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, e- mail me with a topic you’re considering, and we’ll figure it out from there Due no later than December 15 th ! (the last day of class)

Review of November 17 th : 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 Different Airmasses Fronts Asymmetric Very Large Mid-latitudes ~ 30º N ~ 50º 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

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

Thunderstorms Air near the center is heated much more than air at a distance

Air converges at the surface Air diverges at the top

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:

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