Upcoming Schedule

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Upcoming Schedule

• The near future
• Apr 14 Lecture on El Nino and Tropical Cyclones
  (you wont want to miss that)
• Apr 21 Global Warming
• April 28 PGE and Cal Severe Weather
• May 5 Papers Due and ½ Presentations
• May 12 ½ Presentations
• May 19 Final (at regular class time)

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Overview

• Tornadoes are violently rotating columns of air
  that extend from a thunderstorm colud to the
  ground
• Funnel clouds are tornadoes that dont touch the
  ground
• Primarily develop in supercells, but can develop
  in squall line and in hurricanes
• Range in width from 150 ft to ½ mile
• 75 of worldwide tornadoes occur in the US (any
  guess why???)

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Supercell Thunderstorms

• Consist of a single cell instead of a series of
  cells
• Diameter 12-30 miles
• Smaller than MCS and frontal squall lines
• More violent (tornadoes can be produced)
• Short life span (2-3 hours)
• Always rotate

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Supercell Formation Four Ingredients

• Extremely unstable environment
• Humid and warm at the surface
• Cool and dry aloft
• Very moist air in lower troposphere
• Moderate to strong vertical wind speed shear
• A trigger mechanism (frontal lifting)

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SupercellsRotating Thunderstorms
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Stability Index and Supercells

• CAPE (Convective Available Potential Energy) is a
  favored supercell index
• Measures the area where the parcels temperature
  exceeds the environmental temperature
• CAPE is proportional to the maximum updraft speed
  of an unstable parcel
• CAPE between 500-1500 thunderstorm
• CAPE between 1500-3000 supercells with hail and
  tornadoes

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Supercell Cross-section
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Super Cell Note Cloud Rotation
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Super Cell
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Tornado Frequency, 1998
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When and Where

• Any time of year
• Most common east of the Rockies during spring and
  summer
• Southern States March through May
• Northern States Late spring and summer
• Mainly generated from supercell tstorms
• 1 in 1000 storms are supercells
• 1 in 5 supercells produces a tornado

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California Tornadoes
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Tornado Stages (pg 317)

• Dust swirl stage Rotation makes contact with
  ground (A, B)
• Organizing stage Funnel cloud, already on the
  ground, increases in intensity and size (C)
• Mature stage Intensity peaks, size peaks,
  structure nearly vertical (D)
• Shrinking stage Vortex tilts and takes on a
  rope-like appearance (E)
• Decay stage Rope-like structure increases and
  tornado dissipates (F)

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Supercell Tornadoes

• Rotation, tilting and stretching processes are key

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What Causes the Stretching?

• Downdrafts from the forward and rear flank of the
  supercell meet underneath the cell
• When this happens, the low level part of the
  updraft circulation is cut off from the warm air
  souce
• The less buoyant low level air rises more slowly
  than the warm updraft above it
• This stretches out the air column and the low
  level rotation increases dramatically
• Known as vortex stretching

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Other Supercell Tornado Facts

• Tornadoes can stay on the ground from a few
  minutes to an hour
• More than one tornado may be on the ground at the
  same time
• Tornadogenesis may occur several times during the
  life of a supercell

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Tornado Classification

• Fujita Scale Ranking from F0 (weakest) to F5
  (strongest).
• Damage from tornadoes is used to estimate wind
  speed (very few wind measurements inside
  tornadoes)
• F4 and F5 tornadoes are strong enough to pick up
  and throw an automobile

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Fujita Scale Tornado Statistics
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US Tornado Distribution
Tornado Alley
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Tornado WindsHow Do They Get So Strong???

• Law of Conservation of Angular Momentum
• Conservation means in the absence of other
  forces energy in a system remains the same
• For angular momentum the system is made up of
  the radius (r) of the system and the rotational
  velocity (v) of the wall
• For the Law to be upheld, any product of rv
  equals the same value r1v1r2v2r3v3
• Therefore, if the radius gets smaller the
  velocity must increase (visualize an ice skater
  spinning and pulling her arms in and spinning
  faster

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Tornado WindsHow Do They Get So Strong???
r4000 m v2.5 m/s
r1000 m v10 m/s
r100 m v100 m/s
At mesocyclone scale r x v10000 At wall
cloud scale r x v10000 At tornado
scale r x v10000 100 m/sec _at_
200 kt _at_ 230 mph
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Non-Supercell Tornadoes

• Landspouts
• Waterspouts
• Landfall hurricane generated

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Landspouts

• Short lived and not as intense as supercell
  tornadoes
• Called landspouts because they are visually
  similar to waterspouts
• Not due to storm cell rotation like supercells
• These vortices develop when a strong updraft
  stretches into a tight circulation.
• In California strong cold air advection aloft
  causes strong enough instability to create strong
  updrafts
• Nearly all wintertime California tornadoes are
  landspouts (F1 scale usually)

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Waterspouts

• Weak tornadoes commonly observed off coastlines
• Formation mechanism not well understood

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Other Atmospheric Vortices

• Gustnadoes Short-lived and weak, develop in
  strong shear flow along some thunderstorm gust
  fronts
• Cold air funnels Vortices that emerge from the
  base of elevated convective clouds, usually when
  very cold air moves into the upper troposphere
• Dust devils Dry convection over hot surfaces,
  sometimes over 1km in depth

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Tornado Detection

• Storm spotters and Doppler radar are primary
  tools (see pg 333, 334)
• Storm spotters include volunteers, police and
  other emergency management personnel who report
  tornado activity to the National Weather Service
• Doppler radar is superior to ordinary radar
  because it can detect rotational wind patterns

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Doppler Radar and Tornado Detection

• Doppler radar can measure the component of the
  wind moving toward and away from the radar
• Strong rotational winds (20-40 kts) often precede
  tornadogenesis
• When this type of circulation develops it is
  called a mesocyclone signature (Fig 18-21B)
• Forecasters track and extrapolate progress to
  determine its path

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Tornado Forecasting

• Forecasters generally assess the general
  likelihood for tornado activity using stability
  indices (CAPE) calculated from RAOB measurements
  (12 hr- 3 day advance window) CAPE gt 2500,gt4000
• Forecasters then assess cloud development,
  monitor radar (look for supercell development,
  bow echoes, hook echoes) and may issue watches
  (several hours advance windows)
• Forecaster monitor spotter information and
  Doppler radar to issue warnings (few minutes to a
  few hours advance window)

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Ultimate chase
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http//www.stormvideo.com/tornado.html
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A New cloud type Politically Incorrect Cumulus
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Politically Incorrect Cumulus