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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
MET 61

Topic 3

Weather Systems

MET 61 topic 03b

slide2
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

slide3
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

slide4
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

slide5
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

slide6
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

slide7
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

slide8
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

slide9
Smaller-scale phenomena, so depth scale  length scale
  • Strong vertical motions
  • Compare to synoptic-scale storms!

MET 61 topic 03b

slide10
We can identify:
  • Individual convective storms (“single cell storms”)
  • Multicell storms
  • Mesoscale convective systems – larger systems

MET 61 topic 03b

slide11
Required environmental conditions:
  • Conditionally unstable atmosphere

w <   d

  • Boundary layer moisuture available
  • Mechanism to give low-level convergence → lift

MET 61 topic 03b

slide12
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

slide14
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

slide15
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

slide16
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

slide17
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

slide18
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

slide19
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

slide20
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

slide21
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

slide22
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

slide23
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

slide24
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

slide25
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

slide26
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

slide27
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

slide28
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

slide29
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

slide30
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

slide31
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

slide32
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

slide33
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

slide34
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

slide35
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

slide36
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

slide37
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

slide38
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

slide39
In reality? Depends on wind direction shear with height.
  • We examine a hodograph to tell us this

MET 61 topic 03b

slide40

north

700 mb

Example…

900 mb

1000 mb

west

500 mb

Now…connect the ends of the wind “arrows”  hodograph

MET 61 topic 03b

slide41

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

slide42

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

slide43

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

slide44
Net result:
  • Right-moving storms favored when winds veer with height (left-moving storms suppressed).

MET 61 topic 03b

slide45
Storm structure
  • Fig. 8.54 (right-mover)
  • Bounded weak echo region (BWER)
    • Indicates updraft
  • Heaviest rain “behind” (p.355)

MET 61 topic 03b

slide46
Storm structure
  • Fig. 8.55 = surface “map”
  • Mesocyclone (“L”)
  • Gust front
  • Enhances uplift
  • Also enhances shear and thus “spin” generation

MET 61 topic 03b

slide47
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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