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MET 61

MET 61. Topic 3 Weather Systems. Extratropical cyclones a.k.a. Mid-latitude storms Storms have characteristic: Spatial scales Time scales Speeds (of storm motion) Structures Lifecycles Behavior.

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MET 61

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  1. MET 61 Topic 3 Weather Systems MET 61 topic 03b

  2. Extratropical cyclones • a.k.a. Mid-latitude storms Storms have characteristic: • Spatial scales • Time scales • Speeds (of storm motion) • Structures • Lifecycles • Behavior MET 61 topic 03b

  3. The “Norwegian Cyclone Model” of these storms was developed in the early 1900’s based on observations. http://en.wikipedia.org/wiki/Norwegian_cyclone_model http://en.wikipedia.org/wiki/Extratropical_cyclone MET 61 topic 03b

  4. Object in §8.1 is to use a case study to characterize: • Spatial distributions of: • Winds • Pressures • Temperatures • Fronts • Clouds • “weather” MET 61 topic 03b

  5. characterize: • Temporal variations of: • Parameters as a storm travels west  east • Storm’s evolution as it travels west  east • Horizontal structure @ surface and aloft • Vertical structure MET 61 topic 03b

  6. Much of this will be seen again in 170A,B and 171A,B Some material will already be familiar to some of you! Material: • Figures in §8.1 • Text • Summary notes (handout) MET 61 topic 03b

  7. Convective storms • Text §8.3 • We are interested in DEEP convection (Cb – rather than simple Cu) • Tends to occur in preferred locations: • E.g., US midwest; ITCZ • associated with frontal systems (provide dynamics for lift) MET 61 topic 03b

  8. influenced by geography (Gulf, Rockies) • e.g., http://www.weather.gov/outlook_tab.php • VIP for summer rainfall over the midwest • e.g., http://www.climate-zone.com/climate/united-states/iowa/des-moines/ MET 61 topic 03b

  9. Smaller-scale phenomena, so depth scale  length scale • Strong vertical motions • Compare to synoptic-scale storms! MET 61 topic 03b

  10. We can identify: • Individual convective storms (“single cell storms”) • Multicell storms • Mesoscale convective systems – larger systems MET 61 topic 03b

  11. Required environmental conditions: • Conditionally unstable atmosphere w <   d • Boundary layer moisuture available • Mechanism to give low-level convergence → lift MET 61 topic 03b

  12. Conditionally unstable atmosphere • If we can lift an air parcel to the level of free convection (LFC), it will ascend on its own to the equilibrium level (EL) {where T(parcel) < T(environment)} • As air rises, potential energy → kinetic energy {associated with w2/2} MET 61 topic 03b

  13. We define convective available potential energy (CAPE) as follows: MET 61 topic 03b

  14. CAPE is just the area on a skew-T between the sounding and a moist adiabat • Example 8.1 shows typically CAPE value of almost 4000 J kg-1 • See http://en.wikipedia.org/wiki/Convective_available_potential_energy MET 61 topic 03b

  15. CAPE can be computed in multiple ways: • CAPE computed from LFC  EL (as defined above) • MLCAPE = Mean Layer CAPE = CAPE calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level. • SBCAPE = Surface Based CAPE = CAPE calculated using a surface-based parcel. MET 61 topic 03b

  16. CAPE criteria for severe weather (CAPE computed thru the column): • CAPE < 1000 J kg-1  convection chances marginal • 1000 < CAPE < 2500 J kg-1  moderate convection • 2500 < CAPE < 4000 J kg-1  extreme convection MET 61 topic 03b

  17. CAPE and CIN • CAPE is stored energy • We need large values of CAPE stored up since CAPE  kinetic energy (“w”) • Accomplish via a stable layer capping the boundary layer • Inhibits vertical motions until some break through • Thus – allows CAPE to build up (as opposed to “bleeding out”) • These then “eat into” the store of CAPE → deep convection • (see “animations: CAP strength vs CAPE”) MET 61 topic 03b

  18. We can define a “negative CAPE” or “anti CAPE” as CIN = convective inhibition • CIN > 1000 J kg-1  convection unlikely • Daytime heating over land (versus oceans)  deep convection more pronounced over land vs. water • Fig. 6.56 = lightning!!! MET 61 topic 03b

  19. Vertical wind profile • From above, we know there must be CAPE • CAPE is associated with vertical profiles of temperature and humidity • There are also wind profile requirements needed for convective activity • More later… MET 61 topic 03b

  20. Storm speed • Storms move at same speed as a vertically-averaged horizontal speed • We talk of a “steering level” but…(p.347) • Storms can move to the left or right of the steering level flow! MET 61 topic 03b

  21. Storm structure vs winds • Weak vertical shear {where V/z is weak} favors airmass thunderstorms (non-severe) – see below • Stronger vertical shear favors multicell and supercell storms – more severe MET 61 topic 03b

  22. Above relates to speed shear • There can also be directional shear • This can contribute to vorticity enhancement • This can aid the development of rotation in a storm • This can be a precursor to tornadoes! MET 61 topic 03b

  23. Ingredients for deep convection in US • Conditional instability…aided by: • Southerly flow of warm, humid low-level air from the Gulf of Mexico • Westerly flow of dry, conditionally unstable air aloft (source = Rockies to the west) • Strong vertical wind shear • Southerly low-level flow; westerly upper-level flow (veering winds) • Mechanism to create lift • Example: frontal approach MET 61 topic 03b

  24. Structure of deep convection • We think of cells of convection • Cells consist of multiple parcels • Cells grow upward, “eating into” CAPE • Eventually bump into the tropopause → anvil structure • Ice clouds at this level (Ci) MET 61 topic 03b

  25. Single cell storms • Also called airmass thunderstorms • Non-severe • Typical of what we might see over western deserts in summer (NV, AZ) • Arise from local convective instability – do not need to be forced by fronts MET 61 topic 03b

  26. A single cell which → distinct lifecycle • See Fig. 8.48 Cumulus stage • Rising warm, moist air plume + entrainment • w  10 m/s @ top • Top of cloud above FZL • Supercooled droplets MET 61 topic 03b

  27. Mature stage • Precipitation drops large enough to fall out have formed • A strong downdraft develops • Rain droplets + air fall out of cloud (frictional drag) • Falling drops evaporate → evaporative cooling • Thus a cold air downdraft develops • Rain + cold downdraft @ base of cloud MET 61 topic 03b

  28. Dissipating stage • Most of the cloud is now occupied by the downdraft • No more updraft = no more cloud & precip development • Storm is “choked off” Duration - about an hour Non-severe (e.g., no large hail) Can/will have lightning/thunder “animations” MET 61 topic 03b

  29. Multi-cell storms • Consist of a series of cells • Develop in sequence • Development of one aids the development of the next one • Hence storms are more long-lived (several hours) MET 61 topic 03b

  30. Vertical wind shear is crucial: • Airmass thunderstorms form in weak/no vertical shear  updraft & downdraft are not separated • With vertical shear, the updraft can become tilted in the vertical • And the updraft and downdraft can now coexist MET 61 topic 03b

  31. As one cell develops, there is: • Upward growth • Development of precip in the updraft • Growth of large precip droplets which begin to fall out • Development of a cold downdraft in a separate region from the updraft – see Fig. 8.49 • Cold downdraft → Gust Front of cold air @ surface • This gust front provides additional lifting for the development of the next cell! • Hence – the process can continue! • “animations” MET 61 topic 03b

  32. Note that air rises and exits the system @ upper levels ahead of the storm • Air enters @ mid-levels to the rear of the storm! MET 61 topic 03b

  33. Supercell storms • Characterized by a rotating updraft • Most tornadoes come from these • http://www.weather.gov/glossary/index.php?word=supercell+thunderstorm MET 61 topic 03b

  34. Rotation induces a mesolow … small-scale low pressure center which enhances winds (horizontal and vertical) • These storms are short-lived so Coriolis effects are small and can be ignored • As a result, the force balance is between the pressure gradient force (PGF) and the centrifugal force (CE)  cyclostrophic wind with: MET 61 topic 03b

  35. Ex. 8.3  typical p/n of 1 mb/km • Larger pressure gradient  stronger winds • Convergence (induced by surface friction)  rising motions • Ex. 8.4  vertical acceleration of 0.1 m/s per second  6 m/s vertical motion within one minute! MET 61 topic 03b

  36. Development of storm & rotation • Fig. 8.52 • Assume only SPEED SHEAR • Speed shear {u/z}  rotation in a horizontal “tube” of boundary layer air (8.52a) • As this “tube” is fed into the updraft, the rotation becomes oriented about the vertical axis (“vorticity”) MET 61 topic 03b

  37. These induce pressure perturbations (cannot have winds w/o pressure perturbations) associated with each vortex • This makes the storm split into two supercells • Radar example: http://epod.usra.edu/blog/2008/05/splitting-supercell.html MET 61 topic 03b

  38. So we get a right-moving storm (“R”) and a left-moving storm (“L”) • “R” means moving in a direction to the right of the mean wind (and vice versa) • Note the opposite orientations of the two rotating updrafts (clockwise versus CCW) • With only speed shear, they are equally likely • In reality? MET 61 topic 03b

  39. In reality? Depends on wind direction shear with height. • We examine a hodograph to tell us this MET 61 topic 03b

  40. north 700 mb Example… 900 mb 1000 mb west 500 mb Now…connect the ends of the wind “arrows”  hodograph MET 61 topic 03b

  41. north 700 mb Resulting hodograph (blue line)… 900 mb 1000 mb west 500 mb General case: speed and direction change with height MET 61 topic 03b

  42. north Resulting hodograph (blue line)… 800 900 west - Special case: speed shear only (straight line hodograph) - Favors splitting into 2 symmetric storms MET 61 topic 03b

  43. north 700 mb Resulting hodograph (blue line)… 900 mb 1000 mb west 500 mb • Winds veering with height  clockwise-turning hodograph • Favors right-moving storms (observed…theory???) • This is typical of the southern plains MET 61 topic 03b

  44. Net result: • Right-moving storms favored when winds veer with height (left-moving storms suppressed). MET 61 topic 03b

  45. Storm structure • Fig. 8.54 (right-mover) • Bounded weak echo region (BWER) • Indicates updraft • Heaviest rain “behind” (p.355) MET 61 topic 03b

  46. Storm structure • Fig. 8.55 = surface “map” • Mesocyclone (“L”) • Gust front • Enhances uplift • Also enhances shear and thus “spin” generation MET 61 topic 03b

  47. Tornadic storm structure • Fig. 8.56 = tornadic storm • Difficult to measure!!! • Hook echo • http://en.wikipedia.org/wiki/Hook_echo • Read 8.3.3 (tornados, downbursts, derechos) and 8.3.4 (MCCs) yourselves MET 61 topic 03b

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