Lecture 16 Flashcards
Tornado
a violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud
Spanish for turning and thunder
Must be in contact with ground
Evolution of a tornado
Early: warm humid air rises in updrafts
Mature: updrafts continue, strong downdrafts
Dissipating stage: downdrafts dominate, cloud mass shrinks
Hazards associated with tornadoes
High wind speed damages property Uplifting winds create updraft Rapid differences in air pressure 100m-3km in diameter Minutes to several hours
Edmonton 1987 Tornado
EF4 Windspeeds of 460km/h 40km long, 1km wide, 1hr duration 27 killed, 300 injured 2nd worst tornado in Canada >$300 million in damage Triggered development of a provincial emergency public warning system
Warnings (4)
1) Dopplar radar
2) Storm watchers
3) Physical and numerical models
4) Other elements of weather forcasting
Challenges of media coverage (5)
1) Key cell phone towers down
2) Some radio transmitters powerless
3) No way to determine where to go
4) Difficult finding best route
5) Long time before he could find someone in charge
Troposphere
All of Earth’s surface is within this layer
Temperature decreases with increasing altitude
Clouds are present
Weather develops in this layer
Jet stream
High air moving quickly in a stream
Ridges and troughs
When it moves from ridge to trough, air system below tends to rotate clockwise
Low pressure system rotation
Counter clockwise
Cumulonimbus
Clouds with vertical development
Planes need to go around
Less dense air floats higher until it cools and equilibrates - will stop or go back down
Why are tunnels sometimes not seen?
Low relative humidity
Precipitation blocks view
Waterspout
Funnel over water
Mesocyclonic tornado
Associated with larger-scale parent vortex, mesocyclone
Most common in North America
Occur in long-lived convective storms called supercells
Mesocyclone
Term for rotating updraft of a supercell
Force track of supercells to the right of the average tropospheric winds
Supercells
Form when vertical wind shear is 20 m/s in the lowest 6km of storm and there is strong wind at mid atmospheric level
Commonly associated with the jet stream
Lifts streamwise vorticity current
Vertical wind shear
Differences in wind speed with height causes horizontal vorticity (rotation)
Streamwise vorticity current
Tube of streamwise vorticity
Caused by vertical wind shear
North American mesocylone tornado evolution (6)
1) Can form in any severe weather event, e.g. supercell thunderstorms, hurricanes, strong midlatitude cyclones
2) Low-altitude, northerly flow of Gulf of Mexico humid warm air (>22°C) is typical
3) Aloft mid-latitude cold dry air mass, often from Canada or Rocky Mountains, > 80 km/h
4) High altitude jet stream winds greater than 240 km/h
5) These air masses set up shearing and cause spin of the thunderstorm (changes in direction with height)
6) Vorticity generated by storm, most likely horizontal winds, not Coriolis
Landspouts
Other type of tornado, not associated with a mesocyclone
20% of all tornadoes
When vertical shear is weak
Begin at ground level and work upwards
May not have strong funnels and may not appear to be in contact with the ground
Tornado seasons
Most frequent: May - September
Peak frequency: June and July
Rare in winter
Doppler Radar
Objects moving toward antenna increase wave’s frequency
Objects moving away decrease waves frequency
Mitigation (5)
1) Doppler very useful tool for warning, but sometimes only minutes; not so useful at night when people are asleep
2) Prediction of supercell tornadoes easier than non-mesocyclonic tornadoes
3) Preparation measures and warnings in tornado alley
4) Some suggestion of using explosives, but may cause more damage!
5) Some suggestion of storm seeding (AgI) but may generate flooding or more significant hail
