Transcript Begin Severe Weather

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• New Homework
• Exam Results
Review of October 29th: Clouds and the
LCL
Adiabatic Lapse Rate Mixing Ratio
Moist Adiabatic
Lapse Rate
Temperature
Dewpoint
Temperature
Cloud Formation
• Soundings can be used to figure out not
only where clouds currently are, but where
they would form
• The Lifted Condensation Level (LCL) tells
you where clouds would form if you forced
a parcel of air to rise
• To find this, you need to know both the
temperature and mixing ratio
Continued
• In order to find the LCL, use the dry adiabatic
lapse rate (DALR) to find how far up it would
take for the parcel to cool to the dewpoint
• This is done by following the DALR up from the
temperature, and the mixing ratio up from the
dewpoint, and figuring out where they cross
• Once this happens, clouds should form
• If the parcel is forced to keep rising, it will now
cool by the moist adiabatic lapse rate
MALR
Level of Free Convection (LFC)
• The LFC is defined as “The level at which
a parcel of air lifted dry-adiabatically until
saturated and saturation-adiabatically
thereafter would first become warmer than
its surroundings in a conditionally unstable
atmosphere”
From:
http://amsglossary.allenpress.com/glossar
y/search?id=level-of-free-convection1
An Example
The red arrow
indicates the
LCL, while the
yellow arrow
indicates the
LFC
Mountain Effects and Temperature
• The air ends up warmer due to the extra
latent heat release
The same process that causes cyclones to
strengthen leads to cloud formation!
On to severe weather!
Convective Available Potential
Energy (CAPE)
• CAPE is a measurement of energy available to a rising
air parcel
• Large values of CAPE (in the thousands) are often
associated with severe weather
• The value of CAPE is proportional to the area between
the actual environmental temperature profile and the
temperature profile of an air parcel
• Remember that air parcel warmer than its environment
will rise on its own, so CAPE just basically measures for
how much of the atmosphere the parcel will be warmer
(and by how much)
• Therefore, values of CAPE mean stronger updrafts
From: http://www.meteo.mcgill.ca/wxlab/ATOC-546/notes/lesson07.convection/2002091812.mod.gif
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
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