1. MET 61 1 MET 61 Introduction to Meteorology MET 61 Introduction to Meteorology - Lecture 12 Midlatitude Cyclones Dr. Eugene Cordero San Jose State University Reading: Chapter 13 (Ahrens); Pg. 313-320 (W&H) Class Outline:  Polar front theory  Cyclone development  QG Theory

2. MET 61 2 MET 61 Introduction to Meteorology Atmospheric Scales of Motion ScaleTime ScaleDistance ScaleExamples Macroscale -PlanetaryWeeks to years1000-40,000kmWesterlies, trade winds -SynopticDays to weeks100-5000kmCyclones, anticyclones and hurricanes MesoscaleMinutes to days1-100kmLand-sea breeze, thunderstorms and tornadoes MicroscaleSeconds to minutes<1kmTurbulence, dust devils and gusts

3. MET 61 3 MET 61 Introduction to Meteorology Polar Front Theory  Low pressure or cyclones are the principal weather makers at midlatitudes.  Development of a low pressure begins with a small perturbation or disturbance along the polar front.

4. MET 61 4 MET 61 Introduction to Meteorology

5. MET 61 5 MET 61 Introduction to Meteorology Polar Front Theory  Step A –Stationary front with a strong horizontal wind shear –

6. MET 61 6 MET 61 Introduction to Meteorology Polar Front Theory  Step A –Stationary front with a strong horizontal wind shear – Wind shear can produce instability (recall Richardson number).

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8. MET 61 8 MET 61 Introduction to Meteorology Polar Front Theory  Step B: –Under certain conditions a kink or small disturbance forms along the polar front (instability) –A "cold front" of cold air pushes to the south and warm air ("warm front") pushes to the north –The pivot point is the lowest local pressure and is the low pressure centre.

9. MET 61 9 MET 61 Introduction to Meteorology Polar Front Theory  Step C: –Fully developed wave – –Central pressure continues to drop. –Large bands of precip have formed. –A "warm sector" has formed in between fronts. –

10. MET 61 10 MET 61 Introduction to Meteorology Polar Front Theory  Step C: –Fully developed wave –The wave moves east or northeast. –Central pressure continues to drop. –Large bands of precip have formed. –A "warm sector" has formed in between fronts. –Heat transport initiated.

11. MET 61 11 MET 61 Introduction to Meteorology Polar Front Theory  Step D –Faster moving cold front catches up with the warm front, reducing the size of the warm sector.  Step E –Occlusion occurs as cold front catches warm front. – –Widespread precip.

12. MET 61 12 MET 61 Introduction to Meteorology Polar Front Theory  Step D –Faster moving cold front catches up with the warm front, reducing the size of the warm sector.  Step E –Occlusion occurs as cold front catches warm front. –Most intense part of the storm. –Widespread precip.

13. MET 61 13 MET 61 Introduction to Meteorology Polar Front Theory  Step F –Storm dissipates after occlusion. – –Result, weaker temperature gradient –Wave did it’s business!

14. MET 61 14 MET 61 Introduction to Meteorology Polar Front Theory  Step F –Storm dissipates after occlusion. –Storm gradually weakens and dissipates. –Result, weaker temperature gradient –Wave did it’s business!

15. MET 61 15 MET 61 Introduction to Meteorology Fig. 13.2

16. MET 61 16 MET 61 Introduction to Meteorology Divergence  In order for a low pressure to develop upper level divergence must exceed surface convergence.  At upper levels, the flow is parallel to the isobars, and thus non- divergent

17. MET 61 17 MET 61 Introduction to Meteorology  Upper air divergence required for low to develop at the surface 

18. MET 61 18 MET 61 Introduction to Meteorology  Upper air divergence required for low to develop at the surface  Notice tilting with altitude of high and lows

19. MET 61 19 MET 61 Introduction to Meteorology Baroclinic Wave Theory  Upper air flow: interrupted by waves imbedded in the flow –long waves and short waves

20. MET 61 20 MET 61 Introduction to Meteorology Baroclinic Wave Theory  Barotropic –Isotherms (lines of constant temperature) are parallel with isobars. If flow is geostrophic (parallel to isobars), no temperature advection can occur.  Baroclinic –Isotherms cross isobars. Temperature advection occurs (for geostrophic flow)

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22. MET 61 22 MET 61 Introduction to Meteorology Baroclinic Wave Theory  Warm advection –Movement of air from a warm region to a colder region. In upper level flow this typically occurs along the downstream side of a low - air typically heading NE –Warming air causes air to expand and diverge - divergence region - induces upward movement of air, intensifies surface low  Cold advection  Opposite occurs - air moves in from cold region - air cools, contracts, sinks - intensifies a high pressure

23. MET 61 23 MET 61 Introduction to Meteorology Development of a Baroclinic Wave

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25. MET 61 25 MET 61 Introduction to Meteorology Vorticity Advection  Vorticity –Is a measurement of an object's circulation. Counterclockwise (cyclonic, low) is defined as positive vorticity, clockwise (anticyclonic, high) is negative vorticity.  Planetary vorticity –The earth's rotation gives every object some vorticity which is the Coriolis parameter, f. –

26. MET 61 26 MET 61 Introduction to Meteorology Vorticity Advection  Vorticity –Is a measurement of an object's circulation. Counterclockwise (cyclonic, low) is defined as positive vorticity, clockwise (anticyclonic, high) is negative vorticity.  Planetary vorticity –The earth's rotation gives every object some vorticity which is the Coriolis parameter, f. –f is positive for all northern latitudes. f is zero at the equator and maximum in magnitude at the poles.

27. MET 61 27 MET 61 Introduction to Meteorology Vorticity  Relative vorticity –This is an object's local rate of circulation, ignoring planetary vorticity, eg. a skater spinning.  Absolute vorticity –The sum of planetary and relative vorticity.

28. MET 61 28 MET 61 Introduction to Meteorology Vorticity Advection  Vorticity Advection –Air that flows from a high to a a low moves from a low vorticity environment (the high) to a region of high vorticity (a low).   Negative vorticity advection typically enhances a surface high or diminish a surface low. NVA

29. MET 61 29 MET 61 Introduction to Meteorology Vorticity Advection  Vorticity Advection –Air that flows from a high to a a low moves from a low vorticity environment (the high) to a region of high vorticity (a low).  This is called negative vorticity advection (NVA).  Negative vorticity advection typically enhances a surface high or diminish a surface low. NVA

30. MET 61 30 MET 61 Introduction to Meteorology Vorticity Advection PVA  Vorticity Advection –Air that flows from a low to a high moves from a high vorticity environment (the low) to a region of low vorticity (a high).   Positive vorticity advection typically enhances a surface low or diminish a surface high.

31. MET 61 31 MET 61 Introduction to Meteorology Vorticity Advection PVA  Vorticity Advection –Air that flows from a low to a high moves from a high vorticity environment (the low) to a region of low vorticity (a high).  This is called positive vorticity advection (PVA).  Positive vorticity advection typically enhances a surface low or diminish a surface high.

32. MET 61 32 MET 61 Introduction to Meteorology PVA NVA

33. MET 61 33 MET 61 Introduction to Meteorology Relationship to vertical motion field

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35. MET 61 35 MET 61 Introduction to Meteorology Geostrophic Wind

36. MET 61 36 MET 61 Introduction to Meteorology Geostrophic Wind

37. MET 61 37 MET 61 Introduction to Meteorology Quasi-geostrophic (QG) theory  Quasigeostrophic theory—A theory of atmospheric dynamics that involves the quasigeostrophic approximation in the derivation of the quasigeostrophic equations.  Quasigeostrophic theory  However, it cannot accurately describe some atmospheric structures such as fronts or small strong low pressure cells.

38. MET 61 38 MET 61 Introduction to Meteorology Quasi-geostrophic (QG) theory  Quasigeostrophic theory—A theory of atmospheric dynamics that involves the quasigeostrophic approximation in the derivation of the quasigeostrophic equations.  Quasigeostrophic theory is relatively accurate for synoptic scale atmospheric motions in which the Rossby number is less than unity.  However, it cannot accurately describe some atmospheric structures such as fronts or small strong low pressure cells.

39. MET 61 39 MET 61 Introduction to Meteorology Quasi-geostrophic (QG) theory  Aimed to help diagnose observational structures and predict future developments.  The result of this analysis is that for flow is hydrostatic and nearly geostrophic, the three- dimensional wind field can be determined by the isobaric distribution of geopotential height  (x,y,p,t) alone.

40. MET 61 40 MET 61 Introduction to Meteorology Quasi-geostrophic (QG) theory  Aimed to help diagnose observational structures and predict future developments.  QG analysis is great simplification over full primitive equations. The result of this analysis is that for flow is hydrostatic and nearly geostrophic, the three- dimensional wind field can be determined by the isobaric distribution of geopotential height  (x,y,p,t) alone.

41. MET 61 41 MET 61 Introduction to Meteorology Quasi-Geostrophic Theory Omega Equation

42. MET 61 42 MET 61 Introduction to Meteorology Quasi-Geostrophic Theory Omega Equation

43. MET 61 43 MET 61 Introduction to Meteorology Quasi-Geostrophic Theory Vorticity Equation

44. MET 61 44 MET 61 Introduction to Meteorology Idealized Secondary Circulation Associated with a Developing Baroclinic Wave from a Q-G Perspective