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University of Ljubljana Faculty of Mathematics and Physics
Chair of Meteorology
Seminar
LEE CYCLOGENESIS
Jože Baša Supervisor: doc. dr Mark Žagar
Ljubljana, May, 2007
2
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
3
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.
4
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).
5
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
7
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
8
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
10
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).
12
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.
13
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.
14
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.
16
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/