Lecture 16

Question 1

Tornado

Answer 1

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

Question 2

Evolution of a tornado

Answer 2

Early: warm humid air rises in updrafts
Mature: updrafts continue, strong downdrafts
Dissipating stage: downdrafts dominate, cloud mass shrinks

Question 3

Hazards associated with tornadoes

Answer 3

High wind speed damages property
Uplifting winds create updraft
Rapid differences in air pressure
100m-3km in diameter
Minutes to several hours

Question 4

Edmonton 1987 Tornado

Answer 4

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

Question 5

Warnings (4)

Answer 5

1) Dopplar radar
2) Storm watchers
3) Physical and numerical models
4) Other elements of weather forcasting

Question 6

Challenges of media coverage (5)

Answer 6

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

Question 7

Troposphere

Answer 7

All of Earth’s surface is within this layer
Temperature decreases with increasing altitude
Clouds are present
Weather develops in this layer

Question 8

Jet stream

Answer 8

High air moving quickly in a stream
Ridges and troughs
When it moves from ridge to trough, air system below tends to rotate clockwise

Question 9

Low pressure system rotation

Answer 9

Counter clockwise

Question 10

Cumulonimbus

Answer 10

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

Question 11

Why are tunnels sometimes not seen?

Answer 11

Low relative humidity

Precipitation blocks view

Question 12

Waterspout

Answer 12

Funnel over water

Question 13

Mesocyclonic tornado

Answer 13

Associated with larger-scale parent vortex, mesocyclone
Most common in North America
Occur in long-lived convective storms called supercells

Question 14

Mesocyclone

Answer 14

Term for rotating updraft of a supercell

Force track of supercells to the right of the average tropospheric winds

Question 15

Supercells

Answer 15

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

Question 16

Vertical wind shear

Answer 16

Differences in wind speed with height causes horizontal vorticity (rotation)

Question 17

Streamwise vorticity current

Answer 17

Tube of streamwise vorticity

Caused by vertical wind shear

Question 18

North American mesocylone tornado evolution (6)

Answer 18

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

Question 19

Landspouts

Answer 19

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

Question 20

Tornado seasons

Answer 20

Most frequent: May - September
Peak frequency: June and July
Rare in winter

Question 21

Doppler Radar

Answer 21

Objects moving toward antenna increase wave’s frequency

Objects moving away decrease waves frequency

Question 22

Mitigation (5)

Answer 22

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