University of Ljubljana Faculty of Mathematics and Physics

Chair of Meteorology

Seminar

LEE CYCLOGENESIS

Jože Baša Supervisor: doc. dr Mark Žagar

Ljubljana, May, 2007

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Contents

1 Introduction ........................................................................................................................ 4

2 Cyclone............................................................................................................................... 4

3 Lee cyclogenesis ................................................................................................................ 6

3.1 The ALPEX experiment............................................................................................. 6

3.2 The lee cyclogenesis process...................................................................................... 7

3.2.1 Potential vorticity ............................................................................................... 9

3.2.2 Baroclinic instability ........................................................................................ 10

4 Lee cyclone example ........................................................................................................ 12

5 Conclusion........................................................................................................................ 15

References ................................................................................................................................ 16

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Abstract

A lee cyclone is a variety of a cyclone for which the mechanisms of occurrence and

development are not yet known in all details.. They tend to occur in some specific regions in

the middle latitudes, in particular in the lee of the mountains like the Alps, the Rocky

Mountains, and the Andes. This article is going to focus and discuss the lee cyclogenesis in

the Alps. We will cover: the lee cyclogenesis process, the first big Alpine lee cyclogenesis

research experiment ALPEX, look at the physics behind it and try to apply it on a real

example.

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1 Introduction

There are a number of special regions in the middle latitudes that experience an abnormally

high frequency of cyclogenetic events. Perhaps the most remarkable of these is centered on

the Gulf of Genoa just south of the French Alps. A significant step in the understanding of

this process was the Eady (1949) model of baroclinic instability, which described the unstable

growth of small disturbances on a simple baroclinic background state. This result and related

studies found immediate acceptance, as the tilted character and the energetics of these

mathematical solutions agreed qualitatively with observed systems. After several years,

however, it became clear that the application of such a simple model would always be limited,

as real cyclogenesis events rarely start from a nearly undisturbed state and, furthermore, they

are not uniformly distributed in the midlatitude region – a clue that some other controlling

factors might be involved (Smith, 1984).

The idea that the earth's mountains are partly responsible for the uneven distribution of

cyclogenesis events arises from the statistical analysis of Petterssen (1956), Reitan (1974),

Radinovic (1965), Chung (1976) and others which show several regions of high cyclogenesis

frequency located in the lee, with respect to the prevailing wind, of major mountain ranges.

A guiding principle is that lee cyclones are similar in many respects to cyclones which form

without terrain, over the sea for example. So we are going to try to find the main reason for a

lee cyclogenesis (Smith, 1986).

2 Cyclone

Cyclones are mostly circular areas of low pressure. Because of the dynamical demand of

balancing the forces in the cyclone, the wind rotates in the positive direction on the northern

hemisphere and negative or clockwise in the southern hemisphere. There are no extension and

wind speed limits for cyclones. They can be small and have high wind speeds. It all depends

on the pressure gradient and the curve radius. Cyclones have well-defined cold and warm

areas as an opposite to anticyclones which are temperature wise horizontally homogeneous

(Rakovec and Vrhovec, 2000).

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Because of the effect of surface friction and surface air convergence and also because of the

vertical divergence in front of the trough of Rossby wave, we see an air lifting. The result of

this the air spans and cools down. When the temperature reaches the dew point water droplets

are starting to appear and we get precipitations. Warm-core cyclones (such as tropical

cyclones and many mesocyclones) can have their initial start due to a nearby upper trough.

In cyclones we have the warm and the cold front. These are areas with increased temperature

gradient. In areas with big temperature gradients we get high wind speeds. Particularly above

explicitly cold fronts very strong wind can occur (Rakovec and Vrhovec, 2000).

The term cyclone covers a lot of variety of meteorological phenomena such as polar cyclone,

extratropical cyclones, tropical or subtropical cyclones, tornadoes and others.

Figure 2.3: Cyclone Catarina, a rare South Atlantic tropical cyclone viewed from the International Space Station

on March 26, 2004 (Wikipedia).

If we focus on the Europe we can count three regions where so called mid-latitude cyclones

are generated. Two region of high pressure are over the Siberia and the Azores and one region

of low pressure is over Iceland. But if we look at the statistics of generated cyclones we can

see one region around the Alps that looks also as a generating region. This is the area around

the Gulf of Genoa where so called secondary or lee cyclones appear. We do not categorize it

as a constant low pressure area, but because of the influence of the Alps in special

circumstances very low pressure can occur. This is the starting point of lee cyclogenesis

(Rakovec and Vrhovec, 2000).

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3 Lee cyclogenesis

There are about sixty occurrences of low pressure systems in the western Mediterranean

throughout a year. The relatively high frequency can primarily be attributed to the influence

of the surrounding orography on the atmospheric flow. In particular, the blocking influence of

the Alps on north-westerly air streams renders the gulf of Genoa in the Ligurian Sea a

preferred place for lee cyclogenesis. Although the phenomenon had been known since long, it

was relatively recent that conceptual and theoretical models about the formation process of lee

cyclones have been developed. The Alpine Experiment ALPEX (chapter 3.1) conducted in

1982 aimed at gaining more insight and understanding of mountain related flow phenomena

and provided much impetus for research on lee cyclones during the following years

(Tafferner, 1996).

3.1 The ALPEX experiment

The important role that mountains play in determining weather and climate over considerable

areas of the globe was recognized from the outset of a major international meteorological

research investigation, the Global Atmospheric Research Programme (GARP), whose overall

objective was to study the dynamics of atmospheric phenomena in order to extend the range

of useful weather forecasts (Newson, 1987).

The success of this fifteen-year programme, jointly organized by the World Meteorological

Organization (WMO) and the International Council of Scientific Unions (ICSU) in response

to resolutions adopted at the 16th and 17th sessions of the General Assembly of the United

Nations, has led to dramatic progress in meteorology as a whole. In particular, GARP

included a major field investigation, the 1982 Alpine Experiment (ALPEX), the aim of which

was specifically to understand the way in which air flows over or around mountains, the

development of cyclones such as those in the Gulf of Genoa, and local mountain winds.

One of the main characteristics of mountain weather is the small scale, meteorologically

speaking, of the features involved and their sudden generation and disappearance.

Accordingly, ALPEX was designed to gather sufficiently detailed information in space and

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time over the Alpine region. The meteorological services and scientific communities of

twenty nations took part in the Programme, and several years of intensive efforts and detailed

planning culminated in the implementation of a Special ALPEX Observing Period from 1

March to 30 April 1982 (Newson, 1987).

For this, the existing network of observing stations was supplemented by thirty-four

additional stations which provided many extra measurements of pressure and wind at all

levels of the atmosphere. An array of sixty microbarographs, capable of tracing with great

precision the slightest fluctuations in pressure, was set up along the St. Gotthard and Brenner

sections of the Alps. Seventeen aircraft operating from Geneva undertook numerous sorties on

predefined tracks, collecting many observations on wind speed and direction. In the

Mediterranean itself, information was gathered from eleven research vessels and many buoys,

field platforms and tide-gauges. All this was supplemented by images and atmospheric

sounding data from meteorological satellites. This extensive range of observations has been

assembled to form a unique quality-controlled internationally available data set. Never before

have observations of comparable quality and density been produced over a mountain region

(Newson, 1987).

One of the main achievements has been a greatly increased understanding of how mountains

should be treated in the numerical models of the atmosphere, now used routinely for

forecasting the movement of weather systems and the generation of new features such as

depressions and anticyclones.

3.2 The lee cyclogenesis process

Here we want to describe briefly the essential processes during Alpine lee cyclo-genesis from

a synoptic point of view and look at the physics behind the process.

Cyclones in the lee of the Alps frequently occur in consequence of an outbreak of a polar air

mass against the Alps. Prior to lee cyclone development a low pressure trough in the upper

troposphere approaches the Alps from north or north-west in combination with cold air

advection against the Alps in the lower troposphere. The prime effect of the Alps is to block

the low-level flow. Although the cold air could in principle go over the mountains, it will be

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deflected to a good part around the Alps depending on the static stability in place. This

blocking effect is often apparent in the deformation of the cold front at the leading edge of the

cold air mass. During the blocking period of about 6 to 12 hours the upper-level trough moves

over the Alps without hindrance. In this situation the three-dimensional mass balance is

disrupted because the pressure fall induced by the approach of the upper-level trough is no

more compensated by cold air advection at the ground. Therefore, a pressure fall in the lee of

the Alps is found. In principle the mass loss would be compensated as soon as that part of the

cold air which had to flow around the barrier has arrived in the lee. But secondary effects set

in which complicate the figure. It is not only the mass field which experiences a perturbation

by the mountains. At any time there is a tendency in the flow that the wind field is in balance

with the mass field. On the rotating earth this balance is called the quasi-geostrophic

relationship. As a consequence to the disturbed balance there will be forcing of upward

motion in the lee of the Alps which in turn leads to a stretching of the low level air mass.

Thereby a vortex is generated in the pressure fall area in the lee of the Alps (Tafferner,

1996)).

The crude picture given above can strongly look different depending on the flow direction

toward the Alps, the vertical depth of the cold air, the strength of the upper-level potential

vorticity maximum inside the trough, the strength of advection, the moisture supply from the

Mediterranean Sea and the state of the air mass south of the Alps. The intensity of the lee

cyclone, its life cycle and the amount of precipitation are all dependent on these flow

configurations (Tafferner, 1996).

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3.2.1 Potential vorticity

The first very important role in the lee cyclone process is the potential vorticity.

( )( )P f g constpθ

∂θ≡ ζ + − =

∂ 3.1

The quantity P is the isentropic coordinate form of Ertel' potential vorticity. It is defined with

a minus sign so that its value is normally positive in the Northern Hemisphere. According to

the formula potential vorticity is conserved following the motion in adiabatic frictionless

flow. In essence potential vorticity is always in some sense a measure of the ration of the

absolute vorticity to the effective depth of the vortex. In 3.1, for example, the effective depth

is just the distance between potential temperature surfaces measured in pressure units p−∂θ/∂ .

Fig. 3.1 Schematic view of northern flow over a topographic barrier: the depth of a fluid column as a

function of x (Holton, 1992).

The conservation of potential vorticity is a powerful constraint on the large-scale motions of

the atmosphere. This can be illustrated by considering the flow of air over a large mountain

barrier in which p∂θ/∂ undergoes a substantial change along the trajectory. In order to

appreciate some of the consequences of potential vorticity conservation in flow over

topography it is useful to consider first a simpler situation where p−∂θ/∂ is constant so that

the absolute vorticity fη = ζ + is conserved following the motion. Suppose that at a certain

point (x0,y0) the flow is in the zonal direction and the relative vorticity vanishes so that

η(x0,y0) =f0. Then, if absolute vorticity is conserved, the motion at any point along a parcel

trajectory that passes through (x0,y0) must satisfy 0f fζ + = Since f increases toward the

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north, trajectories that curve northward in the downstream direction must have 0 0f fζ = − <� ,

while trajectories that curve southward must have 0 0f fζ = − > .

So in our case as we have a southward flow situation the relative vorticity ζ is always

increasing and the Coriolis parameter decreasing (Holton, 1992).

The situation for northern flow with a vertical cross section of the flow is shown in Fig. 3.1.

We suppose that upstream of the mountain barrier the flow is a uniform zonal flow so that

z=0. If the flow is adiabatic, each column of air confined between the potential temperature

surfaces q0 and q0+dq remains between those surfaces as it crosses the mountain. For this

reason, a potential temperature surface q0 near the ground must approximately follow the

ground contours. A potential temperature surface q0+dq several kilometers above the ground

will also be deflected vertically. But, owing to pressure forces produced by interaction of the

flow with the topographic barrier, the vertical displacement at upper levels is spread

horizontally; it extends upstream and downstream of the barrier and has smaller amplitude in

the vertical than the displacement near the ground.

As a result of the vertical displacement of the upper-level isentropes there is a vertical

stretching of air columns upstream of the topographic barrier. This stretching causes

p−∂θ/∂ to decrease, and hence from 3.1 z must become positive in order to conserve potential

vorticity. Thus, an air column turns cyclonically as it approaches the topographic barrier. As

the column begins to cross the barrier its vertical extent decreases; the relative vorticity must

then become negative. Thus, the air column will acquire anticyclonic vorticity and move

southward. When the air column has passed over the mountain and returned to its original

depth it will be south of its original latitude so that f will be smaller and the relative vorticity

must be positive (Holton, 1992).

3.2.2 Baroclinic instability

Baroclinic instability is associated with vertical shear of the mean flow. Baroclinic

instabilities grow by converting potential energy associated with the mean horizontal

temperature gradient that must exist to provide thermal wind balance for the vertical shear in

the basic state flow.

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A strong dependence of cyclogenesis on initial conditions occurs when a large-amplitude

upper-level potential vorticity anomaly is advected into a region where there is a pre-existing

meridional temperature gradient at the surface. In that case, as shown schematically in Fig.

3.2, the circulation induced by the upper-level anomaly leads to temperature advection at the

surface and upper-level potential vorticity anomalies can become locked in phase, so that the

induced circulations produce a very rapid amplification of the anomaly pattern (Holton,

1992).

Fig. 3.2 A schematic picture of cyclogenesis associated with the arrival of an upper-level positive vorticity perturbation

over a lower-level baroclinic region. (a) The low-level cyclonic vorticity induced by the upper-level vorticity

anomaly. The circulation induced by the vorticity anomaly is shown by the solid arrows, and potential

temperature contours are shown at the lower boundary. The advection of potential temperature by the induced

lower-level circulation leads to a warm anomaly slightly east of the upper-level vorticity anomaly. This in turn

will induce a cyclonic circulation as shown by the open arrows in (b). The induced upper-level circulation will

reinforce the original upper-level anomaly and can lead to amplification of the disturbance (Holton, 1992).

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4 Lee cyclone example

In the earlier chapters we looked at the physic behind lee cyclogenesis. Now it would be nice

to see how it works in real life. To study a realistic example we took data from the re-analysis

project ERA-40. This is a 40 year data set that covers the period from mid-1957 to mid-2002.

The data base is accessible at the ECMWF (European Centre for Medium-Range Weather

Forecasts). The lee cyclone was generated in the first days of the ALPEX experiment in

March 1982. The data range is from 3.march 00UTC till 7.march 18UTC. We are going to

have a closer look what is happening just at the time, when the lee cyclone is starting to

develop. That is the 5.march 00UTC.

5 March 00UTC

Figure 4.1a : Temperature field with wind velocity vectors at 850 hPa show the cold air

stream from the north-west and the warm air stream from the south west toward the Alps.

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Figure 4.1b: Relative vorticity field with wind velocity vectors at 300 hPa. Here we can

see the relative vorticity advection in the upper air level on the west side of the Alps.

Figure 4.1c: Relative vorticity and pressure field at 850 hPa. The connected isobar

represent a cyclonic low pressure area.

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In figure 4 we see the generation of a lee cyclone over the Gulf of Genoa. As shown in the

theory, a lee cyclone will generate if following conditions are satisfied: big temperature

gradient at the lower air level and a vorticity advection in the upper air level. On the Figure

4.1a we can see the cold north and the warm south air flow meeting on the west side of the

Alps and generating a big temperature gradient. The fulfilling second condition can be seen

on the Figure 4.1b where we look at the upper level air mass. We can see a strong vorticity

field over France and its advection toward the Alps. That the conditions really are satisfied is

what the Figure 4.1c shows us. Looking at the lower level air mass we see in the pressure

field a connected isobar line. This represents a generated secondary of lee cyclone. This

consideration is supported with the increase of the relative vorticity over this area.

If we look back at the physic of the lee cyclogenesis, we can see that it matches with the

reality. We have the cold air stream from the north and the warm air stream from the south.

After the cold air hits the barrier it goes partly over the Alps partly around it. The upper-level

air lifting performs vertically stretching and to conserve the potential vorticity the relative

vorticity increases. A vortex on the north side of the Alps is created. On the lee side of the

Alps like shown in the Figure 4.1c we get another increase of relative vorticity because of the

downstream stretching. This is how a low pressure area is generated and consequently a lee

cyclone is born.

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5 Conclusion

There are a lot of theories that tried to explain the process of lee cyclogenesis. A lot of them

were proven wrong. Now, about twenty years after the first big research experiment of lee

cyclogenesis ALPEX, we can approximately say that we understand the main process. In this

article we looked at the present theory and their physics which explained the two main

sources for lee cyclogenesis; the baroclinic instability and the conserve of potential vorticity.

As the example shows the physic can help us explain the reality, although we can not confirm

it totally till we don't make any numerical simulations and than compare it with the reality.

But that also wasn't the purpose of this seminar. We made an overview of the complex

process of lee cyclogenesis and tried to explain it with simple physics.

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References

Jože Rakovec and Tomaž Vrhovec, 2000: Osnove meteorologije – za naravoslovce in tehnike, 2. popravljena izdaja, 202-225

Ronald B. Smith, 1986: Further Development of a Theory of Lee Cyclogenesis,

Journal of the Atmospheric science, 43, 1582-1602

Ronald B. Smith, 1984: A Theory of Lee Cyclogenesis, Journal of the Atmospheric science, 41, 1159-1168

Arnold Tafferner, 1996: Alpine Lee Cyclogenesis, Meteorological Institute, University of Munich

Roger Newson, 1987: The ALPEX experiment; an international study programme on

Alpine meteorology - 1982 Alpine Experiment, UNESCO Courier

James R. Holton, 1992, An Introduction to Dynamic Meteorology, Third Edition,

Department of Atmospheric Sciences, published by Academic Press Limited, 97-102, 228-230

Ronald B. Smith, 1979: Some Aspects of the Quasi-Geostrophic Flow over

Mountains, Department of Geology and Geophysics, 2385-2393

J. Egger, 1988, Alpine Lee Cyclogenesis: Verification of Theories, Meteorologisches Institut der Universität München, 2187-2203

ECMWF homepage. http://www.ecmwf.int/