Download Overview of today`s lecture

Overview of today’s lecture
ƒ What is a mid-latitude cyclone? What are the main
components?
ƒ A (brief) history of where this idea came from
ƒ How do MLCs form (i.e. what are the favorable
“ingredients”?)
ƒ What’s the weather like around an MLC?
ƒ The lifecycle of an MLC: from birth to death
ƒ Where do MLCs like to form? What are their most
common tracks?
ƒ Belts, dry slots, and high wind: the MLC in 3D!
ƒ Synthesis of the MLC and its effect on NC
What’s a Mid-latitude Cyclone?
Cyclone = a closed, cyclonic (N.H.) circulation; ascent of air
Mid-latitudes = 30° to 60° latitude
Associated with stormy weather
Other names:
Extra-tropical cyclone (i.e. NOT tropical = frontal)
Depression (area of low pressure)
Wave cyclone (Rossby wave connection)
ƒ Geographical labels:
“Gulf Low”
“Alberta Clipper”
“Panhandle Hook”
“Hatteras Low” or “Nor’easter”
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Where it all began: The Bergen School
(1920s Norway)
ƒ Early meteorologists: Wilhelm Bjerknes, Jakob Bjerknes,
Halvor Solberg, and Tor Bergeron
ƒ Back in the early 1900s, meteorologists used to wear white
lab coats! No joke!
ƒ Used conventional surface weather observations (e.g.
temperature, wind) and cloud patterns to describe the
structure, weather, and lifecycle of migratory storm
systems (i.e. MLCs)
ƒ Developed the Polar Front Theory = emphasizes the
importance of fronts (from WW I lingo)
ƒ Saw the MLC primarily in 2D space; at that time could not
explain the airflow of the MLC in 3D space (more later…)
Components of the MLC
Frontal boundaries:
Initially a polar front (stationary)
Then a wave develops along the
polar front and we get a…
Warm front
Cold front
N↑
Other components:
• Low pressure center (i.e. depression)
• Air masses (cP and mT primarily) and air flow
• Clouds and precipitation
Occluded front (dying)
Fronts encourage lifting of air =
clouds and precipitation
Air masses don’t mix! Warmer
air will lift over colder, denser air
Where do MLCs form?
What are the “ingredients”?
Beneath the jet stream (Rossby waves) and downstream of a trough
Colder air
Strong contrast
in temperature =
strong jet stream
Warmer air
Can you locate the Jet Stream just by
looking at the 500 mb temperature?
MLC forms
downstream
of trough
Much larger temperature gradient in the mid-latitudes during the winter
Large temperature gradient = large density gradient = large pressure gradient
Why is this a favorable position?
Upper level divergence = rising air = surface pressure falls
What’s the weather like around an MLC?
Wind flow directs warm
and cold air masses
around the system
Cold
Cool
Warm sector
(and humid, mT)
Cold sector
(and dry, cP)
Cool sector
(either humid/dry)
Warm
What are the cloud and precipitation
patterns around an MLC?
“Comma Pattern”
1 “Overrunning”
“Wrap 3
Around”
3
2
2
“Cold Frontal”
1
From Birth to Death: The Lifecycle of an MLC
A
B
C
Time between
stages = 1-2
days
Polar
Front
Frontal
Wave
Steering Wind at 500 mb
D
E
F
Full lifecycle =
as much as
7-10 days
Where do MLCs like to form?
1. “Alberta Clipper”
• Generally fast-moving
L
• Jet stream displaced to
the north
• Light precipitation
▲
• NC generally on the warm
and dry side of the MLC
• Sometimes an MLC forms
off the NC coast as the
clipper moves into the
Northeast
1. “Alberta Clipper”
Where do MLCs like to form?
2. “Panhandle Hook”
• Forms in the lee of the
Rocky Mountains
• Often initiated from
“shortwave troughs”
embedded in the jet stream
L
▲
• Draws in moisture from the
Gulf of Mexico = heavier
precipitation
• Along with the Clipper,
responsible for Great Lakes
windstorms in the fall and
winter
• NC located in the cool
sector, then the warm sector
2. “Panhandle Hook”
Where do MLCs like to form?
3. “The Gulf Low”
• Heavy precipitation along
and north of the main track
• Draws in moisture from
both the Gulf of Mexico and
the warm water off the
Atlantic (south of Hatteras)
▲
L
• Often follows an outbreak
of cold air across the
central and eastern U.S.
• NC remains in the cool
sector and may experience
a snow storm or ice storm
Remember the miserable, cool rain we
had last week? That was the product of
a Gulf Low…
Where do MLCs like to form?
4. “The Hatteras Low”
• Gets its name from its
genesis just off the NC
coast near Cape Hatteras
▲
• Forms in response to a
strong thermal gradient
between the warm Gulf
Stream and cold land mass
L
• Generally gives us windy
conditions with light/mod
precipitation in NC
• However, it makes
forecasters in the
Northeast sweat = SNOW!
4. “The Hatteras Low”
The Conveyor Belt Model: The MLC in 3D
Now, how does the Conveyor Belt model help
explain the cloud and precipitation patterns of
the MLC?
Stratospheric Intrusions and Dry Slots:
Is that why it was so windy???
9-10 km
Adapted from Ackerman and Knox, 2003
Water vapor imagery
Gray = dry air
Blue/green = coldest clouds; heavy precipitation
Synthesis of the MLC and the
Situation in North Carolina:
ƒ Let’s assume a “Gulf Low” is tracking through the
Deep South and heading up the TN River and
Ohio River Valleys
ƒ Where will it be cold and dry?
ƒ Where will it be warm and humid with
thunderstorms?
ƒ Where will it be cool and overcast with rain?
ƒ What types of clouds, if any, will be around?
ƒ From what direction will the wind be blowing?
Temp/Dew Point
1) 45/28
C
2) 71/68
3) 47/40
A
B
C
Temp/Dew Point
1) 45/38
A
B
D
2) 70/68
3) 43/40
4) 62/53
E
C
Temp/Dew Point
1) 50/34
A
2) 48/38
B
D
3) 57/46
4) 46/36
5) 47/45
Want to see the MLC really come to life?
ƒ Some cool graphics and animations from
Penn State…
https://www.e-education.psu.edu/gened/meteo101/Examples/Section9p02.html
ƒ It’s been pretty darn cold…aren’t we due for
an MLC here pretty soon?...
http://www.hpc.ncep.noaa.gov/basicwx/day0-7loop.html