Transcript November 10th: Severe Weather Continued

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• Exam Results
• Median grade: 75%
(88.88% for exam 1)
Range
100%
95-99%
90-94.9%
85-89.9%
80-84.9%
75-79.9%
70-74.9%
65-69.9%
60-64.9%
55-59.9%
50-54.9%
Total
1
1
15
10
37
43
20
30
17
23
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
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
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