Transcript Lecture PowerPoint

Norwegian Cyclone Model Review
Vilhelm and Jacob Bjerknes; Halvor Solberg
Bergen Geophysical Institute ~1920
Before Bjerknes, maps
had no fronts.
There were Highs
and Lows and
even isobars.
So the big
breakthrough of
the Norwegian
Model was fronts.
How did they
come up with that
idea?
In war, up to the 20th century armies were small and wandered around the
countryside looking for enemies to fight.
The battle of
Gettysburg was a good
example. The
Confederate Army was
moving through
southern Pennsylvania
near Washington D.C.
When they spotted the
Union army, they
attacked from the north!
Fronts were a
military concept
during World War I
when armies grew
large enough to
occupy whole
regions
This is the western
front. All troops
on the German
side are alike.
Notice how the
allies also group
together. The big
changes occur
across the front.
The German uniform of WWI
French infantry
Trenches at the
front
Building on his
father’s concepts,
Jacob Bjerknes and
his co-worker Halvor
Solverg proposed a
structure.
The storm doesn’t
look precisely the
way we draw them
now but the main
features are there.
This is how we plot
the features of a
typical cyclone.
Station data is
exaggerated to
show the contrasts.
Air mass types and
the approximate
extent of the region
of overcast skies
are also shown.
Question for the class:
They had maps in 1888.
They had weather
observers. They had
telegraphs and could get
data from across the
country.
Why was the
development of the
Norwegian model of an
extratropical cyclone
such an important
breakthrough for
meteorologists? (It was
so good, we still use the
model today)
Bjerknes and Solberg also
proposed that the cyclones
went through a common
evolution of stages.
In the next slide, a loop of
surface maps shows a storm
going through those stages.
Watch the Low which starts in
western Kansas.
You should also check the
temperatures in the air
masses. Maybe the schematic
wasn’t so exaggerated after all.
Stages of the typical
Extratropical Cyclone
The President’s Day Storm of 2003
Rutgers U
Times Square
Philadephia
The White House
NYO
8.5
Surface Map Loop for Feb 15-18, 2003
To watch for:
Developing waves in Arkansas
Mature cyclone
New Low forms offshore by Carolinas
Arctic air mass with High over New England
Coastal Low
forms
Mature Cyclone
Developing Waves
So the storm was in Arkansas but stalled. A new
low formed off the east coast near North Carolina
and became an intense storm.
Why did it do that? Specifically, we often see storms
travel north along the coast. We call them “Coastal
Storms” (yup). What possible reasons for that
behavior can you think of?
Satellite Loop
Bjerknes didn’t have one. If he had, he would have seen some
characteristic features which show what kind of weather is occurring.
To watch for:
The cloud shield of the warm front is spread out
The cold front has a narrow strip of cloud
Development of “Dry Tongue” behind system.
Backlash means clouds behind the system
Warm front cloud shield
Backlash
Dry Tongue
Cold front cloud shield
Upper Air
This is another asset that didn’t exist in the early 20th century.
Watch the following loop of 300 mb maps.
The solid lines are height lines (like isobars)
The blue shading shows the wind speeds
How does this loop help us to forecast the track of the
surface storm?
Radar Loop
In the Radar loop you can identify more characteristic features of the
Norwegian Model.
What is this?
Class: Identify two features from the 03Z Radar:
Cold air mass with a pressure
trough
Cold front or
pressure trough?
Here are two more. Identify
What is important about those features?
Precip in High pressure ridge
Overrunning
Precip in NJ is closer to the High
than to either Low. Look at the
weather! (not just the fronts)
T-Storms in warm
sector in a squall
line - ● ● -
This symbol means squall line
One more frame:
Backlash is just SW-
Old, dying Low
has very little
precipitation
Coastal
Low, strong
echoes
Bjerknes’ cyclone model was intended to make forecasts.
He had no radar, no satellite, no upper air. So, using the
surface map only, what does the Norwegian Cyclone model
say will happen to the storm near the Gulf of Mexico?
That was a very similar situation to the Feb 2003 storm. Here’s
what it did. (Is this storm evolving as Bjerknes said it would?)
Upper air (300 hPa or mb) in Feb, 2004:
In 2004, the initial storm stayed strong and the coastal storm formed
farther north, over New Jersey. Bjerknes’ model worked but not the
same way as in 2003. What was the difference?
Possibilities (pick one or more):
1. The atmosphere can have a big reaction to a small
difference in initial conditions.
2. The air mass difference was more pronounced in 2003.
3. The jet stream was different.
4. The upper air trough was different.
5. In 2003 the High was much more powerful.
Answer: ALL of these contribute to differences
in the development and tracks of storms.
So, what do these cases tell us about the Norwegian Model?
Every extratropical cyclone in the U.S. has certain features in common
(besides the obvious):
Fronts:
stratiform precipitation shield with warm front
cumuloform precipitation with cold front
Cyclone goes through life cycle from wave on front to dissipating
Every extratropical cyclone is different in some way
You must forecast each on a case-by-case basis
Use as many data sources (maps, numbers, etc.) as you can.
Next we examine the fronts themselves in more detail