Thunderstorms & Tornadoes

What is a thunderstorm?

violent storm that causes heavy precipitation, strong gusts of wind, thunder and lightning. Large thunderstorms can spawn tornadoes, only storm that has thunder and lightning.

How Often Do They Occur?

more than 16M occur in the world as a whole and about 2,000 are taking place at any moment, 100,000 occur in the US, common in southern Ontario

Convective Storm

caused by surface heating, forms cumulonimbus clouds, air rises by convection inside the clouds, as it rises it cools and some water vapour condenses. Releases latent heat condensation, which warms the surrounding air

4 Categories

ordinary, multi-cell, supercell, mesoscale convection complexes (MCC)

Ordinary Thunderstorms

develop within large air masses, not near a frontal system, little vertical wind shear, change of horizontal wind speed with light

Ordinary Thunderstorms Stages

differential surface heating induces upward flow in unstable air, updraft, cumulus cloud formation, mature phase - development of a downdraft with precipitation, gust front develops a downdraft air spreads along horizontal surface

Mature Phase

development of a downdraft with precipitation

Convection Cell

gust front forces more air up into the updraft, updraft and downdraft

What happens when gust front moves past updraft

the updraft weakens, rain starts to fall into the updraft, cutting off the rising humid air

Ordinary Thunderstorm Final Stage

downdraft cuts off updraft and storm loses energy source and dissipates, short lived - 1hr, 1km or less diameter

Multi-Cell Thunderstorm

similar to ordinary except moderate wind shear, storm tilts, downdraft forms downwind of updraft, storms last longer, gust front of one storm initiates another storm

Supercell Thunderstorms

forms with strong vertical wind shear, surface winds (MT AIR) from south/southwest, upper level winds (CP AIR) from north/northwest. Along cold front of a midlatitude cyclone, 100-600M in diameter, tornadoes can spawn

Supercell

downdraft does not cutoff updraft, storm can last for several hours, hail can form, microbursts can also form

Supercell Plane View

areas of updraft and downdraft, gust front, up level blow off (ANVIL EXTENT)

Microbursts Localized Downdrafts

radial bursts of surface wind, horizontal spreading of a microburst creates strong wind shear when it reaches surface

Microbursts Aviation Hazard

plane flying into the headwind of microbursts gets sudden increase in lift, replaced suddenly by a tailwind as it exits the downdrafts. August 1985, Dallas-Fort Worth Airport, 100 lives lost

MCC

defined by oval or roughly circular organized systems containing several thunderstorms, multiple thunderstorms - circular fashion, 100,000 square km, 12hrs or more, self sustaining, heavy precipitation

Squall Line

series of vigorous cumulonimbus clouds that merge to form a continuous line and advances at right angles to the line itself. Strong of thunderstorms start to form along a cold front, 500km, over 10hrs, hair and tornadoes formed. November 15, 1989 - Huntsville tornado is an examples

Lightning

electrical discharge the neutralizes a charge separation that has accumulated within a cloud between two clouds, or between a cloud and the ground.

Charge separation occurs in the cloud

charge is carried by the cloud droplet or ice crystals

Smaller particles tend to

go to top of storm with positive charge, larger ones with negative charge to the bottom

Acts like a magnet

attract positive charge at the surface

Lightning Steps

90% starts at cloud base and goes to surface, path of 50 metres or so ionized by 3Million volts of electricity, further steps of 50 to 100 metres until surface is reached

As surface is neared

positive ions from the surface move upwards

Near the accumulated positive charge

met by a return stroke, carrying positive charge back to the cloud and traveling along. Same path of ionization, returns stroke is main part of the flash

When the two connect

the luminous return stroke is seen, several cm in diameter, process can repeat leading to forked lightning, air heats to 30,000 degrees, generates shock wave - thunder, sound travels more slowly than light, radio waves, Sferics, produced allowing for lightning detection worldwide

Hail

precipitation if form of hailstorms, hard, more or less spherical piece of ice. Largest form of solid precipitation and most damaging.

Canada has a new “heavy-weight” champion when it comes to hailstones

292.71 grams and measuring 123 millimetres in diameter, western university’s northern hail project field team recovered this huge hailstorm after Monday’s storm near Innisfail , atla. 2nd costliest natural weather disaster in Canada after 1998 ice storm

Hail Formation

in cumulonimbus clouds, thought successive deposition of supercooled water onto an ice core. Recycling through up and downdrafts enable hailstone to grow until it is too heavy for the updraft. 10 billion cloud droplets to form a golf ball sized piece of hail

Hail Distribution

anywhere thunderstorms can form, hail days in US range from 0 to 9. Leeward side of rockies has most hail days. Part 3 of lecture examines Canadian distribution of hail

Thunderstorm Distribution in US

maximum number in Florida, land/sea contrast, mid to late afternoon thunderstorm

Worldwide Distribution

DAI (2001), 15,000 stations around the globe, 1975-1997, seasonal distribution of thunderstorm probability summer hemisphere, land versus ocean - mainly over land, tropics and midlatitudes

Tornadoes

rapidly rotating column of air that reaches the ground, “twisters” or “cyclones”. Funnel cloud - precursor to tornado, has not reached the surface

What is a Tornado

mostly counterclockwise, 100-600M diameter, usually last a few minutes, peak winds of 220 knots (400 km/h)

How are Tornadoes formed?

from supercell thunderstorms, often form along the cold front of a midlatitude cyclone

Condition for Tornado formation

CP air aloft, MT air at surface, explosive development when stable level at 800 MB is eroded

Tornado Wind Shear

surface winds from south/southeast, upper level winds from west/northwest, cause air to rotate, vortex tube forms

Tornado Formation updraft

causes distortion of rotating air (vortex tube), rotating column takes on vertical component, this is the start of the tornado. Fully developed tornadic conditions, vortex tube becomes vertical, funnel clouds forms first and becomes tornado when it hits the earth’s surface

Fujita Scale

F0 = weak 64-116, F1 = weak 117-180, F3 = strong 181-252, F4 = strong 253-330, F5 = violent 331-416, F6 = violent >515

Tornado Classification

light damage, moderate damage, considerable damage, sever damage, devastating damage, incredible damage

Ted Fujita

created in 1971 University of Chicago, damage scale - velocities are approximate

Limitations of Fujita Scale

rankings are subjective and based solely on the damage caused by a tornado, difficult to apply with no damage indicators, if tornado hits no structures, large trees. No account of construction quality and variability, no definitive correlation between damage and wind speed, subject to biases of the surveyors.

Enhanced Fujita Scale

Ef-scale was proposed by national weather services to public and the full meteorological community early in 2006. On 1 February 2007, the enhanced Fujita scale replaced the original Fujita scale in all tornado damage surveys in the united states. Enhanced Fujita scale in estimate of peak wind speeds based on damage assessments conducted by trained meteorologists from the NSW.

How are thunderstorms and tornadoes related

tornadoes are spawned from super cell large thunderstorm, tornadoes are formed from the thunderstorm base, thunderstorm is a necessary condition for a tornado but not vice versa

Tornadoes in Canada

emergency preparedness Canada, data base of natural disasters, tornadoes are 6th on the list. 20 damaging tornadoes from 1900 to 1999, 125 fatalities, 1777 injured, 7718 evacuated, 1.2billion dollars of damage

Vaughn Tornado, 2009

up to 45 houses and buildings have been deemed unsafe, more than 600 homes have been damaged, 200 homes were damaged so badly that they will likely be demolished like an F2 Tornado

Tornado Frequency During 1950-2007

upward trend is detected over time with 1.6 tornadoes/decade - statistically significant. Examined the relationships between the multivariate enso index and tornado frequency and found that it is statistically significant. Enso years are associated with more tornadoes