Application of quasigeostrophic arguments in
interpretation of extratropical cyclogenesis
Niels Woetmann Nielsen
Danish Meteorological Institute
● Extratropical cyclones in a global perspective
● Energy conversions
● Simplified quasigeostrophic equations
● Typical flow patterns in extratropical cyclogenesis
● Example
● Exercise
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Why bother about extratropical cyclones
and anticyclones?
because they dominate the weather at
middle and high latitudes.
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Why use QG-theory to explain their
development?
because it is the simplest way to explain
how they develop
because QG-theory, when combined with
a few carefully selected NWP fields, can
explain the dynamics behind the major
cloud patterns seen in satellite images.
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Extratropical cyclones and anticyclones develop
as result of what is called baroclinic
instability.
Baroclinic instability can not occur
everywhere on the globe.
In a global perspective baroclinicity is
generated where the northward relatively warm
and moist branch of the Ferrell circulation
meets the southward relatively cold and dry
branch of the polar circulation
– and baroclinic instability can only occur in
this zone.
Global meridional circulation and surface winds
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During extratropical cyclogenesis mean
available potential energy is converted
to eddy available potential energy on
the cyclone scale and part of the eddy
available potential energy is converted to
eddy kinetic energy in the developing
extra-tropical cyclone.
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Conversion from mean available
potential energy to eddy available
potential energy occurs in a process
where at the same latitude and height
cold air is advected southward and warm
air is advected northward.
Conversion from eddy available
potential energy to eddy kinetic
energy occurs in a process where at the
same latitude and height cold air sinks and
warm air rises.
Energy cycle for Northern Hemisphere winter
“Kinematical” south-north eddy transport of momentum
Upper: trough and ridge axes with positive tilt
Middle: trough and ridge axes without tilt
Bottom: trough and ridge axes with negative tilt
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A developing extratropical cyclone have
sinking cold air penetrating into a
warmer air mass and rising warm air
penetrating into a colder air mass.
How can this be explained by QG-theory?
Typical flow structures in
extratropical cyclogenesis
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Baroclinic instability
Upper-tropospheric wave amplification by
differential temperature advection (cold (warm)
advection below upper trough (ridge) – this
process converts mean available potential
energy (APE) into eddy APE.
Upper-tropospheric wave amplification intensifies
the secondary circulation (ageostrophic winds
and vertical velocity) in such a way that
ascending (descending) motion intensifies in
warm (cold) air downstream of the upper-level
trough (ridge)
- this process converts eddy APE into eddy
kinetic energy (KE)
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A positive feed-back occurs since the intensified
low level circulation (increase in eddy KE) leads
to an increase in the conversion from mean to
eddy APE, the latter promoting increased
conversion from eddy APE to eddy KE and so on.
The feedback occurs through the secondary
circulation (vertical velocity and ageostrophic
wind).
Relative vorticity advection is the most important
process in the upper troposphere.
In the lower troposphere the most important
process is temperature advection and diabatic
heating.
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Baroclinic instability can not occur if the
wave length is too short and static stability
too high.
Under these conditions the secondary
circulation (SC) induced by uppertropospheric processes (relative vorticity
advection) does not reach the lower
troposphere and the SC induced by lowlevel temperature advection does not
reach the upper troposphere. No
interaction between the processes can
take place.
“Small-scale” extratropical cyclogenesis case
18 November 2009
“Small-scale cyclogenesis 18 November 2009
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DMI-HIRLAM analyses of mslp and 300 hPa pot. temp. (K)
Left: 00UTC and right: 12UTCr
“Small-scale” cyclogenesis example (1)
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Incipient stage of cyclogenesis 00UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa
wind
“Small-scale” cyclogenesis example (1)
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Incipient stage of cyclogenesis 00UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and rel.
vorticity and wind at 300 hPa
“Small-scale” cyclogenesis example (2)
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Deepening phase of cyclone 06UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300
hPa wind
“Small-scale” cyclogenesis example (2)
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Deepening phase of cyclone 06UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of
mslp and rel. vorticity and wind at 300 hPa
“Small-scale” cyclogenesis example (3)
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Mature stage of cyclone 12UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa wind
“Small-scale” cyclogenesis example (3)
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Mature stage of cyclone 12UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of
mslp and rel. vorticity and wind at 300 hPa
“Small-scale” cyclogenesis example (4)
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Filling stage 18UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa wind
“Small-scale” cyclogenesis example (4)
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Filling stage 18UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of
mslp and rel. vorticity and wind at 300 hPa
Exercise
Suppose that a forecaster has only access to nwp-analyses of mean sea level
pressure (mslp), wind and relative vorticity (rvor) at 300 hPa and a MSG
channel 5 image valid at analysis time.
The forecaster is asked to consider the surface low (L) west of Spain (see s1) and is
given the following tasks: In connection with L
● identify regions with significant positive and negative rvor advection at 300hPa and
infer qualitatively their contribution to vertical velocity (w)
● identify regions with significant cold and warm advection (mean from surface to
300hPa) and infer qualitatively their contribution to w
● Identify a region with both warm advection and positive rvor advection and a region
with both cold advection and negative rvor advection
● Which way will L most likely move? - and is it likely that L will intensify?
● What is the sign of the correlation between rvor at 300 hPa and bright(+) and dark (-)
colours on the MSG image – is the correlation weak or strong?
DMI-HIRLAM analysis of mslp and wind at 300 hPa valid 12UTC 27 Feb. 2010 (s1)
DMI-HIRLAM analysis of mslp and rvor and T at 300 hPa valid 12UTC 27 Feb. 2010 (s2)
MSG channel-5 12UTC 27 February 2010
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