AOS 100: Weather and Climate Instructor: Nick Bassill Class TA: Courtney Obergfell

Miscellaneous Homework due next Tuesday Extra Credit Offer: - A brief paper (roughly 3-4 pages), with references on a weather topic of your choice - However, it must be related back to something we’ve done in class - I will grade it, and the grade will equal 50% of your lowest exam grade - More details to follow soon!

Review of November 10 th : Thunderstorms Thunderstorms are simply storms which include thunder Most thunderstorms aren’t severe (i.e. damaging), although they may have gusty winds, heavy rain, or small hail Severe thunderstorms can have strong winds, large hail, or tornadoes, and require specific atmospheric conditions to exist Areas in the southeastern part of the country have the most thunderstorms per year, but areas in the central plains have the most severe thunderstorms per year

Review Continued We commonly call non-severe thunderstorms “air-mass thunderstorms” These thunderstorms begin with warm, moist air rising (the “updraft”) However, once rain starts falling, the updraft is cut off, and the air near the surface is cooled (by evaporation) This causes the thunderstorm to dissipate after about an hour

Review Continued Severe thunderstorms generally require certain atmospheric characteristics: large values of CAPE, wind shear, and lots of moisture Wind shear can be speed shear, directional shear, or both Veering winds indicates the winds rotate clockwise with height, and backing winds indicate the winds rotate counterclockwise with height Severe weather is generally never found with backing winds (either veering winds or just speed shear is most conducive)

Speed Shear

Directional Shear

From:

Air Mass Thunderstorms The cold air from the rainfall cuts off the updraft, causing the thunderstorm to die

Severe Thunderstorms For severe thunderstorms, the change in the wind with height forces the downdraft to be separate from the updraft, which allows the storm to live longer and become severe

Common Setup

Detecting Severe Weather For meteorological considerations, there are two primary types of remotely sensed observations: (1)RADAR (2)Satellite

RADAR RADAR is an acronym for “RAdio Detection And Ranging” RADAR uses radiowaves or microwaves to detect objects RADAR dishes send out a pulse of electromagnetic radiation, which can be reflected back by objects The length of time it takes for the pulse to return, as well as the strength of the return pulse indicate how near/far and how big the object(s) are - From:

From: ay/Dictionary/radar/DI90G1.jpg Pictures

Thunderstorms over Michigan

Doppler RADAR Doppler RADAR makes use of the “Doppler Effect” to determine whether objects are traveling towards or away from the radar site Doppler RADAR measures the change in wavelength of the incoming signal (compared with the signal that was sent out)

Velocity (from the Doppler RADAR) This feature is extremely useful for detecting tornadoes – many tornadoes are first “detected” using this method

Types of Severe Weather For our purposes, we can consider two types of severe weather: (1)A squall line is mostly linear continuous line of thunderstorms associated with speed wind shear, and they often produce strong straight line winds (2)Supercells are smaller storms associated with directional wind shear, and often cause tornados and large hail

A Squall Line

A Supercell

Reflectivity: Velocity: From: We call this feature a “velocity couplet”, and it often signifies a tornado

Tornadoes Tornadoes are caused by the creation of a rotating updraft

Fig , p. 402 A Supercell (with tornado)

TornadoWall Cloud }