TornadoesTornadoes

• Name : ______________ ( )

• Class : _______________

• Date : _______________

Fig. 10-39, p. 292

is a rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground.

it’s circulation is present on the ground either as a funnel-shaped could or as a swirling cloud of dust and debris.

Definition and CharacteristicsDefinition and Characteristics

Sometimes called twisterscan assume a variety of shapes and forms that range from twisting rope-like funnels, cylindrical-shaped funnels. A funnel cloud, is a tornado that has not reached the ground.Short lived (similar to thunderstorms)On a local scale, the tornado is the most intense of all atmospheric circulations

CharacteristicsCharacteristics

The diameter of most tornadoes is between 100 and 600 m although some are a few meters wide Winds in tornadoes are very destructive. Most have winds that are 125 knots or less, but the most powerful have winds up to 220 knots!

CharacteristicsCharacteristics

A Point to Note :

A visible condensation funnel is NOT necessary to have a tornado

However, just a funnel without a circulation in contact with the ground is NOT a tornado

Table 10-2, p. 288

Tornado Winds

Table 10-1, p. 287

Formation of Tornadoes

1. Spinning vortex tubes created by wind shear.

2. The strong updraft in the thunderstorm carries the vortex tube into the thunderstorm

Dynamic pipe effect• Constricted mid-level flow in stretched portion of vortex

forces lower levels to constrict as well• Process proceeds toward the ground

Formation of Tornado

Bottom-up process• Descending air behind gust front and ascending air ahead of

gust front creates rotation

Tornado Formation

• As gust front moves under updraft, low-level mesocyclone rotation stretches beneath mid-level mesocyclone to form a tornado

Vortex breakdown• Downdraft descends in

central core of existing tornado

• Strong shear between updraft and central downdraft leads to smaller suction vortices

• Worst damage occurs from suction vortices

• The same process occurs in a multiple vortex tornado

Tornado Formation

Formation of Tornadoes• Spawned in thunderstorms• Vertical shear of horizontal wind

Horizontal vorticity formed (aligned with the average wind direction - streamwise vorticity)

• Convection (updrafts) in thunderstorms distort the vortex tube from horizontal to vertical

• Regions of rotating updraft air in the thunderstorms formed (mesocyclones)

• The rotations in mesocyclones concentrated to form tornadoes (similar to the water vortices - vertical stretching)

In A Nut Shell:In A Nut Shell:

Features associated with a tornado-breeding supercell thunderstorm

Which way do tornados rotate?

Nature of Tornadoes

• Vortices in atmosphere• Funnel shapes with

diameters on the scale of 100s of meters (Coriolis force is negligible in such small system)

• Rotate anti-clockwise in many cases (the North Hemisphere)

• Low pressure centres (capricious effect may be caused)

• Lifespan measured in minutes

Tornado Variations and Waterspouts

• Some tornadoes may form during the early stages of rapidly developing thunderstorms

• Tornadoes may appear nearly transparent until dust and debris are picked up

• Occasionally, two or more tornadoes may occur at the same time

• Tornadoes (weak) that form over warm water are called waterspouts

• Waterspouts occasionally move inland becoming tornadoes causing damage and injuries

A Waterspout Occurred during Typhoon Durian （ 2001)

Power of Tornadoes

• In an average year, 800 tornadoes are reported, resulting in 80 deaths and over 1,500 injuries in USA

• Wind speeds can approach 800 km/hr.

• Damage paths can be in excess of one mile wide and 50 miles long

• Lifetime of violent tornadoes can exceed 1 hour

Tornado Wind Speeds

The total wind speed of a tornado is greater on one side than on the other.

When facing an on-rushing tornado, the strongest winds will be on your left side.

Fig. 10-31, p. 287

A powerful multi-vortex tornado with three suction vortices.

Fig. 10-36, p. 290

A tornado-spawning supercell thunderstorm .A hook echo in its rainfall pattern on a Doppler radar screen. The colors red and orange represent the heaviest precipitation.

Fig. 10-41, p. 294

Doppler radar display of winds associated with the supercell storm that moved through parts of Oklahoma City during the afternoon of May 3, 1999.

The close packing of the horizontal winds blowing toward the radar (green and blue shades), and those blowing away from the radar (yellow and red shades), indicate strong cyclonic rotation and the presence of a tornado.

Forecasting Tornadoes

• Impossible to predict exact location of a tornado (so far…)

• No operational forecast models resolve tornadoes

• Examine specific indices from forecast models to determine locations favorable for supercell development