š Extremely rapidly rotating winds that form below the base of cumulonimbus clouds.
š Generally cyclonic (although too small to be affected by Coriolis effect, the Coriolis effect plays are role in their formation).
š Variety of shapes (thin ropes, cylinderical columns, funnels), sizes (100 – 1500 m), and strengths (wind speeds 65-450 km/hr).
š Move at speeds around 50 km/hr and last a few minutes (although can last hours), covering distances of 3-4 km.
š Develop at frontal boundaries, MSCs, supercells, and tropical cyclones.
š Although most contain a single central core, some of the most violent involve multiple small, intense “suction” vortices.
Most destructive tornadoes form from supercells, so concentrate on formation of these tornadoes.
Formation occurs in 3 stages:
The conditions for formation are those for formation of severe thunderstorms:
· low-level flow of moist, warm air,
· upper-level flow of drier, cool air,
· passage of cold front (lifting + rotation).
These conditions are frequently meet in the “Tornado Alley”, especially during spring (warming of surface in winter-summer transition, and favorable synoptic conditions.
Most of the damage from tornadoes is caused by the strong winds (and flying objects).
It was thought that the large pressure change (100 hPa over 100m) caused houses to explode (and hence old advice to open windows), but it is now known this is not the reason.
There
are very few measurements of tornadoes (they are localized and destroy most
instruments), so tornadoes are classified according to their destruction.
The scale used is the Fujita scale (or F-scale) which consists of 6 types: F0 – F5 (weak – strong – violent). Weak tornadoes (F0,F1) are most common (74%), and violent tornadoes (F4,F5) are rare (1%).
Weather centers put out two types of forecasts:
Tornado Watch: possibility that tornadoes may develop.
Tornado Warning: a tornado has been sighted.
Extremely strong tropical storms (winds > 120 km/hr) are called:
· Hurricanes over Atlantic and eastern Pacific Oceans
· Typhoons over western Pacific Ocean
· Cyclones over Indian Ocean and Australia
From name obvious that occur within tropics, but not uniformly:
o Not within 5O of equator.
o Not in SH Atlantic / Eastern Pacific.
o Most common in western Pacific.
Atlantic Hurricanes have a distinct seasonal cycle: June – November, with peak in Aug-Oct.
For hurricane formation need:
š
Large-scale flow
pattern
Need low level convergence and upper level divergence.
Most commonly (for Atlantic) produced by easterly waves.
š
Moist unstable
air
š
No vertical
shear
Strong vertical shear inhibits deep convection.
š
Warm ocean water
š Non-equatorial location
Corilois force needed for rotation (and prevention of “filling in” of low pressure).
All above conditions are needed for formation. Hence lack of hurricanes near equator and in cooler tropical oceans.
Start as regions of convective activity (“tropical depressions”), which intensify and gain cyclone rotation (“tropical storm” when winds > 60 km/hr). Tropical storm becomes a hurricane when winds > 120 km/hr. Names are also used in demise of hurricanes.
Because all above conditions are required only a small percentage of tropical depressions turn into hurricanes.
When tropical storm is formed, it is named …
Winds weaker than tornadoes but are larger (~600km) and last longer (~week).
General wind flow is low-level convergence, rising in convective clouds, and divergence at upper levels.
Ascent occurs in a spiraling ring of towering thunderstorms (the “eyewall”), where the maximum winds and precipitation occur. Eyewall is around 10-20 km from center.
Inside eyewall is a distinctive region of nearly clear skies and slow descent … the “eye”.
Beyond hurricane’s cloud mass there is large scale descent -> clear skies which make the hurricane prominent.
Hurricanes have a warm core (subsidence in eye), and strongest pressure gradients and winds occur near the surface.
Hurricanes obtain their energy from latent heat release by condensation (hence need for deep layer of warm water).
Surface air gains heat as swept towards hurricane center.
As air rises in eyewall it cools moist adibatically.
At top of hurricane air diverges, losses energy through longwave radiation, and subsides.
If air subsides all the way to surface, loop begins again.
Movement of tropical depressions is dominated by trade winds, i.e. systems migrate westward.
But influence of trade winds diminish as become tropical storms … location of warm waters and the upper-level winds become important.
Hurricanes move on erratic paths, but in general they move west then northward, and eventually northeastward (effect of upper level westerlies).
Hurricanes diminish whenever
Move over ocean water that cannot supply warm moist air.
Move over land (no moisture)
Reach a region with unfavorable large scale flow.
The Saffir-Simpson scale is used to rank hurricanes (scale 1 to 5).
The damage due to a hurricane can be divided into 3 classes.
Storm Surge:
š most devastating damage
š 60-160 km wide dome of water that sweeps coast
š caused by strong onshore winds and low pressure
š largest of right side of hurricane
Wind Damage:
š winds > 110 km/hr cause extensive damage
š largest of right side of hurricane
š also can spawn tornadoes
Flooding:
š heavy rains can lead to inland flooding
A hurricane watch is issued for coastal areas where hurricanes pose a threat within 36 hrs.
A hurricane warning is issued for areas where winds > 110 km/hr are expected within 24 hrs.
To
illustrate the above we examine Hurricane Andrew, August 23-28, 1992.
š
Days 1-3:
Tropical Storm Genesis.
Satellite images show that
Easterly wave sweeps off African continent.
Clouds form in convergence in easterly flank of wave
Distinct cyclonic structure, with spiral bands, by day 3.
Weather reconnaissance aircraft flights begin on day 2.
š
Days 4-5: Slight
Strengthening.
Convection builds, and on day 4 winds > 60km/hr and becomes a tropical storm.
Storm receives name “Andrew” (first of year).
š
Days 6-7: Andrew
Tracks Northwestward.
Andrew tracks northward around day 6, and is more organized (although aircraft reconnaissance shows high pressure of 1015hPa).
Andrew weakens slightly on day 7 (weaker winds, stronger p).
š
Days 8-10:
Andrew becomes a Hurricane.
A dramatic change occurs on day 8. Upper level winds shift (westerley->easterly), and Andrew moves into a region of easterly flow at nearly all levels. => deep convection occurs.
On day 9 Andrew becomes a hurricane.
By day 10 pressure has dropped to 925hPa and winds increased to 240 km/hr. … Category 4 hurricane.
From days 8 to 10 path turned from northwest to southwest and Andrew is heading toward Florida.
š
Days 11-14:
Landfall and Dissipation.
Florida landfall occurs on day 11. Southern Florida hit with extreme winds (>200 km/hr) and torrential rain.
As Andrew moves over Florida it weakens to Category 3, but when it moves over warm water of the Gulf of Mexico it re-intenisfies.
Between days 12 and 13 dramatic change in path, and Andrew head northward. Andrew makes second landfall (over Louisiana) on day 13.
As Andrew moves north over continent it weakens dramatically, and “dies”.