MET 61 Introduction to Meteorology - Lecture 12

1. 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 MET 61 Introduction to Meteorology

2. Scale Time Scale Distance Scale Examples Macroscale -Planetary Weeks to years 1000-40,000km Westerlies, trade winds -Synoptic Days to weeks 100-5000km Cyclones, anticyclones and hurricanes Mesoscale Minutes to days 1-100km Land-sea breeze, thunderstorms and tornadoes Microscale Seconds to minutes <1km Turbulence, dust devils and gusts Atmospheric Scales of Motion MET 61 Introduction to Meteorology

3. 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. MET 61 Introduction to Meteorology

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5. Fig. 13.2 MET 61 Introduction to Meteorology

6. 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 MET 61 Introduction to Meteorology

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

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

9. 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) Baroclinic Wave Theory MET 61 Introduction to Meteorology

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11. 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 MET 61 Introduction to Meteorology

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

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14. 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. MET 61 Introduction to Meteorology

15. 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. MET 61 Introduction to Meteorology

16. 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 MET 61 Introduction to Meteorology

17. Vorticity Advection • 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. PVA MET 61 Introduction to Meteorology

18. PVA NVA MET 61 Introduction to Meteorology

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

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21. Geostrophic Wind MET 61 Introduction to Meteorology

22. 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. MET 61 Introduction to Meteorology

23. 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. MET 61 Introduction to Meteorology

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

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

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