Transcript November 19th: Tropical Cyclones

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• Extra Credit Offer:
- If you’re interested in an extra credit project, email me with a topic you’re considering, and
we’ll figure it out from there
• Due no later than December 15th! (the last day
of class)
Review of November 17th: Thunderstorms
Continued
• Supercells are small storms that can
produce tornadoes and large hail
• They often have a “hook” on their
southwest side where any tornado would
likely occur
• Tornadoes can be identified on doppler
radar by a velocity couplet
• Tornadoes are given ratings from EF0
(weakest) to EF5 (strongest) depending on
how much damage they cause
Review Continued
• Tornadoes form when:
- Wind shear produces a horizontal rotating tube
of air
- Then the updraft of the storm will turn this
rotating tube vertical, allowing a tornado to
develop
• The location where the tornado meets the base
of the cloud often has a wall cloud, which is a
location where the cloud base is a little lower
• Seeing a wall cloud is a good indication that a
tornado may be forming
A Squall Line
• Dry air coming in behind the storm causes rain to evaporate,
quickly cooling the air
• This makes the air very dense, causing it to sink rapidly to
the surface, which leads to strong straight-line winds
Approaching Squall Line
Cold Air
Incoming Dry Air
Squall Line Motion
Cold Air
A Squall Line
• Dry air coming in behind the storm causes rain to evaporate,
quickly cooling the air
• This makes the air very dense, causing it to sink rapidly to
the surface, which leads to strong straight-line winds
Hurricanes!
(a.k.a. Tropical Cyclones)
A Review of Extratropical Cyclones
• Recall that extratropical cyclones:
- Frequently form in the mid-latitudes
- Develop from pre-existing horizontal
temperature gradients
- Have fronts
- Are not very symmetric
- Have sizes up to 1000 or more
kilometers in any direction
An
Extratropical
Cyclone
Different Airmasses
An
Extratropical
Cyclone
Different Airmasses
Fronts (temperature
gradients)
An
Extratropical
Cyclone
Different Airmasses
Fronts
Asymmetric
An
Extratropical
Cyclone
Different Airmasses
Fronts
Asymmetric
Very Large
An
Extratropical
Cyclone
~ 50º N
Different Airmasses
Fronts
Asymmetric
Very Large
Mid-latitudes
~ 30º N
Then What Are Tropical Cyclones?
• In many ways, tropical cyclones have the opposite
characteristics of extratropical cyclones:
- They mostly form between 30º N and 30º S
- They develop in areas without a horizontal
temperature gradient
- They do not have fronts
- They are quite symmetric
- They typically can only strengthen when over
warm water, and weaken over land
• However, like extratropical cyclones, tropical
cyclones are also areas of low pressure
Hurricane
Isabel - 2003
The Storm
Center, or “Eye”
How Do Tropical Cyclones Develop?
• First, tropical cyclones require warm water
(typically greater than 80º F)
• The environment must have little vertical
wind shear (change in wind speed or
direction with height)
• Some initial disturbance, such as a complex
of thunderstorms, which may slowly develop
• If these conditions persist for several days, a
tropical cyclone may form
Common Development Regions Are In Orange
Development Frequency
Tropical Cyclone Frequency In The
North Atlantic
How Do Tropical Cyclones Strengthen?
• Most simplistically, tropical cyclones
strengthen and maintain themselves through
latent heat release
• Large quantities of latent heat release warm
the air, causing it to be less dense than
surrounding air
• The ideal gas law tells us that the pressure
should then decrease
• The storms must be over a warm ocean
surface to supply enough moisture
A 3-D Look
Air near the center is heated much
more than air at a distance
Thunderstorms
Air diverges at the top
Air converges at the surface
Comparison of
extratropical
vs. tropical
pressure
distributions
Stages Of Hurricane Development
• So far, only Hurricanes have been shown
• However, when these disturbances first
appear, they are called “Tropical
Depressions” and do not receive a name
• Once the disturbance is sufficiently strong, it
is deemed a “Tropical Storm” and given a
name, which occurs when the surface winds
become stronger than 39 mph
• After further strengthening, the system is
upgraded to a “Hurricane” after the surface
winds strengthen to greater than 74 mph
From: http://www.pennwellblogs.com/tropics_watch/wp-content/uploads/2008/10/saffir-simpson.gif
Storm Surge Explained
• As the winds converge toward the storm center,
they help “pile up” ocean water
• Also, the decreased surface pressure raises the
sea level slightly
• Strong waves also can increase the water height
What Causes The Most Damage?
• Many different aspects of a hurricane can
cause damage:
- Very strong winds can damage structures
- Heavy rainfall can cause flooding
- Storm surge associated with rising ocean
waters can inundate low-lying areas
- Occasionally, lightning strikes and/or
tornados can also cause damage
The Answer Is: Storm Surge
• Generally storm surge causes the most
damage and deaths, particularly in strong
hurricanes
Damage from Hurricane Ivan