Met Office College - Course Notes
Conceptual Models of Cyclogenesis
Contents
1.
Introduction
2.
Types of cyclogenesis
2.1
2.2
2.3
Emerging cloud head cyclogenesis
Instant occlusion cyclogenesis
Comma cloud cyclogenesis
3.
Cyclogenesis – summary
4.
Further reading
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Principal, Met Office College, FitzRoy Road, Exeter, Devon. EX1 3PB. UK.
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1. Introduction
This course note builds on the ideas described in the Met. Office College
course note entitled Conveyor Belt Conceptual Models. These two notes
should be used together.
Cyclogenesis in extra-tropical latitudes usually occurs in association
with an upper trough. The shape and orientation of this trough can vary
greatly from cyclone to cyclone. Despite this, the component parts of
most cyclogenesis events can be described in terms of conveyor belts
and dry intrusions associated with the jet streaks around the trough.
Several classification schemes have been developed which aim to
categorise cyclogenesis into characteristic types. Perhaps the most
widely used is that of Young (1993) which is described in detail in
chapter 5 of Images in Weather Forecasting. Young’s scheme can be further
simplified, and here we will look at just 3 classes of cyclogenesis, as
listed below.

Emerging cloud head cyclogenesis

Instant occlusion cyclogenesis

Comma cloud, or cold air cyclogenesis
Within these 3 categories a wide range of configurations are possible
depending on the shape of the trough and the interaction between the
different flows in the developing system.
2. Types of cyclogenesis
2.1 Emerging cloud head cyclogenesis
In this category, cyclogenesis occurs in association with a warm
conveyor belt flow on the forward side of an upper trough. Fig. 1
summarises the main stages in this type of cyclogenesis.
(i)
W1 is a pre-existing WCB on the warm side of jet J1, ahead of the
upper trough. Associated with this is a band of cloud known as the
polar front cloud band. This can be oriented meridionally as in the
satellite image in figure 3, or more zonally depending on the shape
of the upper trough. J2 indicates a jet-streak to the rear of the trough
axis, within the dry descending air.
(ii)
The first sign that cyclogenesis may occur is the development of
an ‘S’ shaped curve to the sharp poleward edge of the cloud in the
WCB. The presence of a jet left exit associated with the jet streak in
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the dry intrusion flow (J2) induces ascent and cyclonic vorticity near
the surface on the flank of the WCB. The evidence for this in satellite
imagery is the appearance of an emerging cloud head, formed by a
branching of the WCB at low levels, labelled W2. The cloud tops in
this emerging cloud head will typically be lower than the cloud in
W1.
(iii)
At a later stage the emerging cloud head may be reinforced by
the development of a cold conveyor belt flow (CCB) feeding
moisture into the cloud head. The presence of a CCB also indicates
easterly system relative flow. The dry intrusion is now cutting into
the centre of the system between W1 and the cloud head. This is
often evident as a clear slot on IR imagery or a dark slot on WV
pictures. If this dark slot becomes darker with time it indicates that
the dry intrusion is descending to lower levels of the system,
possibly undercutting the high w air in W1/W2, forcing more
intense ascent and deepening of the system.
Figure 1. Emerging cloud head cyclogenesis. J2 is the jet streak
associated with the dry air to the rear of the upper trough. W1 is the
pre-existing warm conveyor belt flow associated with jet J1. W2 is the
secondary branch of the WCB which initially forms the emerging
cloud head. CCB is the cold conveyor belt which develops once
cyclogenesis is underway, indicating system-relative easterly flow at
low levels.
Figure 2 shows an IR satellite image of a developing cyclone at the stage
of fig. 1(ii). At this stage the depression centre is located beneath the
WCB at L, but as development continues the low centre will move
towards the poleward edge of the WCB, eventually moving into the
cloud free dry slot between the polar front cloud band and the emerging
cloud head.
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Figure 2. Emerging cloud head cyclogenesis as seen in an IR satellite
image. This stage corresponds to fig. 1(ii). F indicates the polar front
cloud band associated with the WCB. E indicates the emerging cloud
head. At this stage the low centre is located at L.
Young’s scheme further divides this category of cyclogenesis depending
on whether the trough is confluent, diffluent or symmetrical. However,
all 3 types exhibit similar signals at the early stages of development. In
the case of a diffluent trough the W2 flow is likely to become more
extensive as development continues and is often over-run by the dry
intrusion producing a split cold front.
2.2 Instant occlusion cyclogenesis
A long wave upper trough will often have a short wave trough
embedded in the flow on the upstream side. On the forward side of this
short wave trough there will be an area of positive vorticity advection
(PVA) leading to ascent. This is often shown in satellite imagery by an
area of enhanced convection in the cold air, with higher cloud tops than
the surrounding cumulus cloud cells. Instant occlusion cyclogenesis
involves an interaction of this cloud area in the cold air and the main
polar front cloud band conveyor belt. The sequence of events leading to
cyclogenesis is shown in fig. 3.
(i)
Although the cloud area in the cold air has convective cells
embedded within it, it still shows some characteristics of a WCB,
with slantwise ascent along the isentropic surfaces. This WCB is
labelled W2. W1 is the main polar front cloud band WCB.
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(ii)
As W2 rounds the base of the long wave trough and approaches
W1, new cloud is observed in the gap between the two. This shows
that a CCB flow is emerging from beneath W1. PVA and warm
advection associated with W2 have induced low level cyclonic
vorticity and a system relative easterly flow.
(iii)
Eventually W2 merges with W1. The resulting cloud formation
resembles the Norwegian model occlusion of a mature depression,
but studying a sequence of imagery shows that the processes which
lead to this state are completely different to the Norwegian model
occlusion process. Once again, the dry intrusion is cutting into the
system between the cloud head and W1.
Figure 3. Instant occlusion cyclogenesis. J2 is the jet streak associated
with the dry intrusion flow. W1 is the main WCB associated with jet
J1. W2 is a WCB associated with a short wave trough embedded in the
flow to the rear of the long wave trough axis, which then moves
around the base of the main trough. CCB is the cold conveyor belt that
forms once cyclogenesis is under way.
Figure 3(iii) is very similar to fig. 1(iii) for the case of emerging cloud
head cyclogenesis. However, in this case the cloud head did not emerge
from beneath W1, but started out as a completely separate entity in the
cold air.
Figure 4 shows an IR image prior to instant occlusion cyclogenesis. C is
the cloud area associated with W2 and F is the polar front cloud band
associated with W1. As the image shows, C consists of a merged area of
convective clouds, but there is also evidence for slantwise conveyor belt
ascent. The cloud tops to the south west of C are low, and there appears
to be a coherent band feeding into C. The highest cloud tops are on the
northern side of C.
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Figure 4. IR satellite image prior to instant occlusion cyclogenesis. C is
the cloud area in the cold air associated with W2. F is the polar front
cloud band associated with W1.
The dry intrusion between W1 and the cloud head can result in overrunning and potential instability on the inside edge of the cloud head.
Mass ascent of this potentially unstable air can result in the rapid
formation of heavy convective precipitation and thunderstorms. Such a
case is shown in fig. 5, with convective cloud forming in the dry slot of
an instant occlusion.
Figure 5. An instant occlusion cyclone in IR imagery. The dry
intrusion is over-running air in the cloud head (C), resulting in
potential instability and CB activity indicated by CON.
In Young’s classification scheme, a further type of cyclogenesis is
described which is a combination of the emerging cloud head and
instant occlusion types. The cloud area C with its associated forcing for
cyclonic development moves towards W1, but in this case, a cloud head
emerges from beneath W1 without the two cloud areas (C and F)
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merging. Cloud area C often begins to dissipate at this stage. Young
describes this as induced wave cyclogenesis as the ascent associated with C
induces a wave on F without the two areas merging. This type is
described more fully on pages 259-266 of Images in Weather Forecasting.
2.3 Comma cloud cyclogenesis
This category of cyclogenesis occurs entirely within the cold air to the
poleward side of the polar front cloud band and so is sometimes
referred to as cold air cyclogenesis. Figure 6 illustrates the sequence of
events leading to cyclogenesis in such a case.
(i)
This picture is identical to fig. 3(i) for the case of instant occlusion
cyclogenesis. Once again there is a WCB (W2) associated with a short
wave trough to the poleward side of the main WCB W1.
(ii)
W2 with its associated cloud area, which often resembles a
comma punctuation mark, moves around the base of the long wave
trough. Studies suggest that if W2 approaches to within 300km of
W1 then cyclogenesis will occur beneath W1 as in the instant
occlusion case. However, if W2 remains more than 300km to the
poleward side of W1 then cyclogenesis will occur beneath W2
without any interaction with the main polar front cloud band.
(iii)
Once W2 has rounded the base of the trough, the cloud area
expands due to mass ascent and convection, and may start to rotate
cyclonically as it moves. This indicates that cyclogenesis has
occurred beneath W2, independently of the polar front cloud
associated with W1. At this stage the polar front cloud band may
actually start to dissipate as the comma cloud develops further.
Figure 6. Comma cloud cyclogenesis. J2 is the jet streak associated
with the dry intrusion flow. W1 is the main WCB associated with jet
J1. W2 is a WCB associated with a short wave trough embedded in the
flow to the rear of the long wave trough axis, which then moves
around the base of the main trough. As the cyclone develops under
W2, the cloud associated with W1 will often start to dissipate.
Comma cloud cyclones tend to have quite a distinctive appearance on
satellite imagery. There will often be convective cloud elements
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embedded within the comma, and no high cirrus shield due to the lack
of warm advection. The frontal structure of a comma cloud cyclone is
not well marked since it forms completely within the cold air. A surface
trough with a well marked wind veer and pressure kick but no clear
dewpoint contrast takes the place of the surface cold front.
Figure 7. IR satellite images of a comma cloud cyclone. (a) shows the
system in its early stages, with the cold air cloud area C and the polar
front cloud band F. (b) shows the system at a stage corresponding to
fig 6(iii), 15 hours later. C has developed a typical comma shape with
very cold cloud tops. F has started to dissipate.
Figure 6 shows a comma cloud cyclone at 2 different stages of
development.Figure 6(a) is very similar to fig. 4 for the instant occlusion
case, with the cloud area C and polar front cloud band F. In this case C
does not merge with F but, as shown in 7(b), it develops into a comma
cloud cyclone polewards of F, which has started to dissipate in this
picture. The cloud tops in C cool considerably in the 15 hours from 7(a)
to 7(b).
3. Cyclogenesis – summary
The classes of cyclogenesis described in this section are typical of the
way many cyclones develop. Sometimes the real atmosphere is more
complex than these models. Even so, most cyclogenesis events in midlatitudes will contain some elements which are recognisable from these
simple conceptual models. Monitoring satellite imagery allows us to
spot the early signs of cyclogenesis, and we can then apply simple
conceptual models to make some assessment of how the system will
develop in the future.
By being able to recognise patterns of cyclogenesis from satellite
imagery and other data, we have an early indication that development is
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occurring. One of the most useful applications of this is in verifying an
NWP model forecast. If the patterns we see in imagery are consistent
with cyclogenesis predicted by the model, we can use the forecast
products with some degree of confidence. However, if we can see
cyclogenesis occurring in reality that has not been predicted by the NWP
model then we know that some modifications to the forecast will be
necessary. A classic example of when signs of cyclogenesis in satellite
imagery were not recognised by forecasters or predicted by the NWP
model was the October Storm in 1987. Ten years later, on Christmas Eve
1997, the Chief Forecaster at NMC did spot rapid cyclogenesis occurring
that had not been predicted by the NWP model. He was able to adjust
all the forecast products and the forecasts that were issued to customers
were remarkably accurate.
4. Further reading
Although this is a fairly lengthy course note, it should be regarded as an
introduction to the subject of fronts and cyclogenesis. It is certainly
worth doing more reading on the subject to get a better idea of some of
the concepts introduced here. This is a short list of some good books and
papers that cover the subject in more detail, or are of interest for
historical reasons.
T.W. Harrold. 1973. Mechanisms influencing the distribution of precipitation
within baroclinic disturbances. Quarterly Journal of the Royal
Meteorological Society. 99. 232-251. A pioneering paper on the subject of
air flows within developing depressions. The first reference to conveyor
belts.
K.A. Browning. 1990. Organisation of clouds and precipitation in extratropical cyclones. Erik Palmen Memorial volume. American Met. Soc. An
excellent summary of conveyor belt ideas.
K.A. Browning. 1997. The dry intrusion perspective of extra-tropical cyclone
development. Meteorological Applications. 4. 317-324. A descriptive paper
discussing the importance of the dry intrusion in cyclogenesis and the
evolution of fronts.
M.J. Bader, G.S. Forbes, J.R.Grant, R.B.E. Lilley and A.J. Waters. 1995.
Images in Weather Forecasting. Cambridge University Press. A
comprehensive description of fronts and cyclogenesis from the
viewpoint of features in satellite imagery. Section 4.2 deals with split
cold fronts and section 5.2 describes Young’s scheme of cyclone
classification.
T.N. Carlson. 1991. Mid-Latitude Weather Systems. Routledge. Chapter 12
consists of a very detailed discussion of airflow through depressions,
including a section on system-relative isentropic flow.
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A. T. Semple 1998. Conceptual Models of Cyclogenesis. JCMM Internal
report no. 92. A summary of the various conceptual models of fronts and
cyclogenesis developed over the last 60 years, concentrating on
conveyor belt ideas. Young’s cyclogenesis classes are described and
unified into a life cycle model that includes the full spectrum of
cyclogenesis events.
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