1. COLD FRONT - CLOUD STRUCTURE IN
SATELLITE IMAGES
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The satellite image shows a cyclonically curved synoptic scale cloud band
usually a few hundred kilometres wide; in the VIS image the cloud band
mostly shows white grey shades, demonstrating high albedo;
in the IR image the grey shades can vary between white and grey, depending on
the existing conceptional sub-model (for instance Ana Cold Front or Kata Cold
Front) (see Meteorological physical background) as well as on the stage of
development within a single conceptual model;
as with the IR image, the grey shades in the WV image also can vary between
white and grey, according to classical conceptual model ideas: in the case of an
Ana Cold Front the cloud band should be characterized by high pixel values
(white), but lower pixel values (grey) in the case of a Kata Cold Front;
in reality this differentiation is not so strict:
o the typical Ana Cold Front cloudiness can have a structured but often
also smoothed appearance;
o the typical Kata cold front shows, in the IR and WV images, a structured
appearance for the cloud band, sometimes even representing a typical
Split Front character (see Cold Front - Split Front ); at the rear part of the
cloud band grey shades are darker accompanied by lower pixel values,
which often continuously increase northward to the point of the
Occlusion; the leading part of the cloud band varies in IR and WV grey
shades from light grey to white (see Meteorological physical
background); within the cloud band there can be subsynoptic areas
forming a continuous substructure; mostly the IR image shows there a
vertically and horizontally increased cloud area leading to brighter grey
shades. Typical examples are Waves, Front Intensifications by Jet
Crossing or merely situations with superimposed PVA maxima (see Key
parameters, Wave and Front Intensification By Jet Crossing
Figure 1: Typical grey shades from anafronts and katafronts in VIS, IR and WV satellite
images
Figure 2: Example of a structured Anafront
The image shows a typical example of a Cold Front stretching from the western
Mediterranean across France and the British Isles. As is very common, the cloud band
shows two areas of substructures consisting of colder cloud tops and forming convex
cloud bulges. So three conceptual models can be identified within the same cloud band:
Cold Front (CF), Wave development (WAVE) and an intensification of frontal cloudiness
by crossing of a jet streak (FI BY JET).
Figure 1: Example of a well developed Anafront (upper left: VIS, down left: IR, right: WV)
This case is an example of a well developed Ana cold front stretching from the southwestern corner of the image (Atlantic approximately 35N/35W) to the West coast of
Scotland. The cloud band in the VIS image is characterized by brighter grey shades in
the leading and middle part of the band, while the rear part is much more broken with
transparent cloudiness (easy to recognize from approximately 40N/30W to
approximately 50N/20W). In the IR image this is quite opposite with bright grey shades
denoting colder and therefore higher cloud tops from the middle to the rear part. This
indicates multi-layered cloudiness with more and higher cloud tops to the rearward edge
and fits well with the classical ideas described in the meteorological physical
background. These higher cloud tops are even more recognizable in the WV image. The
most characteristic features in this channel are the sharp rear cloud edge and the black
stripe of dry air parallel to the cloud band (pronounced gradient from white to black).
Figure 2: Example of a well developed Katafront (upper left: VIS, down left: IR, right: WV)
This case is an example of a well developed Kata Cold Front stretching from the southeastern coast of England across the Baltic Countries, Finland and Russia to the White
Sea. The cloud band in the VIS image shows brighter grey shades in the rear part of the
band, for instance from north Germany across the Baltic Sea to the south coast of
Finland.
This is completely opposite to the IR and WV images where the lower grey shades,
denoting warmer cloud tops, can be found. Dry air in the WV image overflows the rear
part of the cloud band. Such a situation is in accordance with the classical ideas of the
Kata Cold Front describing cloud dissolution by advection of dry sinking air (see
Meteorological physical background). This is also opposite to the Ana Front model.
2. COLD FRONT - METEOROLOGICAL PHYSICAL
BACKGROUND
If cold and warm air are situated next to each other an inclined boundary (oriented
downward from high to low layers) can be found between the two air masses. The main
physical process for the development of Cold Fronts is the movement of the colder
against the warmer air. As a consequence of this movement, and relative to it, the warm
air tends to ascend this air mass boundary while the cold air tends to sink below it. This
upward motion often leads to condensation and subsequently the development of clouds
and precipitation.
Figure 3: Schematic picture of Ana front and Kata front
Many conceptual models have been developed to depict fronts; the most classical ones
are the for Ana Cold Front and the Kata Cold Front. From the viewpoint of satellite
meteorology, the conveyor belt model presents new ideas. Therefore within the following
paragraphs these cold front types are described.
Ana Cold Front
According to classical ideas cold air moves rapidly against warm air, thereby creating
convergence at the surface line between the two airmasses. This convergence forces
the warm, moist air to ascend on the frontal surface. The cloud band develops, inclined
rearward from the surface cold front. Consequently, in this case the main zone of
cloudiness and precipitation appear behind the surface front (indicated by the TFP).
Satellite images confirm this structure (see Cloud structure in satellite image). Only in
the case where there are high upper level winds, does the high cloud extend
downstream ahead of the surface front, leading to a TFP maximum within the cloud
band.
If one describes the situation using conveyor belt theory, the frontal cloud band and
precipitation are related to an ascending warm conveyor belt. This conveyor belt has
a rearward component relative to the movement of the front. This leads to the same
result mentioned above, with the frontal cloud band and precipitation appearing
behind the surface front. Parallel to the warm conveyor belt there is a dry stream (dry
intrusion). The sharp rear cloud edge of frontal cloudiness marks the transition
between the two relative streams and is accompanied by a limiting stream line.
As mentioned in literature, real examples of CF do not always show these model
characteristics, but sometimes even show parallel or even forward inclined warm
conveyor belts. While the rearward component can be explained by ageostrophic
wind flow in the planetary boundary layer due to friction, the parallel or even
foreward sloping warm conveyor within the middle and higher levels is in accordance
with the geostrophic wind relation.
Figure 4: The conveyer belt theory for an Ana Cold front
Kata Cold Front
According to classical ideas, in this type of front the warm air follows processes similar to
the Ana Cold Front, however, the ascent of air is restricted by dry descending air
originating from behind the front and, consequently, dissipating the higher clouds. In this
case, the main zone of cloudiness and precipitation appear in front of the surface front.
Satellite images show parts of this process of cloud decay (see Cloud structure in
satellite image). It is now generally considered that a Kata Cold Front evolves from an
Ana Cold Front.
In contrast to the Ana Cold Front, the ascending warm conveyor belt is overrun by
dry air, which is transported within the relative stream of the dry intrusion. The air of
this intrusion originates from upper levels of the troposphere or even the lower levels
of the stratosphere and crosses the Cold Front from behind. Due to this process, the
warm conveyor belt acquires a component which is forward inclined relative to the
movement of the Cold Front. Therefore, frontal clouds and precipitation tend to lie
ahead of the surface front. The cloud tops within the area of the dry sinking
tropospheric and/or stratospheric air are lower (warmer) than in the case of an Ana
Cold Front. At the leading edge of this dry air, an increase of the cloud tops can be
observed (see Cloud structure in satellite image). This area indicates the so-called
upper Cold Front. The air mass which is advected by the dry intrusion is colder than
the air within the warm conveyor belt. The intrusion cools air above and, later, also
ahead of the Cold Front. Furthermore, the air of the upper relative stream is
indicated by lower values of equivalent potential temperature. The result of this
situation is the development of a conditionally unstable layer close to the leading
edge of the frontal cloud band. As a result of ascent, this area is suitable for the
development of pronounced instability which is often observable by a change of
cloud type from stratiform to cumuliform (see Weather events).
Figure 5: The conveyer belt theory for a Kata Cold front
Schematic Summary
Ana Cold Front
Kata Cold Front
Figure 6: Schematic structure of Ana and Kata Cold front
The schematics above summarise several aspects of conveyor belt theory and compare
the different behaviour of Ana and Kata Fronts.
The most important feature is always the orientation of the jet streak relative to that of
the cloud band, i.e. parallel to an Ana Front but across the cloud band of a Kata Front
and descending.
Figure 7:
04 October 1995/12 UTC - IR image (Ana Cold Front);
relative streams on the isentropic surface of ThetaE =
310K; frontal lines are in accordance with the maximum
of the thermal front parameter (TFP) 500/850 hPa; lines:
dashed blue: jet axis in accordance with the zero line of
the shear vorticity at 300 hPa but corrected with WV
imagery, yellow: isobars, magenta: relative streams system velocity: 267° 9 m/s, white: position of vertical
cross section
Figure 8:
04 October 1995/12 UTC - WV image (Ana Cold Front);
relative streams on the isentropic surface of ThetaE =
318K; frontal lines are in accordance with the maximum
of the thermal front parameter (TFP) 500/850 hPa; lines:
dashed blue: jet axis in accordance with the zero line of
the shear vorticity at 300 hPa but corrected with WV
imagery, yellow: isobars, magenta: relative streams system velocity: 267° 9 m/s
The above images show an example of an Ana Cold Front. On the lower isentropic
surface (top) a warm conveyor belt can be observed crossing (rearwards) the frontal line
approximately northward of the Bay of Biscay. Under its influence is the cloud band in
front of the TFP as well as the relevant part behind it stretching from France across
south-east England into the North Sea. The second relative stream is a broad one, from
the north-western part of the trough behind the front. Frontal cloudiness to the rear of the
TFP, from north-west Spain across the Bay of Biscay and Brittany into the English
Channel is, increasingly, under its influence.
If this situation is compared with the vertical cross section below (upper cross
section), it can be seen that the zone of high humidity ahead of the front, at the 310K
surface around 800 hPa, represents the warm conveyor belt, while humidity values
on this surface, between 800 and approximately 550 hPa, represent the relative
stream approaching from the rear. This height also marks the position of the
rearward edge of the cloud band, which coincides with a sharp decrease in IR and
WV pixel values.
On the higher isentropic surface (lower vertical cross section) the relative stream
from the rear can be divided into two parts. The one near the anticyclonic side of the
jet axis originates from moist regions in the warm sector of the consecutive frontal
system and is associated with Cold Front clouds to the rear of the TFP; the other
drier relative stream can be found on the cyclonic side of the jet axis. Both are
parallel to the frontal cloud band. On this higher surface the warm conveyor belt
crosses the TFP much less than on the lower surface.
Looking again at the vertical cross section (upper cross section), the humidity
maximum in front at the 318K surface between 500 and 400 hPa represents the
warm conveyor belt (accompanied by peaks in IR and WV pixel values) while on this
isentropic surface further upward, near 350 hPa, cloudiness is associated with the
moist part of the relative stream from the rear side (accompanied by a second IR
and WV peak in pixel values). The dry part of this stream shows up in the cross
section as very low humidity values (around 300 hPa).
Figure 9:
04 October 1995/12.00 UTC - Vertical cross section; Ana
Cold Front; black: isentropes (ThetaE), blue: relative
humidity, orange thin: IR pixel values, orange thick: WV
pixel values
Figure 10:
29 February 1996/06.00 UTC - Vertical cross section;
Kata Cold Front; black: isentropes (ThetaE), blue:
relative humidity, orange thin: IR pixel values, orange
thick: WV pixel values
The Ana Cold Front (top) shows a backward inclined zone of high humidity from low to
high levels while the Kata Cold Front (bottom) shows a similar zone but forward
inclined. The driest air, in the case of the Ana Cold Front, lies behind and below the
frontal surface, but within and below the frontal surface in the case of the Kata Cold
Front.
Figure 11:
29 February 1996/06 UTC - IR image (Kata Cold Front);
relative streams on the isentropic surface of ThetaE =
286K; frontal lines are in accordance with the maximum
of the thermal front parameter (TFP) 500/850 hPa; lines:
dashed blue: jet axis in accordance with the zero line of
the shear vorticity at 300 hPa but corrected with WV
imagery, yellow: isobars, magenta: relative streams system velocity: 326° 10 m/s, white: position of vertical
cross section
Figure 12:
29 February 1996/06 UTC - WV image (Kata Cold Front);
relative streams on the isentropic surface of ThetaE =
300K; frontal lines are in accordance with the maximum
of the thermal front parameter (TFP) 500/850 hPa; lines:
dashed blue: jet axis in accordance with the zero line of
the shear vorticity at 300 hPa but corrected with WV
imagery, yellow: isobares, magenta: relative streams system velocity: 326° 10 m/s
In the case of the Kata Cold Front, the zero line of shear vorticity, marking the jet axis,
crosses the cloud band leading less bright (i.e. lower height) frontal clouds at the
cyclonic side. This is the main difference from the situation of the Ana Cold Front
described before, where the jet axis and frontal cloud band are parallel. On the lower
isentropic surface (top image) a warm conveyor belt can be observed which is nearly
completely restricted to the area of high cloudiness ahead of the TFP. Behind the TFP,
relative stream lines are from the rear. Comparing with the vertical cross section (above,
lower cross section) the moist zone ahead of the front, at the 286K surface from the
ground up to about 800 hPa, is composed of two relative streams: the warm conveyor
belt air mass only exists in the lowest layer (below about 900 hPa) and the air mass of
the moist relative stream from behind, dominates above 900 hPa.
The second image above shows the situation on a higher isentropic surface with a
crossing of the relative stream lines from the north-west over the TFP of the Cold
Front. This belongs to the maximum of relative humidity in the vertical cross section
(above, lower cross section) ahead of the front at the 300K surface (approximately
500 hPa). Consequently, the high clouds, indicated by the maximum of pixel values,
is formed in the moist branch of the relative stream to the rear, while the warm
conveyor belt is only associated with a layer of low level cloudiness.
The lower cloud tops on the cyclonic side of the jet axis should be the result of the
dry part of the relative stream, which is confirmed in the vertical cross section; dry air
above 300K associated with the IR peak at about 400 hPa.
In this case it is not easy to distinguish between the origins of the two branches of
the relative stream from the north-west because the stream lines cross the jet axis to
the rear of the trough. One possible explanation is that the system velocity is
computed for the Cold Front and does not match that of the subsequent Warm Front
system. The following image is the result of a system velocity computation based on
propagation of the approaching Warm Front system. The differentiation between the
dry and the moist branches in the upstream region is now much clearer.
Figure 13:
29 February 1996/06 UTC - IR image (Kata Cold Front); relative streams on the isentropic
surface at ThetaE = 300K; frontal lines are in accordance with the maximum of the thermal
front parameter (TFP) 500/850 hPa;lines: dashed blue: jet axis in accordance with the zero
line of the shear vorticity at 300 hPa but corrected with WV imagery, yellow: isobares,
magenta: relative streams - system velocity: 278° 9 m/s, white: position of vertical cross
section
3. COLD FRONT - KEY PARAMETERS
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Equivalent thickness
Crowding zone of the equivalent thickness:
o Ana Cold Front: within the cloud band
o Kata Cold Front: behind the cloud band
Thermal front parameter (TFP)
Maximum of the TFP accompanies the cloud band:
o Ana Cold Front: within the leading area of the cloud band
o Kata Cold Front: within the rear area of the cloud band
Temperature advection (TA)
The field of TA shows (weak) WA in front of, and (pronounced) CA behind the
surface front. Depending on the conceptual model the zero line of TA should
be found within or very close to the cloud band:
o Ana Cold Front: cloudiness usually within CA, TA=0 in front of cloud
band
o Kata Cold Front: cloudiness usually within WA, TA=0 within the cloud
band
Positive vorticity advection (PVA) in upper levels
PVA maxima can be found near the rear edge of the cloud band indicating the
propagation of the upper level trough and/or the approach of a jet streak (see
Front Intensification By Jet Crossing ).
Isotachs at 300 hPa
o Ana Cold Front: the jet is situated behind and parallel to the Cold Front
cloudiness, the cloud band is on the anticyclonic side of the jet, the jet
crosses the frontal system in the area of the point of the Occlusion
o Kata Cold Front: the jet crosses the surface cold front at a sharp angle;
according to the crossing point the cloud band can be on the anticyclonic
or cyclonic side of the jet
As already mentioned, there are similarities between Kata Cold Front and Split
Front. The main difference can be found in the orientation between jet and front
because in the case of a Split front the jet approaches from the rear side nearly
at right angles.
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Shear vorticity at 300 hPa
Zero line of the shear vorticity:
o Ana Cold Front: the zero line is close to and parallel to the rear cloud
edge
o Kata Cold Front: the zero line crosses the Cold Front, usually it
accompanies the transition of the high cloud tops on the anticyclonic side
from the lower cloud tops on the cyclonic side of the zero line; the latter
are the result of cloud dissolution within dry air on the cyclonic side see
Cloud structure in satellite image).
4. COLD FRONT - TYPICAL APPEARANCE IN
VERTICAL CROSS SECTIONS
The isentropes of the equivalent potential temperature show a downward inclined
crowding zone, which extends through the whole troposphere, with the warmer air found
in front and above, the colder air at the rear and below the crowding zone (see
Meteorological physical background). Besides the surface front, often an upper level
front, which is situated ahead of the surface front, can be observed.
The relative humidity has high values in front of and low values behind the crowding
zone. With the help of the models of Ana Cold Front and Kata Cold Front, the field of
humidity can be interpreted with the warm ascending air in front of and cold descending
air behind the frontal surface. Furthermore, in the case of an Ana Cold Front the field of
humidity is characterized by high values (approximately 80%) extending on the warmer
side of the crowding zone with a backward inclination from the lower to the higher levels
of the troposphere. In contrast to this, the distribution of the relative humidity in the case
of a Kata Cold Front shows high values (approximately 80%) in front of the warmer side
of the crowding zone from the lower levels with a forward inclination into higher layers of
the troposphere, thereby moving away from the highest surface of the crowding zone.
The dry cold air (with minimum values of about 20%) extends in the case of an Ana Cold
Front from the highest layer of the frontal surface downward below the frontal surface. In
contrast to this, in the case of a Kata Cold Front the dry air extends downward below as
well as within the frontal surface, thereby leading to cloud dissolution (see
Meteorological physical background).
The field of temperature advection shows WA in front (ahead) of the crowding zone,
indicating warm ascending air of the warm conveyor belt and/or the upper relative
stream from behind the front. CA can be found behind (below) the frontal surface. In
general, the CA is much more pronounced than the WA.
The whole crowding zone from the lower to the upper levels of the troposphere is
characterized by positive vorticity advection. In the ideal case the values of the
vorticity advection are continously increasing. Therefore its maximum (often situated
at 300 hPa) can be found in the upper levels of the troposphere and behind the
surface front line.
The field of divergence shows a pronounced zone of convergence within and a zone
of divergence above the frontal surface.
As a consequence of this distribution the field of omega is characterized by negative
values (upward motion) within and above the crowding zone. As convergence has its
highest values in the lower troposphere the greatest negative values of omega can
be found in the mid-levels of the troposphere. Weak positive values (downward
motion) can be found within the cold air at the rear side below the crowding zone.
The pixel values of the satellite image show in the VIS image high values for both the
Ana Cold Front and Kata Cold Front cases. In contrast to this the distribution of the
pixel values of the IR and WV images are dependant on the existing cold front type. In
the case of an Ana Cold Front the pixel values of the IR and WV images are increasing
backwards i.e. behind the TFP, but in the case of a Kata Cold Front the pixel values
mostly are increasing forewards i.e. ahead of the TFP (see Cloud structure in satellite
image).
Figure 14:
26 September 1995/06.00 UTC - Meteosat IR image; Ana
Cold Front; position of vertical cross section indicated
Figure 15:
29 February 1996/06.00 UTC - Meteosat IR image; Kata
Cold Front; position of vertical cross section indicated
The first two figures show the main differences in the humidity distribution between Ana
and Kata Cold Fronts. The other parameters do not distinguish between the two types of
Cold Front and are therefore chosen from the Kata Cold Front type example. All fields
are very close to the schematics representing ideal situations with the exception of the
isentropes in the Kata Cold Front case which unusually consists of two branches of the
frontal zone, one reaching the ground (approximately at 57N/15E) and an upper level
front slightly ahead of the surface front reaching down to about 750 hPa.
Figure 16:
26 September 1995/06.00 UTC - Vertical cross section;
Ana Cold Front; black: isentropes (ThetaE), blue:
relative humidity, orange thin: IR pixel values, orange
thick: WV pixel values
Figure 17:
29 February 1996/06.00 UTC - Vertical cross section;
Kata Cold Front; black: isentropes (ThetaE), blue:
relative humidity, orange thin: IR pixel values, orange
thick: WV pixel values
Figure 18:
29 February 1996/06.00 UTC - vertical cross section; Ana
and Kata Cold Fronts; black: isentropes (ThetaE), red
thick: temperature advection - WA, red thin: temperature
advection - CA, orange thin: IR pixel values, orange
thick: WV pixel values
Figure 19:
29 February 1996/06.00 UTC - vertical cross section; Ana
and Kata Cold Fronts; black: isentropes (ThetaE), green
thick: vorticity advection - PVA, green thin: vorticity
advection - NVA, orange thin: IR pixel values, orange
thick: WV pixel values
Figure 20:
29 February 1996/06.00 UTC - Vertical cross section;
Ana and Kata Cold Fronts; black: isentropes (ThetaE),
cyan thick: vertical motion (omega) - upward motion,
cyan thin: vertical motion (omega) - downward motion,
orange thin: IR pixel values, orange thick: WV pixel
values
5. COLD FRONT - WEATHER EVENTS
Weather events are highly variable and can differ from season to season. Two type of
fronts are distinguished: Ana and Kata Cold Fronts
Ana Cold Front
Parameter
Description
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Moderate to heavy showery precipitation
In winter season snow is possible.
Quite often thunderstorms are observed at all seasons.
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Falls rapidly after the passage of the front.
Over land in wintertime temperature can rise after front
passage.
Wind(incl. gusts)
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Around embedded Cbs strong gusts are possible.
Veering of the wind at the front passage
Other
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Precipitation in narrow cloud band just ahead of front (line
Precipitation (incl.
thunder)
Temperature
relevant
information
convection)
Enhanced areas with precipitation behind surface front Risk
of moderate to severe icing and turbulence. Predestined
areas for heavy weather are regions with superimposed
PVA, e.g.left exit region of a jet streak
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Figure 21:
26 September 1995/12.00 UTC - Meteosat VIS image; Ana
Cold Front; weather events (green: rain and showers,
blue: drizzle, cyan: snow, red: thunderstorm with
precipitation, purple: freezing rain, orange: hail, black:
no actual precipitation or thunderstorm with
precipitation); blue: thermal front parameter 500/850 hPa
Kata Cold Front
Parameter
Precipitation(incl.
thunder)
Temperature
Wind (incl. gusts)
Other relevant
information
Description
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Precipitation ahead of surface front
Precipitation sometimes in narrow bands ahead of
surface front
Sometimes thunderstorms are observed at all seasons.
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Falls rapidly after the passage of the front
Over land in wintertime temperature can rise after front
passage.
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Around embedded Cbs strong gusts are possible.
Veering of the wind at the front passage
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Risk of moderate to severe icing (risk of freezing rain)
Risk of moderate to severe turbulence
Figure 22:
29 February 1996/12.00 UTC - Meteosat VIS image; Kata
Cold Front; weather events (green: rain and showers,
blue: drizzle, cyan: snow, red: thunderstorm with
precipitation, purple: freezing rain, orange: hail, black:
no actual precipitation or thunderstorm with
precipitation); blue: thermal front parameter 500/850 hPa