Chapter 19 Tornadoes Tornado Violently rotating column of air that extends from the base of a thunderstorm to the ground Tornado Statistics Over (100, 1000, 10000) tornadoes reported in the U.S. every year Average of 56 deaths occur and 815 injuries Estimated $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.? 1
Tornado Occurrences 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 rotating dust swirl on the ground Second stage (organizing stage) is the funnel cloud making contact with the ground and increasing in intensity 2
Tornado Life Cycle Third Stage (mature) intensity of the vortex peaks rotation is strongest and tornado is typically at it s widest point and most vertically erect Fourth Stage (shrinking) Vortex begins to tilt over more and the tornado begins to shrink Fifth Stage (decay) Also known as the rope-out stage Supercell Tornadoes Supercells rotate because of a process known as vortex tilting. What leads to vortex tilting? Vertical Wind Shear What is vertical wind shear? Vertical Wind Shear in the 0-6km layer (also known as the surface-500mb shear) most commonly used to determine supercell probability 3
Vertical Wind Shear 0 1 or 0 3 km shear is used to determine tornadic vsnon-tornadic environments Vortex Tilting Tornado Development Supercell and mesocyclone rotation origins are well-understood. Tornado formation 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? 4
Theorized Tornadogenesis Process Believed to occur in 3-steps 1 st step is development of a mid-level rotation (mesocyclone) 2 nd step is development of low-level rotation (low-level meso, associated with wall cloud development) 3 rd step is rotation at the ground Step 1 What causes the mid-level rotation to form? Step 2 What causes the 2 nd step to occur? 5
Step 3 Three proposed mechanisms for tornado production Bottom up process Top down proces s Vortex breakdown Bottom 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 tilted up by the updraft as it moves under the mesocyclone Can produce columns of cyclonic and anticyclonic rotation Bottom Up Process 6
Bottom Up Process Anti-cyclonic tornadoes can form in instances where the primary tornado is large and violent Vortex stretching of the cyclonic portion can often lead to tornadogenesis The tornadic potential of the surface-based air rotation arriving at the ground depends critically 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 acting on this region? What keeps them in balance? Top Down Process Air moving upward in the mesocyclone must continually be replaced by air below Air constricts as it reaches the entry point If this air is also rotating, it too will constrict to balance the forces, which lowers the pipe until it reaches the surface 7
Top Down Process Vortex Breakdown Based on data collected from VORTEX Occurs when a central downdraft develops in the center of the mesocyclone Downdraft 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 downdraft inside the mesocyclone merges with the rotating air in the outer part of the surface mesocyclone Only one storm/tornado thus far has shown this behavior What is the downdraft called that occurs in the vicinity of the mesocyclone? 8
Vortex Breakdown Tornado Duration Lifecycle typically concludes when the RFD wraps around the tornado circulation Longer-lived tornadoes have warmer RFD s Strong outflow winds from the thunderstorm, in addition to the storms movement, eventually displace the near-surface portion of the tornadic circulation 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? Sometimes more than one tornado can be on the ground at the same time and in some rare instances, older tornadoes may merge with new, developing tornadoes 9
Non-Supercell Tornadoes Often referred to as Non-supercell tornadoes, landspouts, waterspouts, mesovortices 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 rotation 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 10
Landspout Tornadoes Fujita Scale Originally designed to be a damage scale estimate 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? 11
EF-F Scale Comparison Spotters Radar Tornado Detection Radar Detection includes RFD Debris ball Hook echo Mesocyclone signature Tornado vortex signature 12
RFD detection Contains precipitation that wraps around the echo-free updraft Typically responsible for the hook appearance in radar reflectivity fields Hook Echo Debris Ball Theorized to be a result of debris being lofted by a tornado New science is disputing this 13
Debris Ball Mesocyclone Signature Tornado Vortex Signature TVS Indicated on radar by the beam (gate) of the radar that has dramatically higher winds than the beams (gates) around it 14
Tornado Forecasting CAPE Measure of how unstable an updraft is Storm-Relative Helicity (SRH) measure of horizontal rotation in the lower atmosphere relative to the motion 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? 15