Chapter 19 Tornadoes 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 Vortex Til4ng 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 1 •  What causes the mid-­‐level rota4on to form? Step 2 •  What causes the 2nd step to occur? 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 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 Top Down Process 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? Vortex Breakdown 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 Landspout Tornadoes Fujita Scale •  Originally designed to be a damage scale es4mate of tornadic wind speeds •  Had several drawbacks, what were they? EF-­‐Scale •  Came into use in 2007 •  What EF-­‐5 tornado was the first ranked by this scale? EF-­‐F Scale Comparison Tornado Detec4on •  Spoders •  Radar 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 Hook Echo Debris Ball •  Theorized to be a result of debris being lo_ed by a tornado •  New science is dispu4ng this Debris Ball Mesocyclone Signature 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) Tornadoes and Climate Change •  How might Tornadoes be affected by climate change?