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Seven sins in dynamical meteorological education

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Seven sins in dynamical meteorological education

The mathematics is always correct, the computers are given the right equations, but the explanations do not only contradict Nature but also the mathematics they are supposed to illuminate

Seven sins in dynamical meteorological education

Professor Richard Reed, Univ. Of Seattle 1988:

-Our understanding of the cyclogenesis process has increased tremendously during the last 40 years – at least the computers seem to understand!

3. The Rossby Wave

“-I have never understood what a Rossby waves is…”

Professor Harold Jeffreys on his deathbed 1987

Lunch discussion at ECMWF 1995:

Scientist: -How is the weekend going to be?

AP: -Fine, a Rossby wave is seen coming in!

Scientist: -But you can’t see Rossby waves??

-Well, Rossby could see them - at least on Christmas Day 1940

Rossby’s wave formula (inspired by Ekman, 1932)

C= phase speed, U= zonal flow at 500 hPa, L=wave length, =df/dy

c < 0 for large L c >0 for small L

How the Rossby wave was initially misunderstood by Rossby himself

The isobaric channel illustration used by

Rossby et al (1939)

Only when the paper was published did Rossby realize that he could not use gradient wind balance - it is only applicable on stationary patterns

Jack Bjerknes´ 1937

gradient wind explanation of the progression of waves

L

Conv

Div

H

H

Short waves - the curvature effect dominates

Carl Gustaf Rossby et al (1939) used

Bjerknes´ gradient wind idea to illustrate the retrogression of waves

High latitude

L

Div

Conv

H

H

Low latitude

Long waves - the latitude effect dominates

A very common misunderstanding:

This is NOT a Rossby wave!

low high f

+f==const

high low f

…but a Constant Absolute Vorticity Trajectory!

There are 5-6 other misleading or erroneous explanations of the Rossby wave before it finally was explained in a kinematically consistent way

Rossby (1940), Petterssen (1956), Persson (1993)

Very few seem to have taken

notice of a Rossby (1940)

correction - and even fewer

understood what he meant

(Trajectories

represented by

PV isolines)

Relation between stream lines and

trajectories in a progressive flow

L

H

H

Larger amplitudes

and wave lengths

Relation between stream

lines and trajectories in a

retrogressive flow

L

H

H

Shorter amplitudes

and wave lengths

Let us go back to the Constant Absolute Vorticity Trajectory (which is NOT a Rossby wave)

+f==const

Rossby asked: -Which non-stationary streamlines would correspond to this trajectory?

One and the same CAV trajectory satisfies two types of streamlines (waves)

+f==const

Short progressive waves

Long retrogressive waves