Meteo 3: Chapter 12/13

1. Meteo 3: Chapter 12/13 The Cyclone Model: Common characteristics and evolution of mid-latitude lows Read pages 488-489, 492-494, 519-532

3. Mid-latitude cyclones • Recall weather is caused by uneven radiative heating of the earth • These cyclones (low-pressure centers) are another mechanism by which the atmosphere tries to mitigate temperature contrasts • Cold air brought equatorward, warm air brought poleward

5. Terminology • Advection: The horizontal transport of some atmospheric quantity by the wind (i.e. temperature, moisture) • Vorticity: Measure of amount of rotation

6. Origins of a mid-latitude cyclone • Upper-trough must approach and bring upper-level divergence for pressure to lower (column weight to decrease) • This begins cyclogenesis- formation of low pressure center

7. Cyclonic (positive) vorticity in troughs, anticyclonic (negative) vorticity in ridges • Cyclonic = counterclockwise • Anticyclonic = clockwise • Assumption: Air parcels moving faster than trough-ridge system

9. Stationary front: A storm’s breeding ground

10. More on cyclones- Advection • Warm sector: Lies between cold and warm front, mild with nearly uniform temperature and moisture • Cold advection behind cold front • Warm advection ahead of warm front • Warm air forced to rise over denser, cold air => overrunning…lifting mechanism supports widespread clouds and precipitation

11. Fronts as 2D surfaces: Dense cold air wedges under warm air

12. Self-development • Low pressure systems help themselves strengthen by concentrating temperature gradients along fronts and promoting temperature advection • Temperature advection sharpens the upper-level trough, increasing the vorticity maximum, increasing the divergence aloft to the east of the trough, thereby lowering the surface pressure

17. Occlusion: Low reaches its strongest point, but begins to decay…cold front overtakes warm front…no more warm advection over low

19. Motion of mid-latitude cyclones • Pressures lower the most (a low moves in the direction) where upper-level divergence and warm advection ahead of a low sum to create the largest pressure falls • WAA lowers air density, lowering column weights/pressure • Lows usually move toward area of negative pressure tendencies via these two processes • Highs move toward positive pressure tendencies behind cold front

20. Today’s surface analysis

21. Weather ahead of a warm front • Patches of cirrus • Then cirrostratus (with halo?) • Lowering/thickening clouds • Falling pressure • Steady, long duration (stratiform) precipitation from nimbostratus

22. Warm front cross section

23. Conveyor Belts

24. Conveyor belts…view from satellite

25. A little on the cold conveyor belt • Initially, a cold, dry, low-level flow of air moving west to the north of a warm front • Moistens via falling precipitation evaporating • Begins to ascend upward as it approaches low • In winter, produces heavy snow to NW of low

26. Weather associated with cold fronts • Convective precipitation (showers/thunderstorms) • Decreasing pressure as it approaches…rising pressure after its passage • Increasingly warm and humid air ahead of cold front….instability! • Wind shift at frontal passage • Drier, cooler air behind front • Sinking air • Decreasing clouds

27. Cold front cross section

28. Generated by sinking air motion west of cyclone Stream of dry, cold air from higher in atmosphere sinks as it flows southward = dry conveyor belt Drawn east into cyclone’s circulation Leads to clearing & end of precipitation Easily tracked on water vapor imagery Dry slot- Creating the comma shape

29. Mixing of cold, dry air with warm air…a cyclone’s demise