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Tornado
• Violently rota4ng column of air that extends from the base of a thunderstorm to the ground
Tornado Sta4s4cs
• Over (100, 1000, 10000) tornadoes reported in the U.S. every year
• Average of 56 deaths occur and 815 injuries • Es4mated $87.4 million in damages occurs each year
• In the most recent several years alone, this amount has exceed $1 billion
• What percentage of all tornadoes worldwide occur in the U.S.?
Tornado Occurrencs
• Primarily form under Supercells • Can also form in squall lines, end of bow echoes, land-‐falling hurricanes, ordinary thunderstorms
• What are they typically called when they form under ordinary thunderstorms?
Tornadoes
• Typically range from 150 feet to 0.5 miles wide and have wind speeds that range from 65 mph to over 200 mph
• Can exceed 1 mile in diameter • What is the current record-‐holder for width? • What was the previous one?
Tornado Life Cycle
• Most are short-‐lived (<10 minutes) • Long-‐lived ones undergo a 5-‐stage life cycle • First stage is the development of a funnel cloud and/or rota4ng dust swirl on the ground
• Second stage (organizing stage) is the funnel cloud making contact with the ground and increasing in intensity
Tornado Life Cycle
• Third Stage (mature) intensity of the vortex peaks – rota4on is strongest and tornado is typically at it’s widest point and most ver4cally erect
• Fourth Stage (shrinking) Vortex begins to 4lt over more and the tornado begins to shrink
• Fi_h Stage (decay) Also known as the rope-‐out stage
Supercell Tornadoes
• Supercells rotate because of a process known as vortex 4l4ng.
• What leads to vortex 4l4ng?
Ver4cal Wind Shear
• What is ver4cal wind shear?
• Ver4cal Wind Shear in the 0-‐6km layer (also known as the surface-‐500mb shear) most commonly used to determine supercell probability
Ver4cal Wind Shear
• 0 – 1 or 0 – 3 km shear is used to determine tornadic vs non-‐tornadic environments
Tornado Development
• Supercell and mesocyclone rota4on origins are well-‐understood.
• Tornado forma4on is not • Key Process is vortex stretching, what is this and how does it occur?
• Why don’t we have a good understanding of how tornadoes form?
Theorized Tornadogenesis Process
• Believed to occur in 3-‐steps • 1st step is development of a mid-‐level rota4on (mesocyclone)
• 2nd step is development of low-‐level rota4on (low-‐level meso, associated with wall cloud development)
• 3rd step is rota4on at the ground
Step 3
• Three proposed mechanisms for tornado produc4on – Bodom up process – Top down process – Vortex breakdown
Bodom Up Process
• Occurs when the RFD moves under the mesocyclone
• Believed to be the most common • RFD causes air to rotate horizontally at the surface, which gets 4lted up by the updra_ as it moves under the mesocyclone
• Can produce columns of cyclonic and an4-‐cyclonic rota4on
Bodom Up Process
• An4-‐cyclonic tornadoes can form in instances where the primary tornado is large and violent
• Vortex stretching of the cyclonic por4on can o_en lead to tornadogenesis
• The tornadic poten4al of the surface-‐based air rota4on arriving at the ground depends cri4cally on the air’s temperature – why? What dictates what the air temperature will be?
Top Down Process
• “Dynamic Pipe Effect” • Occurs when the mid-‐level mesocyclone begins to stretch
• What two forces are ac4ng on this region?
• What keeps them in balance?
Top Down Process
• Air moving upward in the mesocyclone must con4nually be replaced by air below
• Air constricts as it reaches the entry point • If this air is also rota4ng, it too will constrict to balance the forces, which lowers the “pipe” un4l it reaches the surface
Vortex Breakdown
• Based on data collected from VORTEX • Occurs when a central downdra_ develops in the center of the mesocyclone
• Downdra_ is a result of such extreme low pressures in the center of the mesocyclone that air is forced to descend towards the low pressure
Vortex Breakdown
• Tornado occurs when the central downdra_ inside the mesocyclone merges with the rota4ng air in the outer part of the surface mesocyclone
• Only one storm/tornado thus far has shown this behavior
• What is the downdra_ called that occurs in the vicinity of the mesocyclone?
Tornado Dura4on
• Lifecycle typically concludes when the RFD wraps around the tornado circula4on
• Longer-‐lived tornadoes have warmer RFD’s • Strong oujlow winds from the thunderstorm, in addi4on to the storms movement, eventually displace the near-‐surface por4on of the tornadic circula4on away from cloud-‐base – why does this occur? what happens to the tornado at this point?
Tornado Families
• Refers to tornadoes that developed from the same parent storm – what are these types of storms called?
• Some4mes more than one tornado can be on the ground at the same 4me and in some rare instances, older tornadoes may merge with new, developing tornadoes
Non-‐Supercell Tornadoes
• O_en referred to as Non-‐supercell tornadoes, landspouts, waterspouts, mesovor4ces or gustnadoes
• Most typical name is landspout
Landspouts
• Appear visually similar to waterspouts • Short-‐lived, not as intense as supercell tornadoes
• Form from thunderstorms lacking obvious signs of mid-‐level rota4on
Landspout Thunderstorms
• Triggered along advancing fronts or gust fronts
• Waterspouts are theorized to form in storms that form in the same manner over the water
Fujita Scale
• Originally designed to be a damage scale es4mate of tornadic wind speeds
• Had several drawbacks, what were they?
Radar
• Detec4on includes – RFD – Debris ball – Hook echo – Mesocyclone signature – Tornado vortex signature
RFD detec4on
• Contains precipita4on that wraps around the “echo-‐free” updra_
• Typically responsible for the hook appearance in radar reflec4vity fields
Debris Ball
• Theorized to be a result of debris being lo_ed by a tornado
• New science is dispu4ng this
Tornado Vortex Signature
• “TVS” • Indicated on radar by the beam (gate) of the radar that has drama4cally higher winds than the beams (gates) around it
Tornado Forecas4ng
• CAPE – Measure of how unstable an updra_ is • Storm-‐Rela4ve Helicity (SRH) – measure of horizontal rota4on in the lower atmosphere rela4ve to the mo4on of the thunderstorm
• Energy-‐Helicity Index (EHI) – combines CAPE and SRH (= CAPE x SRH / 160000)