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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

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  • 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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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

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Object in §8.1 is to use a case study to characterize:

  • Spatial distributions of:

    • Winds

    • Pressures

    • Temperatures

    • Fronts

    • Clouds

    • “weather”

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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

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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)

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  • 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)

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  • 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/

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  • Smaller-scale phenomena, so depth scale  length scale

  • Strong vertical motions

  • Compare to synoptic-scale storms!

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We can identify:

  • Individual convective storms (“single cell storms”)

  • Multicell storms

  • Mesoscale convective systems – larger systems

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Required environmental conditions:

  • Conditionally unstable atmosphere

    w <   d

  • Boundary layer moisuture available

  • Mechanism to give low-level convergence → lift

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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}

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  • We define convective available potential energy (CAPE) as follows:

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  • 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

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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.

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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

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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”)

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  • 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!!!

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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…

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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!

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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

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  • 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!

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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

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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)

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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

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  • 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

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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

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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”

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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)

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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

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  • 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”

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  • Note that air rises and exits the system @ upper levels ahead of the storm

  • Air enters @ mid-levels to the rear of the storm!

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Supercell storms

  • Characterized by a rotating updraft

  • Most tornadoes come from these

  • http://www.weather.gov/glossary/index.php?word=supercell+thunderstorm

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  • 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:

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  • 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!

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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”)

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  • 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

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  • 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?

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  • In reality? Depends on wind direction shear with height.

  • We examine a hodograph to tell us this

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north

700 mb

Example…

900 mb

1000 mb

west

500 mb

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

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north

700 mb

Resulting hodograph (blue line)…

900 mb

1000 mb

west

500 mb

General case: speed and direction change with height

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north

Resulting hodograph (blue line)…

800

900

west

- Special case: speed shear only (straight line hodograph)

- Favors splitting into 2 symmetric storms

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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

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Net result:

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

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Storm structure

  • Fig. 8.54 (right-mover)

  • Bounded weak echo region (BWER)

    • Indicates updraft

  • Heaviest rain “behind” (p.355)

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Storm structure

  • Fig. 8.55 = surface “map”

  • Mesocyclone (“L”)

  • Gust front

  • Enhances uplift

  • Also enhances shear and thus “spin” generation

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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

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