of the surrounding air will deviate in density from its surroundings:  and thus experience the lifting force: F per unit volume: and the vertical acceleration: Numerical example:  = +1 K,  = 300 K  az = 3.310-2 ms-2 The air-parcel will cover the distance from the earth’s surface to the tropopause (assumed at 10 Km altitude) in the time t:

  36. 1) Navier-Stokes Eq. in Components In components, with Note: Anisotropy between horizontal and vertical flow, because of gravity and different lenght scales Þ frequently: v1 ~ v2 ~ vh >> v3 = vz

  37. 2) Geostrophic Flow With f = 2W·sinj (=Coriolis parameter): or:

  38. 1010 1020 1000 East-West North-South N-S O-W 3) Geostrophic Flow Geostrophic flow: Stationary flow without friction From the equilibrium pressure gradienten force = Coriolis force we obtain the geostrophic flow velocities: Geostrophic flows: perpendicular to the pressure gradient.  They do not reduce pressure differences! Rule of thumb for the direction of geostrophic flows: N-Hemisphere: Higher pressure to the right of the flow S-Hemisphere: The other way around

  39. 4) Definition of Vorticity • The curl of the wind vector field is only important in the horizontal since the vertical extent of the atmosphere is very small • The relative vorticity (relative vortex strength) is defined as the z-component of the curl of the wind vector field:with:

  40. r 5) Example: Vorticity of a Rigid Rotator • Velocity at distance r from the axis: • „Circulation“: • Thus, the vorticity is •  The vorticity of a rigid rotator is twice its angular velocity • Example: High pressure system, R = 500 km, v=10 ms-1  = v/r ≈ 10/5·105 = 2·10-5 s-1  = 2  = 4·10-5 s-1

  41. 6) The Absolute Vorticity • The (large scale) dynamics of fluids on Earth is described in a rotating coordinate system • The Earth is a rigid rotator with angular velocity Ω • Local vorticity in z-direction at latitude φ is given by the Coriolis parameter f = 2 Ω sin φ(see previously discussed vorticity of a rigid rotator) • Thus the absolute vorticity η of the wind field is the sum of the relative vorticity ζ (measured relative to the terrestrial coordinate system) and the Coriolis parameter f:  = Geographical latitude

  42. Frontal Zones Frontal zones form between two air masses of different temperature. In a narrow inner region temperature gradients can become very large (up to 5K/100km); this region is called a front. At a given altitude the horizontal ‘extent’ of a front is several 10 km. Since fronts are tilted (as will be detailed), their total horizontal extent can reach several 100 km. Within a front the isotherms are strongly tilted; there is a pronounced baroclinic structure. This is frequently called a hyperbaroclinic structure. Air masses, frontal zone, front al region, and front. Malberg, 1994

  43. cold air warm air warm air cold air motion motion Cold Front – Warm Front

  44. Tilt of Fronts – Margules‘ Formula • The tilt of a front is determined by two counteracting forces: • - Due to ist lower density the warmer air mass has the tendency to move above the colder air mass. • The Coriolis force generated by the dominating wind direction parallel to the front acts against the former force. • Taking into account that at either side of a stable front there has to be equal pressure. Thus, the change of pressure along an infinitesimal distance ds (see Figure) on the warm and cold side has to be equal: We assume a front oriented along the x-direction(being infinitely extended in x-direction).

  45. vw z warm v k ds cold y a x ds dz  dy Tilt of Fronts – Margules‘ Formula (1) Taking into account that at either side of a stable front there has to be equal pressure. Thus, the change of pressure along an infinitesimal distance ds (see Figure) on the warm and cold side has to be equal:

  46. Margules‘ Formula (2)

  47. Margules‘ Formula (3)

  48. Margules‘ Formula (4) Example: T = 10 K, v = 50 m/s For 50° geographical latitude (f = 11.2 10-5 s-1) follows: Thus the frontal zone will extend horizontally over 8 km/0.171 ≈ 470 km at 8000 m vertical extent. In reality there are also wind components perpendicular to the front, thus the front will move. In addition, friction at the surface will then play a role.

  49. Fronts in Motion Cold Front Warm Front Mean Flow Roedel 2000 Typical structures of cold fronts and warm fronts

  50. Air Masses, Cold Front, and Jet Stream during a Storm on Nov. 12, 1972

More Related