severe weather and tornadoestcpf.fiu.edu/internship/met3502_synoptic_lec26_f10.pdf · severe...
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MET 3502 Synoptic MeteorologyMET 3502 Synoptic Meteorology
Lecture 26: November 30, 2010
Severe Weather and
Tornadoes
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What is severe weather?What is severe weather?
Large, damaging hail (20
Any storm
Large, damaging hail (20 mm, or ¾ in)
Damaging winds (60 mph, 50 kt, or 25 m s-1)
Tornado
Or all of the above
Mesoscale with strong updrafts!
Thunderstorm: Any storm that produces thunder
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Frontal Frontal
SquallSquall
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MidMid--Latitude Squall Line AnatomyLatitude Squall Line Anatomy
Backshear anvil can be much more extensive
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Mesoscale Convective ComplexesMesoscale Convective Complexes
Form in late afternoon or evening
Several thunderstorms combine
Many convective cells
Outflow boundaries trigger new convection
Characteristics:
◦ 100,000 km2 with cloud top temperature < -32oC
◦ Must stay that large for 6 h
◦ Elliptical, (minor axis)/(major axis) > 0.7
Cause nighttime rainfall max in Great Plains
Some severe weather, but mostly beneficial rain
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SUPERCELLSUPERCELL A single entity with long-lasting, rotating
updraft
Supercell storms account for most tornadoes and damaging hail
Characteristics
◦ Single vertically aligned cell
◦ Hook-shaped radar echo in plan view
◦ Middle level reflectivity overhangs updraft (Weak Echo Region)
◦ Matures in 90 minutes
◦ Updraft rises in “Echo-free vault or Bounded Weak Echo Region.
◦ Precipitation (Hail) carried across storm to other side.
◦ Thus it never loads the updraft
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Supercell Evolution in Plan ViewSupercell Evolution in Plan View
Initially more or less round
Moves with surrounding wind
Becomes elongated parallel with shear.
Hook-shaped radar echo
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Mature Supercell ThunderstormMature Supercell Thunderstorm
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Anatomy of a Supercell ThunderstormAnatomy of a Supercell Thunderstorm
Require shear and powerful instability
Shear causes sloping updrafts
Hydrometeors fall from updrafts
So precipitation loading is less of a factor
Crossing mid-level and surface inflows form front- and rear flank downdrafts (FFD & RFD)
Which generate permanent merging outflow bondaries
That maintain a long-lasting (hours) updraft
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Supercells and TornadoesSupercells and Tornadoes
Most F2-F5 tornadoes form in supercells
Rotating updraft forms a mesocylone
Tips horizontal vorticity(low-level shear) into the vertical
Causing cyclonic and anticyclonic rotation on the flanks of the updraft
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Tornado CharacteristicsTornado Characteristics Must have a funnel cloud extending from cloud base to the surface
Maximum winds can be as strong as 100 m/s, or 200 mph
Size ranges a few meters to nearly a kilometer, 100m typical
Typically about 1000 per year (20-50 of which a “violent”)
Type I Tornadoes
◦ Form within the Mesocyclone of a supecell thunderstorm
◦ Strongest tornadoes are Type I
Type II Tornadoes
◦ Form along stationary or slowly moving wind-shift lines
◦ Much weaker
Operational Enhanced Fujita Scale
(Feb 2007, uses 3 s gust))
◦ Weak F0, 65-85F1, 85-110 mph
◦ Strong F2,111-135 mphF3, 135-165 mph
◦ Violent F4, 166-200 mph F5, > 200 mph
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Shelf cloud and precipitation shaftShelf cloud and precipitation shaft
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Shelf cloud and rotating wall cloudShelf cloud and rotating wall cloud
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Mesovortex Wall CloudMesovortex Wall Cloud
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TornadoTornado
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When do Tornadoes Occur During the When do Tornadoes Occur During the
Day?Day?
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When Do Tornadoes Occur During When Do Tornadoes Occur During
the Yearthe Year
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Tornado MotionTornado Motion
Form in warm sector or in warm air above the cold frontal surface
Flow is from the SW so that Tornadoes generally move SW-NE
Often with a looping or cycloidal motion
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Tornado ForecastingTornado Forecasting Starts with forecasting
instability based upon large scale flow.
◦ Jet stream
◦ Air masses
Set up Watch Boxeswithin which Tornado formation is likely
Based upon radar and visual observation the Watch becomes a Warning
Meaning tornado in the ground.
Radar cannot detect tornadoes, but it can tell where to look
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Mesocyclone
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SUMMARY SUMMARY Severe Weather: Hail > 20 mm, 25 m/s wind, or a tornado
Mesoscale Convective Complex: Nearly circular mass of self-sustaining convection in heartland
Supercell Thunderstorms
◦ Shear causes tilting updraft that unloads condensate
◦ Intertwining inflow-updrafts distribute rain to flanking downdrafts
◦ Intersecting outflow boundaries sustain updraft
◦ Spawn Type I Tornadoes
Tornadoes
◦ Funnel cloud reaches the ground
◦ Form afternoon and evening
◦ May & June are the “cruelest months” in the lee of the rockies
◦ Move SW to NE
◦ 10s of m to a km in diameter
◦ Fujita Scale F0 to F5
Forecasting
◦ Watch Boxes set based upon synoptic situation
◦ Warning when a it touches down on the ground
◦ Radar is a key tool
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Lee Lee CyclogenesisCyclogenesis
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Where do Frontal Cyclones Form?Where do Frontal Cyclones Form?
Lee of the Rocky
Mountains
◦ Alberta
◦ Colorado Wyoming,
New Mexico
Gulf Coast
◦ TX-LA boarder
◦ Off Georgia-
Carolinas Coast
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Jet Max (or Jetstreak) passes over Jet Max (or Jetstreak) passes over
the Mounainsthe Mounains
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What happens when the wind blows What happens when the wind blows
over a mountain range?over a mountain range?
We assume that the Rossby number is small
Troposphere is shallower over the mountain.
Air moves faster
But how does it deflect meridionally?
Would the flow be different if the wind came from the East?
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East Wind in the MountainsEast Wind in the Mountains
Flow starts to deepen just a bit before the mountain
Initially turns cyclonically
Flow becomes shallower, compressing the vorticity tubes, as it passes over the mountain
The air both turns anticyclonically and bends to lower latitude where the Coriolis parameter is smaller
Straightens out on the lee side of the mountains
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West Wind in the MountainsWest Wind in the Mountains
Flow starts to deepen just a bit before the mountain
Initially turns cyclonically
Flow becomes shallower, compressing the vorticity tubes, as it passes over the mountain
The flow turns anticyclonicallyand bends to higher latitude where the Coriolis parameter is greater---opposing effects.
Overshoots on the lee side of the mountains forming a Lee Trough
This is where Lee Cyclogenesisstarts
In the steady state (never reached) a train of vorticity(Rossby) waves extends downwind
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Some New FeaturesSome New Features
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The Upper Front and the Dry LineThe Upper Front and the Dry Line
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Upslope Snow (& Sometimes Rain)Upslope Snow (& Sometimes Rain)
Cyclonic circulation passes
S or SE of the end of N-S
mountain range.
As through the mountain
gap in NM & AZ
East wind around the
northern flank flows up the
slope, cools adiabatically
and snows
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Patterns of WeatherPatterns of Weather
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Figure 9.1Figure 9.1
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Relation to Upper Trough & Upper FrontRelation to Upper Trough & Upper Front
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Occlusion StageOcclusion Stage
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SummarySummary
Over North America, extratropical cyclones form◦ Lee of the Rockies
◦ Gulf Coast
◦ Over the Gulf Stream
Lee Cyclogenesis◦ Anticyclonic flow over mountain
◦ Cyclonic trough form in the lee only under a west wind
Jet MAX (streak) passes over the Rockies
Deepening Low◦ Dry Line separates warm moist from warm dry air at the
surface
◦ Upper Front separates sinking warm dry air aloft from ascending warm moist air
◦ Upper front can catch the warm front in the occlusion process
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Nor’eastersNor’easters
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Snowstorms Snowstorms
in the in the
NortheastNortheast
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What Makes EastWhat Makes East--Coast Snowstorms Coast Snowstorms
So Intense?So Intense?
Strong thermal contrast between sea and land in winter and early spring
Superposition of Jet Streaks from the Polar-Front and Subtropical Jets (More during El Niño)
Ocean is a source of sensible and latent heat
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EastEast--Coast Storm TracksCoast Storm Tracks
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If the Cyclone Stays West of the MountainsIf the Cyclone Stays West of the Mountains
Only one Jet Streak
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Figure 10.4Figure 10.4
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Precursor Front Sets Up ColdPrecursor Front Sets Up Cold--Air Air
Damming as Jet Streaks ApproachDamming as Jet Streaks Approach
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Jet Streaks in Phase as Cyclone Jet Streaks in Phase as Cyclone
Deepens OffshoreDeepens Offshore
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Merged Jets and a Strong, Merged Jets and a Strong,
Occluded Coastal StormOccluded Coastal Storm
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Setting the Stage for a GulfSetting the Stage for a Gulf--Coast Coast
Nor’easterNor’easter
Precursor cyclone that
pushes the front far south
and two merging Jet Streams
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Two Jet Streaks Approach as the Two Jet Streaks Approach as the
Cyclone FormsCyclone Forms
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Open Wave With Merging Jet StreaksOpen Wave With Merging Jet Streaks
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Jet Streaks Merge SE of Jet Streaks Merge SE of
Occluded LowOccluded Low
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Example: The Blizzard of 1888. Stalled Example: The Blizzard of 1888. Stalled
for Two Days off Long Island. Shut the for Two Days off Long Island. Shut the
City Down. Led to underground utilities City Down. Led to underground utilities
in New York & Bostonin New York & Boston
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New Years Blizzard of 2000New Years Blizzard of 2000
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SummarySummary
Gulf coast and offshore sources
Intense because of:
◦ Contrast between land and sea
◦ Merging Jet Streaks from Polar-Front and Subtropical Jets
◦ Sea is a source of latent and sensible heat
Cold air damming and onshore flow cause heavy snow