AOS 100: Weather and Climate

Instructor: Nick Bassill

Class TA: Courtney Obergfell

Miscellaneous

• Exam Results• Median grade: 75%

(88.88% for exam 1)

Range Total

100% 1

95-99% 1

90-94.9% 15

85-89.9% 10

80-84.9% 37

75-79.9% 43

70-74.9% 20

65-69.9% 30

60-64.9% 17

55-59.9% 23

50-54.9% 12

Review of November 5th: The LCL, LFC, and CAPE

• The Lifted Condensation Level (LCL) tells you where clouds would form if you forced air to rise

• The Level of Free Convection (LFC) indicates where the parcel will become warmer than the environment around it, and will rise on its own

• Convective Available Potential Energy (CAPE) indicates how much energy a parcel has to create a strong updraft. It represents the area between the temperature line and the parcel’s temperature

An Example

The red arrow indicates the LCL, while the yellow arrow indicates the LFC

From: http://www.meteo.mcgill.ca/wxlab/ATOC-546/notes/lesson07.convection/2002091812.mod.gif

Mountain Effects and Temperature

• The air ends up warmer due to the extra latent heat release

Thunderstorms

• Thunderstorms (rain, lightning, gusty winds, etc.) are most commonly not severe

• We call a thunderstorm “severe” when it has very strong winds, large hail, or tornadoes

• Severe thunderstorms require certain atmospheric conditions

• Non-severe thunderstorms are commonly called “air mass” thunderstorms

Thunderstorm Climatology

Thunderstorm days per year …

Hail days per year …

Tornadoes per year, per state

Tornado frequency per

month …

Forcing

• All of the things we learned about last class (the LCL, LFC, CAPE) only matter if there is something to force an air parcel to rise

• There are many different types of forcing• Some commonly associated with thunderstorms

are:- Strong surface heating- An approaching cold front- A seabreeze front- And many more … some of which we’ll learn about later

Air Mass Thunderstorms

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

Commonly Found Characteristics of Severe Weather

• (1) Large values of CAPE (as mentioned earlier) contribute to the strength of an updraft

• (2) Wind shear is the change in wind speed or direction with height- Speed shear often causes severe weather (often called a “squall line”) with strong straight-line winds- Directional shear often causes supercells and tornadoes by causing the updrafts to rotate

• (3) Large amounts of moisture often allow for the LFC to be reached at a lower level in the atmosphere, and thus often result in more CAPE

• In order for severe weather to occur, generally all of these need to be present in some form

Directional Wind Shear

• We would say that the wind is “backing” if the wind direction rotates counterclockwise with height

• We would say that the wind is “veering” if the wind direction rotates clockwise with height

• Generally, severe weather will only be found if the wind is veering with height

Speed Shear

Directional Shear

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

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: http://en.wikipedia.org/wiki/Radar

http://www.bergenskywarn.org/Pages/BergenSkywarnUptonOpenHouse2001.htm

From:http://www.centennialofflight.gov/essay/Dictionary/radar/DI90G1.jpg

Pictures

http://www.tropicalstorms.us/current/radar.gif

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)

http://www.grc.nasa.gov/WWW/K-12/airplane/Images/doppler.gif

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

http://apollo.lsc.vsc.edu/classes/met130/notes/chapter14/graphics/squall_line_ex/squall_line1.gif

http://www.ucar.edu/news/releases/2004/images/bowecho.radar.gif

A SupercellReflectivity: Velocity:

From: http://kkd.ou.edu/METR%202603/supercell%20velocity.jpg

We call this feature a “velocity couplet”, and it often signifies a tornado