norwegian cyclone model review vilhelm and jacob bjerknes; halvor solberg bergen geophysical...

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.

French infantry

The German uniform of WWI

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 UTimes Square

The White House

Philadephia

NYO8.5

Surface Map Loop for Feb 15-18, 2003

To notice:

Lows on first frame in Arkansas are developing waves

Mature cyclone organizes in northern Alabama by 9Z Feb 16

Cyclone occludes by 18Z. New Low forms offshore by Carolinas

By Feb 17, old Low is no longer associated with a front while the

coastal Low strengthens and becomes mature.

Arctic air mass associated with High drifts over New England

Coastal Low occludes by 00Z feb 18.

Developing WavesMature Cyclone

Coastal Low forms

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:

Cloud shield is large and spread out north of warm front

Narrow overcast area with cold front.

Usually have strong enhancements with cold front (not obvious in this case)

Development of “Dry Tongue” behind system.

Backlash becomes more pronounced with time.

Warm front cloud shield

Cold front cloud shield

Dry Tongue

Backlash

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 LoopIn the Radar loop you can identify more characteristic features of the Norwegian Model.

Cold front or pressure trough

Frozen stratiform precipitation in cold airmass

Class: Identify two features from the 03Z Radar:

T-Storms in warm sector in a squall line - ● ● -

Precip in High pressure ridge

Here are two more. Identify

This symbol means squall line

Precip in NJ is closer to the High than to either LowOverrunning

What is important about those features?

Old, dying Low

Coastal Low

Strong gradient of precip from heavy snow to nothing

One more frame:

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