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Cold Front - Meteorological Physical Background

by ZAMG and FMI

Fronts generally form within a baroclinic boundary between cold and warm air masses. The main physical process for the development of Cold Fronts is the movement of the cold air against warm air. The warm air ascends along the boundary while the cold air sinks below it. If there is enough humidity, the upward motion leads to condensation and to the development of clouds and precipitation.

Cold Fronts can be devided into two types: Ana and Kata Cold Fronts. These types can be described both in terms of classical frontal theory and in terms of conveyor belts.

The main feature which separates the different types of Cold Front is the orientation of the jet relative to the front in the middle and upper levels of the troposphere:

In the case of an Ana Cold Front, the jet axis and dry intrusion are parallel to the frontal cloud band, and form a well pronounced rear cloud edge.

In the case of a Kata cold front, the jet axis crosses the frontal cloud band.

Discussion

In the literature, as well in the studies carried out by ZAMG and FMI, there are some uncertainties about the structure of Ana and Kata types:

It is not always completely clear whether a Cold Front is Ana or Kata type. Even within the same front Ana and Kata structures can be observed; in these cases

Ana features can be found close to the occlusion point, whereas Kata structures prevail in the parts of the front farther away.

The frontal cloudiness, especially in Kata Cold Front, is often not produced solely by the Warm Conveyor Belt. The rear parts of the cloud band are produced by an upper relative stream orginating from the trough behind the Cold Front. This moist rising stream can sometimes be followed backwards as far as the area of Warm Front clouds. Thus, the upper relative stream is added to the schematics describing the conveyor belt model of the Ana and Kata Fronts.

There can be parallel or even forward inclined Warm Conveyor Belts. The rearward component at lower levels is due to the ageostrophic wind within the boundary layer, while the parallel or even forward sloping Warm Conveyor Belt in the middle and upper levels is in accordance with the geostrophic wind relationship.

Ana Cold Front

According to the classical theory: The cold air moves rapidly against warm air, creating convergence within the baroclinic

zone between the two air masses.

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Convergence forces the warm, moist air to ascend along the frontal surface. The developing cloud band is inclined rearward with height.

The main zone of cloudiness and precipitation is located behind the surface front. An exception is a case with strong upper winds which force the high clouds to extend

downstream ahead of the surface front.

According to the conveyor belt theory: The frontal cloud band and precipitation are related to an ascending Warm Conveyor

Belt, which has a rearward component relative to the movement of the front, causing the frontal cloud band and precipitation to appear 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.

04 October 2005/12.00 UTC - Vertical cross 04 October 2005/12.00 UTC - Meteosat 8 IR

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section; black: isentropes, blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

10.8 image; magenta: relative streams 308K - system velocity 236° 15 m/s, yellow: isobars 308K; position of vertical cross section indicated

The 308K isentropic surface is close to the upper boundary of the frontal zone reaching through the whole troposphere. The frontal cloudiness is under the influence of the Warm Conveyor Belt.

Kata Cold Front

According to classical theory: The ascent of warm air is restricted by dry descending air originating from behind the

front and, consequently, dissipating the higher clouds. The main zones of cloudiness and precipitation appear in front of the surface front.

According to the conveyor belt theory: The ascending Warm Conveyor Belt is overrun by the dry intrusion. The dry air originates from upper levels of the troposphere or even from the lower

levels of the stratosphere, and crosses the Cold Front from behind.

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The warm conveyor belt acquires a component which is inclined forwards relative to the movement of the Cold Front. Therefore, frontal clouds and precipitation tend to lie ahead of the surface front.

The cloud tops in the area of the dry airstream are relatively low, whereas on the leading edge of this area the cloud tops are higher. 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 has lower equivalent potential temperature. The result is the development of a conditionally unstable layer close to the leading edge of the frontal cloud band. This can be observed as a transformation of layered clouds into convective ones.

19 September 2005/12.00 UTC - Vertical cross section; black: isentropes, blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

19 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 312K - system velocity 236° 15 m/s, yellow: isobars 312K; position of vertical cross section indicated

The 312K isentropic surface is close to the upper boundary of the frontal zone. The relative streams show that most parts of the frontal cloud band are under the influence of the moist

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upper relative stream coming from behind. The Warm Conveyor Belt is only associated with some cloudiness in the leading edge of the cloud band.

There are a lot of similarities between Kata Cold Fronts and Split Fronts (see Split Front ). The main difference is the orientation between the jet and the front.

It is generally considered that a Kata Cold Front evolves from an Ana Cold Front. As baroclinic disturbances often develop over the Atlantic, the newly developed Ana Fronts can mainly be found there, whereas older, continental fronts are mostly Kata type. Another reason for the spatial differences might be that the lower parts of the front are decelerated due to the friction of the continent, while the upper parts continue with higher speed.

Split Front - Meteorological Physical Backgroundby ZAMG and FMI

The conceptual model of a Split Front is strongly associated with jet streaks and sinking of very dry stratospheric air.

The initial stage of a Split Front is generally an Ana Cold Front type (see Cold Front - Meteorological physical background ). In contrast to the Ana Cold Front, the Warm Conveyor Belt is overrun aloft by the relative stream of the dry intrusion. This process takes place as the warm air ascends ahead of the surface cold front with a forward component relative to the frontal system.

Looking at the situation on isentropic surfaces, the meteorological process which leads to the typical appearance of a Split Front in the satellite images can be explained as follows: together with a jet streak approaching the frontal cloud band, dry stratospheric air is advected on the cyclonic side, and dry tropospheric air on the anticyclonic side of the jet axis. Both air streams are sinking at this stage of development. Relative streams on the isentropic surface are parallel to the jet axis within the jet streak but show a characteristic splitting in the exit region into a northwards oriented and a southwards oriented component. While the southern branch is still sinking, the northern branch is rising again. During an interaction of jet streak and frontal cloud band this configuration of relative streams causes dissolution of cloudiness from above and the Split Front character then appears.

One way to classify the rear edge of the low cloud band is to regard it as a surface front and the rear edge of the high cloud band as an upper level front (see Cloud structure in satellite image). Between these frontal surfaces a shallow moist zone remains (see Weather events). A characteristic feature of the upper level front is that this frontal surface is a moisture boundary and not a thermal boundary (see Typical appearance in vertical cross sections).

Associated with the approaching jet streak, a PVA maximum situated in the left exit region may be superimposed upon the low cloud band of the Split Front. Within this area, the development of the above mentioned EC-like cloudiness can often be observed (see Front Intensification By Jet Crossing - Cloud structure in satellite image ).

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30 August 2005/06.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 318K - system velocity 236°15 m/s, yellow: isobars 318K, position of vertical cross section indicated

30 August 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

Looking at the vertical cross section, the humidity maximum in front and above the 318K surface between 500 and 350 hPa represents the warm conveyor belt (accompanied by peaks in IR and WV pixel values) while on this isentropic surface further upstream, near 350 hPa one can see drier air, which is connected to the relative streams bringing dry stratospheric air over the frontal region.

Front Intensification By Jet Crossing - Cloud Structure In Satellite Images

The satellite image shows an area of increased cloudiness (vertically as well as horizontally) within the frontal cloud band which has superimposed a PVA maximum at 300 hPa (see Key parameters );

VIS, IR and WV images show bright grey shades, indicating thick cloudiness (see Typical appearance in vertical cross section );

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the increased cloudiness can appear in two forms: o lumpy structure, which indicates embedded CBs o Wave-like configuration, as a consequence of formation by cyclonic

vorticity; this cloud feature is clearly brighter than the surrounding frontal cloudiness

(see Key parameters ); at the rear of the frontal cloud band, WV imagery indicates a jet axis

pointing approximately perpendicular to the cloud band by a Black Stripe as well as Cloud Fibres which may be seen also in the IR image.

Split Front - Cloud Structure In Satellite ImagesDefinition: A Split Front is accompanied by a cyclonically curved cloud band, which, contrary to a classical Cold Front (see Cold Front ), contains a distinct double banded structure with cold cloud top temperatures at the leading edge and warmer cloud top temperatures at the rear edge:

In the thick cloud band at the leading part of the Split Front VIS signals are white to grey, IR and WV signals are white, representing a thick, multilayered cloud band;

In the low cloud band at the rear part of the Split Front VIS signals are white, IR signals are dark grey to grey and WV signals are either black (if one is using the Meteosat 8 WV 6.2 µm , which shows higher level water vapour) representing a low cloud band with very dry air above or fairly white/light grey if the Meteosat 8 WV 7.3 µm representing the lower atmosphere is used;

In the ideal case the boundary between both cloud bands is marked by a sharp gradient of IR and WV pixel values. In reality only a gradual change of IR and WV grey shades exists;

Some features of a life cycle can be observed: o often EC shaped cellular cloudiness develops within and above the low cloud band on the cyclonic side of the jet axis; o in the WV imagery a black area sometimes develops on the anticyclonic part of the jet axis over the low cloud band of

the front, indicating sinking dry air connected with relative streams (see Meteorological physical background); o in a multilayered leading cloud band the higher cloud fibres are shifted downstream and the high and low cloud bands

become decoupled. Note: In literature (especially US) the name Split front has been used in relation to an upper level Cold Front. This is comparable to "Frontal

delay by mountains" and "Decoupling of cloud layers at different heights" in this manual (see Orographic Effects on Frontal Cloud ).

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30 August 2005/06.00 UTC - Meteosat 8 IR 10.8 image 30 August 2005/06.00 UTC - Meteosat 8 WV 6.2 image

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30 August 2005/06.00 UTC - Meteosat 8 VIS 0.8 image The leading high cloud band can be observed in the IR and WV images over the Atlantic from 30W/45N to 18W/57N; in contrast to the schematics this is a case where the leading cloud band consists predominantly of high level clouds. The long rear cloud band, with warm tops, can be seen from 32W/46N to 20W/57N. The jet axis is indicated in the WV image by the two dark lines behind the front. In this case the jet axis is almost perpendicular to the front. The convective cloudiness in the rear cloud band on the cyclonic side of the jet or within the jet is not very striking (but it is there!), but there is dry sinking air above the rear cloud band on the anticyclonic side of the jet. Surface and upper level fronts are in accordance with the schematic, as shown in the vertical cross section described in more detail later on.

Split Front - Key Parameters Temperature advection (WA): The ridge of WA is superimposed on the high level cloudiness representing the warm air rising on

the upper level frontal surface. Jet streak and positive vorticity advection (PVA):,A jet streak approaches the cloud band at a large acute angle accompanied by a

PVA maximum in the left exit region. Humidity: Very dry values in the upper and middle troposphere above the low level cloud band and a strong gradient between the

two cloud bands at different heights can be observed. Relative streams and potential vorticity (PV): Typical configuration of relative streams as described in the Meteorological physical

background, PV anomaly on relevant isentropic surfaces indicating stratospheric air.

30 August 2005/06.00 UTC - Meteosat 8 IR 10.8 image; red: positive vorticity advection (PVA) 300 hPa, yellow: isotachs 300 hPa

Split Front - Typical Appearance In Vertical Cross SectionsIn the ideal case the isentropes of the equivalent potential temperature show two frontal gradient zones, an upper level front and a surface front. Both zones have Cold Front inclinations. While the upper level front is connected to the high leading cloud band, the surface front

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represents the low-level cloudiness to the rear of the frontal system. Whereas the zone of the surface front is pronounced, that of the upper level front is quite weak. Therefore, the upper level front is not characterized as a thermal boundary but rather as a moisture boundary (see Meteorological physical background). The field of temperature advection often shows pronounced WA in front of and above the upper level frontal zone, which is connected with the upper level cloudiness. CA, typical for Cold Fronts, is situated below the surface front. The most characteristic feature of the humidity distribution is a dry area at higher levels between the two frontal zones. High values of humidity can be found in front of the frontal zones. In the case of superimposed EC cloudiness, a distinct isotach and PVA maximum can be found above the surface front at approximately 300 hPa (see Front Intensification By Jet Crossing - Typical appearance in vertical cross section ). Looking at the distribution of humidity the satellite signals, IR and WV images show the highest pixel values in front of the upper level front and, if existing, within the EC - like cloud. To the rear of the upper level front the VIS signals are usually higher while IR and WV signals are much lower than those in front of the upper level front (see Cloud structure in satellite image). Compare the chapter Cloud structure in satellite image, where the cross section is indicated in the image. In contrast to the ideal case the surface Cold Front has a superadiabatic layer in the lower levels of the troposphere. The upper level Cold Front is well developed. The distribution of humidity shows the described insertion of drier air between the two frontal zones. The IR and WV images show the pronounced decrease of temperature from the low to the high cloud part.

30 August 2005/06.00 UTC - Meteosat 8 IR 10.8 image; position of vertical cross section indicated

30 August 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values

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30 August 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), green thick: vorticity advection - PVA, green thin: vorticity advection - NVA, orange thin: IR pixel values, orange thick: WV pixel values

30 August 2005/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), yellow: isotachs, orange thin: IR pixel values, orange thick: WV pixel values

Split Front - Weather Events

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Weather events are highly variable and have to be split into two parts: the upper level front and the shallow moist area.

Upper FrontParameter Description

Precipitation Moderate to heavy showery precipitation Quite often thunderstorms are observed.

Temperature No significant change

Wind (incl. gusts) Around embedded Cb's strong gusts are possible.

Other relevant information The upper front at the transition between high and low cloud part Hail and thunderstorms possible during whole year Risk of moderate to severe icing and turbulence

Shallow moist area, including the surface frontParameter Description

Precipitation Slight to moderate rain or drizzle

Temperature Falls after the passage of the surface front

Wind (incl. gusts) Veering of the wind at the front passage

Other relevant information

Precipitation in area of shallow moist zone behind upper front Risk of moderate icing With superimposed PVA-max showers and thunderstorms are possible. With showers moderate to severe icing and turbulence

28 December 2004/12.00 UTC - Meteosat 8 IR 10.8 image; 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);

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