Transcript AOS 100: Weather and Climate

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 10th: 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: http://www.noaanews.noaa.gov/stories2009/images/statewidetrank_200910.jpg
From: http://www.noaanews.noaa.gov/stories2009/images/statewidetrank_200910.jpg
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: http://en.wikipedia.org/wiki/Radar
Pictures
From:http://www.centennialofflight.gov/ess
ay/Dictionary/radar/DI90G1.jpg
http://www.bergenskywarn.org/Pages/BergenSkywarnUptonOpenHouse2001.htm
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 Supercell
A Supercell
Reflectivity:
Velocity:
We call this
feature a
“velocity
couplet”, and it
often signifies a
tornado
From: http://kkd.ou.edu/METR%202603/supercell%20velocity.jpg
Tornadoes
• Tornadoes are caused by the creation of a
rotating updraft
A Supercell (with tornado)
Fig. 14-46, p. 402
}
Tornado
Wall Cloud