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# Lecture11 oct16-bb

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### Lecture11 oct16-bb

1. 1. 1
2. 2. Geostrophic Wind when the isobars are straight, parallel lines, and the only two forces acting on a parcel are the PGF and the CF, then the wind is called geostrophic wind L 475 mb • PGF and CF are equal in magnitude and opposite in direction • Geostrophic winds are always parallel to the isobars PGF CF Wind 500 mb H 500 mb Surface 2
3. 3. Strength of Geostrophic Wind geostrophic conditions are that PGF and CF are equal in magnitude and opposite in direction P G F  C F  2  V g sin  Vg  PGF L 475 mb PGF CF 2  sin  Wind 500 mb H strength of geostrophic wind is determined by PGF not by CF, why? 3
4. 4. Vg  PGF 2  sin  The Coriolis force can only change a parcel's direction, it CAN NOT affect its speed. 4
5. 5. Centripetal force and gradient wind 5
6. 6. Gradient Wind counter lines are relatively straight Counter lines are curvature Geostrophic Wind 6
7. 7. Gradient Wind Centripetal force if you attach a string to a ball and swing it in a circular manner, then the force required to keep the ball moving in a circular path is called centripetal force magnitude of the centripetal force is centripital force  V 2 r V=speed of a parcel (object) r= distance from the center direction of the centripetal force is directed inward, towards the axis of 7 rotation V
8. 8. recall geostrophic wind L 475 mb PGF Wind CF 500 mb H 8
9. 9. Gradient Wind Centripetal force Gradient wind is produced by the balance between PGF, CF and centripetal force PGF PGF PGF PGF 9
10. 10. Free troposphere and Boundary layer 10
11. 11. above 850 mb level, the flow is either geostrophic wind or gradient wind(free troposphere) near the surface about 1-2 km (boundary layer), we must include the effect of friction and therefore, the flow is no longer geostrophic or gradient balance 11
12. 12. Q: As a result, which force becomes smaller, the PGF or the CF? the atmosphere above the boundary layer is called free troposphere friction has a significant impact on wind in boundary layer (1-2 km) 12
13. 13. Four forces in the atmosphere 1. Pressure gradient force 2. Coriolis force  Phigh  Plow distance  2  V sin( ) 3. Centripetal force  V 2 r 4. Friction   kV which force is not caused by wind? which two forces do not affect wind speed? Coriolis force and centripetal force 13 PGF
14. 14. at the center of a surface low, the air converges, and then must rise H L at the center of a surface high, the air diverges, and must come from aloft due to sinking motion 14
15. 15. Why temperature is the most important to understand weather?
16. 16. What is temperature definition? Why earth’s radiation is infrared while solar radiation is short-wave? Wien’s Law  m ax  3000 T (K ) ( m)
17. 17. The earth’s radiation energy is determined by what? Stefan-Boltzmann Law E  T 4
18. 18. The capacity of air for holding water vapor is determined by what?
19. 19. Saturation vapor pressure (mb) Temperature determines the capacity of holding water vapor in the air 120 110 100 90 80 70 60 50 40 30 20 10 0 * H 2O P This curve indicates how much water vapor can be held in the air at a given temperature 17.27 T PH 2 O (T )  0.6108 e T  273.3 * T=44 oC T=10 oC T=30 oC 0 10 20 30 40 Temperature (oC) 50
20. 20. Thermal circulation is generated by pressure gradient Pressure gradient is produced by different heating
21. 21. Rule Cool surface  air sinking  high surface pressure Warm surface air rising  low surface pressure Wind is blowing from high pressure to low pressure Where is clear sky? Cool High P Warm Low P
22. 22. Scales of Atmospheric Motions Time and space scale of atmospheric motions Typical size Global scale 5000 km Synoptic scale 2000 km Mesoscale 20 km Microscale 2m Typical life span 23
23. 23. Sea breeze
24. 24. isobars 7 AM 820 mb 860 mb 900 mb 65oF 65oF
25. 25. Sea breeze isobars 820 mb 860 mb Cold Warm 780 mb 900 mb 90oF 70oF
26. 26. isobars 820 mb 860 mb 900 mb 90oF 70oF Cold Warm 780 mb