Tornadoes

Outline Motivation Definition When and Where Ingredients Forecasting Historical Events

Year Deaths In mobile homes #Injuries Estimated damage 1985 94 30% 1986 15 47% 1987 59 41% 1989 50 24% 2490 $1088M 1990 53 13% 1150 $668M 1991 39 51% 854 $798M 1992 1300 $764M 1993 33 39% 3990 $368M 1994 69 38% 1067 $518M 1995 30 647 $408M Most tornado outbreaks result in less economic losses than other natural disasters

However, tornados account for a higher number of fatalities on average than hurricanes.

This is largely due to two reasons: 1)Warning time 2)Catastrophic nature of damage

Storm Structure (from the hurricane lectures) Tropical Cyclone Size Typical hurricane strength tropical cyclones are about 300 miles (483 km) wide although they can vary considerably. The relative sizes of the largest and smallest tropical cyclones on record as compared to the United States. What about a tornado’s size????

Four reasons why people ignore tornado warnings Many victims (in tornado alley) live in mobile homes, and do not have access to a safe evacuation shelter People tend to ignore any warnings that fall outside of the ‘normal’ tornado season (tornadoes may, in fact, occur during any month of the year) Optimism bias: ‘Bad things only happen to other people’ Warnings are often ignored, because there may be no planned evacuation plan, or because people think that the storms do not pose a significant threat

In general, the weakest tornados have wind speeds of hurricane force.

The overall number of fatalities associated with tornados continues to decrease

This decrease in fatalities is being realized even as the number of reported tornados is increasing

This decrease in fatalities is being realized even as the number of reported tornados is increasing

What is a tornado? A tornado is a violently rotating column of air extending between, and in contact with, a cloud and the surface of the earth. The stronger tornadoes attain an awe-inspiring intensity, with wind speeds that exceed 200 mph and in extreme cases may approach 300 mph.

How do we know how strong the wind speeds in a tornado are? The most common way to identify the strength of a tornado is through an examination of the damage caused. This concept led to the development of the Fujita and Enhanced Fujita Scales (http://www.spc.noaa.gov/efscale/).

EF Scale Damage EF1 – 31.6% of all reports EF2 – 10.7% of all reports

EF Scale Damage EF3 – 3.4% of all reports EF4 – 0.7% of all reports

EF Scale Damage EF5 – .1% of all reports

How do we know how strong the wind speeds in a tornado are? Another way is to remotely measure the wind speeds by using a mobile doppler radar.

How do we know how strong the wind speeds in a tornado are? Doppler radars can measure the speed of wind because the beam that is received by the radar will have a different frequency depending on the motion of the water drops

How do we know how strong the wind speeds in a tornado are? This is an image of the wind velocities from a doppler radar scan. The circle indicates a possible tornado.

When and Where do Tornadoes Form?

Map of World-Wide Tornado Fromation

Over 85% of world-wide tornado reports occur in North America The geography of North America is uniquely favorable for the formation of tornados. Warm-moist air source Cold air source All significant mountain ranges are oriented north-south.

The Gulf of Mexico and the western North Atlantic Ocean are important sources of water vapour for North American thunderstorms

States with the largest number of tornadoes are located in ‘tornado alley’ Tornado alley corresponds to a northeast-southwest orientation that corresponds to the orientation of strong fronts traveling across the central United States in the spring and early summer, and to the orientation of the upper-level ‘jet stream’ These states include Texas, Oklahoma, Nebraska, Kansas, Iowa, Missouri, Illinois, Indiana, Mississippi, and Florida

States with the largest number of tornadoes are located in ‘tornado alley’ However, the traditional tornado alley isn’t where deaths are most frequent. Instead, most deaths occur in the deep south. Why?

Tornadoes (when?) Most frequent in the spring and summer

Tornadoes (when?) Most frequent in the spring and summer

Tornadoes (when?) Most frequent in the spring and summer

Tornadoes (when?) Most common in the evenings

Ingredients

All tornados form within thunderstorms These thunderstorms can be found in: Landfalling hurricanes Within supercell thunderstorms In squall lines, often located ahead of cold fronts

Thunderstorms Defined by its scale (short and short-lived) Typically 5-10 km horizontally and vertically An individual cell lasts only 30-60 minutes

How do thunderstorms form? Require three elements: 1. A source of moisture 2. A conditionally unstable atmosphere 3. A mechanism to trigger a thunderstorm updraft, either through forced lifting or heating Try this: In order to get a thunderstorm you have to go out on a “lim”. l=lift, i=instability, m=moisture

How do thunderstorms form? Typically, the source of moisture is the Gulf of Mexico. Cool dry air aloft often comes from the Plains or the Rockies. This combination of warm moist air at the surface and cool dry air aloft leads to instability.

So what do we mean by instability?

What is a lapse rate? A lapse rate is defined as the rate of change in temperature observed while moving upwards through the Earth's atmosphere.

Key points Rate at which a temperature decreases with height. Units: generally C/km, sometimes K/km

Stability of Atmosphere This describes the tendency for the atmosphere to either resist or enhance vertical motions. The stability of the atmosphere is directly related to the changes of temperature with height. First we need to consider the temperature changes experienced by rising air. As a parcel of air rises it moves into regions of lower pressure. This means that the surrounding air is pushing on the parcel with less force. So the air in the parcel will expand, and the volume will become larger.

When the air expands, the molecules must now cover a larger volume When the air expands, the molecules must now cover a larger volume. This means that the air in the parcel must perform work to inhabit the increased volume. The work done by the parcel will result in lower kinetic energy, and the temperature must fall.

Now, what happens when air becomes saturated? Consider: condensation causes a release of latent heat. So the expansion of the air will induce a cooling, but this will be partially offset by heat release from the condensation of water. As a result, the rate of temperature change of rising air that is saturated is smaller than for dry air. This is called the Moist Adiabatic Lapse Rate, and it is not a constant value. This is because the rate of condensation changes with height.

Stability and Saturated Air Recall that when rising air becomes saturated, latent heat is released, and slows the rate of cooling. The moist adiabatic rate is variable, but always less negative than the dry adiabatic rate.

Conditional Instability Air is stable to a certain height, however, if a “lifting mechanism” can cause air to rise, to a level where condensation is reached the air is now saturated.

What is a “trigger” or lifting mechanism? Any mechanism which lifts a parcel of air up to the point where it becomes buoyant is can trigger a thunderstorm.

Examples of lifting mechanisms

Tornadoes migrate northward from spring to summer: Triggering in the form of forced lift occurs along the front range of the Rocky Mountains and along the sea-breeze convergence zone in Florida

Airmass thunderstorms occur in three stages 1. Cumulus (warm, buoyant plume with updrafts) 2. Mature (combination of both updrafts and downdrafts) 3. Dissipating (falling precipitation shuts off the updrafts…all downdrafts and system self destructs

Stages of a thunderstorm

Severe thunderstorms However, airmass thunderstorms don’t typically produce tornados and almost never produce strong tornados. The reasons that airmass thunderstorms don’t produce thunderstorms are two fold: Updrafts are suppressed by the weight of the water There is no inherent rotation to work with

Both liabilities can be overcome by shear Speed shear Directional Shear

The updraft then tilts the rotation from the horizontal axis to the vertical axis

Also the updraft is tilted by the shear, removing the updraft from the precipitation area. At this point the thunderstorm is called a “supercell”.

Eventually, this rotating updraft produces a small scale low pressure system called a mesocyclone

About 10% of the time, this rotation becomes “stretched” by the rear flank downdraft and concentrated into a tornado

About 10% of the time, this rotation becomes “stretched” by the rear flank downdraft and concentrated into a tornado

An animation of tornado formation from vertical wind shear is at http://esminfo.prenhall.com/science/geoanimations/animations/Tornadoes.html

Overhead view of a supercell T

Tornado forecasting Tornado forecasting is based largely on the recognition of the patterns which are conducive to tornado formation.

Tornado forecasting However, we can not pinpoint actual locations of tornado formation because the process is somewhat stochastic. Consequently, forecasts are for generalized regions with probabilities of events.

The tornado case of November 10-11, 2002 illustrates the relatively large-scale control on the location of the tornado outbreaks •88 tornadoes touched down in areas from Alabama to Pennsylvania •36 fatalities •one of the most extensive tornado outbreaks in several years •the outbreak is especially unusual during the month of November •Much of the activity occurred between the hours of 3PM and 9PM EST during the afternoon and evening of November 10, 2002

A map summarizing the severe weather reports for Nov. 10, 2002

An image of a tornado near West Mansfield, Ohio on Nov. 10, 2002:

Vehicles having been thrown across Main Street in Clark, Pennsylvania:

An evacuation of patrons at a movie theatre just minutes prior to the tornado saved lives in Van Wert, Ohio:

There was very little left of a mobile home in Fruitvale, Tennessee:

Canada’s most life-threatening tornadoes: Regina, Saskatchewan; June 30, 1912; 28 dead, hundreds injured Edmonton, Alberta; July 31, 1987; 27 dead, hundreds injured Windsor, Ontario; June 17, 1946; 17 dead, hundreds injured Pine Lake, Alberta; July 14, 2000; 12 dead, 140 injured Valleyfield, Quebec; August 16, 1888; 9 dead, 14 injured Windsor, Ontario; April 3, 1974; 9 dead, 30 injured Barrie, Ontario; May 31, 1985; 8 dead, 155 injured Sudbury, Ontario; August 20, 1970; 6 dead, 200 injured St-Rose, Quebec; June 14, 1892; 6 dead, 26 injured Buctouche, New Brunswick; August 6, 1879; 5 dead, 10 injured

The Edmonton tornado of July 31, 1987 Wind speeds up to 420 km per hour (a minimal F5) and stayed on the ground for an hour 27 people were killed with several hundred injuries 300 homes destroyed $330 million in total property damage One of the worst natural disasters in Canadian history

The tornado droppped down from a supercell thunderstorm at approximately 1458 and stayed on the ground for about an hour; the path was about 40 km long.

An aerial damage survey of several of the industrial sites in listed in the previous slide:

A view of the Edmonton tornado from the shores of the North Saskatchewan River:

A view of a residential area being affected:

And just after the damage was done:

Mitigation Residents accessing either radio or television were warned as much as 15 minutes prior to the tornado’s impact What about other more recent cases?

First, what is a tornado warning? A tornado warning means that a tornado is believed to be present in the warning area The warning may be based upon a Doppler radar signature, or on public spotter reports Storm spotters can be the first line of defence against tornadoes, particularly in regions outside of Doppler radar coverage

Warning times for tornadoes: Longest warning times are based upon mesoscale vortex signatures seen only one hour or more in advance The best way to be informed is to listen to a Weather radio (20 dollars and up) The theatre manager in van Wert, Ohio evacuated his customers after hearing the warnings being broadcast on the weather radio This alert action saved many lives

Tornado watches provide more potential lead time: A tornado watch states that conditions are favorable for tornadoes to form These watches may be given as much as 12 hours in advance of an anticipated event

The Canadian radar network in its present form has many gaps in coverage:

Tornado safety measures: When a tornado watch is issued, listen to a weather radio If at home, move to a designated shelter, such as a basement If there is no basement, move to an interior room and avoid windows (put as many walls as possible between you and the tornado) If outside, move to a low elevation away from any possible airborne object (including vehicles and mobile homes!)

Tornado myths: The skyscrapers in cities prevent tornados from forming in or striking downtown areas.

Tornado myths: Highway overpasses are a safe place to shelter if you are on the road when you see a tornado.

Tornado myths: It is commonly and mistakenly thought that if the condensation funnel of a tornado does not reach the ground, then the tornado can not cause substantial damage.

Tornado myths: It has been thought in the distant past that tornadoes moved almost exclusively in a northeasterly direction.

Readings: http://www.mb.ec.gc.ca/air/summersevere/tornadoes.en.html http://www.nssl.noaa.gov/edu http://www.fema.gov/library/tornado.htm Bluestein, H. B., 1999: Tornado alley: Monster storms of the Great Plains. Oxford University Press. Zebrowski, Chapter 8. Rauber, Walsh, and Charlevoix, chapters 17-18.