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Appearance in Satellite Data

Usually Cold Fronts are associated with cold advection behind the frontal zone and warm advection ahead of it, but sometimes the whole front is under cold advection. These are called Cold Fronts in Cold Advection, abbreviated CF in CA.

CF's in CA form mostly over the Atlantic, and are more common in the winter than during the summer season. The stage of CF´s in CA last generally 0,5-2 days. In the winter there are sometimes 3-4 day developments, however.

CF's in CA are often old, dissolving kata-type fronts in occluded cyclones. The upper cloudiness is slight or nonexistent. Also in the earlier stages there is often a significant dark stripe behind the front due to the very cold dry air.

The appearance of CF in CA in different channels:

  • In VIS images a white/light grey cloud band with some broken structure
  • In IR images grey, broken cloud band, which is clearly darker than other frontal parts (like warm front or occlusion) of the system
  • In WV images a grey cloud band with often a black stripe behind it. In small and dissolving systems the black stripe is missing. Also in some cases other systems like jet clouds or an upper wave can hide the black stripe. Generally in the end of the CF in CA stage the dark stripes weaken and disappear.
  • In the combination (HRVis;HRVis;IR10.8i) greyish yellow cloud band with some fractured fibres of light blue cirrus
  • In convection product (WV6.2-WV7.3;IR3.9-IR10.8,NIR1.6-VIS0.8) the front looks almost nonexistent in the area of warm advection, with only some grey-blue patchy cloudiness

On the 20th of January 2007 at 12.00 UTC, there was a CF in CA in an old occluded cyclone, reaching from Denmark towards the southwest.

20 January 2007/12.00 UTC - Meteosat 8 IR10.8 image
20 January 2007/12.00 UTC - Meteosat 8 VIS0.6 image
20 January 2007/12.00 UTC - Meteosat 8 WV6.2 image
20 January 2007/12.00 UTC - Meteosat 8 (hrvis,hrvis,IR10.8i) image
20 January 2007/12.00 UTC - Meteosat 8 (WV6.2-WV7.3;IR3.9-IR10.8,NIR1.6-VIS0.8) image

When defining the type of the CF in CA, the following aspects are to be taken into account:

  • Along the front there can be several types of Cold Fronts present, that is CF, CF in CA and CF in WA.
  • In the earlier stages there might still be some upper cloudiness connected to the front.
  • Quite often (in over 70% of the cases) some other upper cloudiness overruns CF in CA: most common is Jet Fibres, the other possibility is an Upper Wave. This might make the images look like an ordinary Cold Front, and hide the black stripe behind the CF in CA.
  • CF in CA can change back into CF or even into CF in WA before it is dissolved.
  • Sometimes in the north CF in CA can be found already in a wave before the occlusion stadium. These fronts look like conventional Cold Fronts in the beginning, but the cloudiness reduces rapidly.
  • The dark stripes are clear only in the beginning of the development, afterwards they grow lighter and smoother.

The last stage of CF's in CA can be:

  • most often a common CF, after the cold advection has reduced, or warm advection from a WF or WCB is effecting it
  • CF in CA can also dissolve in itself when the cyclone is very old and there are no other features nearby having any influence
  • a Split Front, although this is rare.

Meteorological Physical Background

The most typical feature in many cases is a sinking dry intrusion from behind. It splits into two branches over the rearward and middle part of the cloud band of the CF in CA: one branch is oriented to the south-southwest and the other branch to the north-northeast. The southern part is sinking from high levels to middle or even low levels, whereas the northern part stays on high levels. This feature is not always clear, however, especially in the earlier stages of development. The warm conveyor belt is usually weak.

The great majority of CF in CA's are either weakening or remaining the same as they are. This is because the strong cold advection triggers the weakening of the fronts.

In the summer the CF in CA's may be accompanied by MCS development on the leading edge of the cloud band (see Convective Cloud Features in Typical Synoptic Environments: At the Leading Edge of Frontal Cloud Bands ). This is due to the unstable stratification caused by the warm conveyor belt below and cold dry intrusion above. This development occurs regularly in the area of northern Adriatic Sea, where the topography is further contributing to the instability.

On the 3rd of September 2007 at 00 UTC there is a Cold Front in Cold Advection reaching from Southern Sweden to the English Channel. The relative streams at 316 K show a dry intrusion coming from the cold, dry northern air and splitting over the frontal cloudiness into two branches: one is turning to follow the cloud band towards the northeast, while the other turns to southwest while sinking.

03 September 2007/00.00 UTC - Meteosat 8 IR10.8 image; magenta: relative streams 316K, yellow: isobars 316K

Key Parameters

In this chapter the key parameters of Cold Front in Cold Advection reaching from Denmark towards the southwest are described and compared to the corresponding parameters of a Cold Front over Scotland.

Positive vorticity advection at 500 hPa and temperature advection at 700 hPa:

  • In a CF in CA there is a maximum of PVA over the rear edge of the cloud band.
  • In a CF in CA there is cold advection over the whole frontal cloudiness as well as behind it. Ahead of the front the temperature advection is usually negligible.
  • In a CF there is warm advection ahead of the cloud band and strong cold advection behind it. The cloud band is mostly under cold advection.

20 January 2007/12.00 UTC - Meteosat 8 IR 10.8 image; green: PVA 500 hPa, red: temperature advection 700 hPa
19 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; green: PVA 500 hPa, red: temperature advection 700 hPa

Thermal frontal parameter and equivalent thickness:

  • TFP accompanies the cloud band in both cases, although in CF in CA it is weaker.
  • Equivalent thickness shows a crowding zone near the rear edge of the cloud band, and a ridge ahead of it.
20 January 2007/12.00 UTC - Meteosat 8 IR 10.8 image; blue: TFP, green: equivalent thickness
19 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; blue: TFP, green: equivalent thickness

Shear vorticity and wind at 300 hPa:

  • The zero line of shear vorticity follows the rear edge of the cloud band.
20 January 2007/12.00 UTC - Meteosat 8 IR 10.8 image; black: zero line of shear vorticity 300 hPa, yellow: isotachs 300 hPa
19 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: zero line of shear vorticity 300 hPa, yellow: isotachs 300 hPa

Typical Appearance In Vertical Cross Sections

Comparison of cross sections between a Cold Front and a Cold Front in Cold Advection (see Cold Front - Typical Appearance In Vertical Cross Sections ):

  • Temperature Advection: in CF warm advection ahead of the frontal zone and strong cold advection behind it, in CF in CA cold advection within and behind the frontal zone, weak warm advection ahead of it.
  • Omega: strong ascend ahead of the frontal zone and weak descend behind it.
  • Positive Vorticity Advection: the frontal zone is within PVA, which increases with height.
  • Divergence: convergence within the frontal zone, divergence above it. Features are similar in both types.
  • Relative humidity: Humid air ahead of the frontal zone, dry air intruding into it from behind.

Cold Front in Cold Advection: 20 January 2007/06.00 UTC - Meteosat 8 10.8 image; position of vertical cross section indicated
Cold Front: 10 November 2006/06.00 UTC - Meteosat 8 10.8 image; position of vertical cross section indicated

 

Temperature advection

20 January 2007/06.00 UTC - Vertical cross section; black: isentropes, red thick: warm advection, red thin: cold advection
10 November 2006/06.00 UTC - Vertical cross section; black: isentropes, red thick: warm advection, red thin: cold advection

 

Omega

20 January 2007/06.00 UTC - Vertical cross section; black: isentropes, cyan thick: upward motion, cyan thin: downward motion
10 November 2006/06.00 UTC - Vertical cross section; black: isentropes, cyan thick: upward motion, cyan thin: downward motion

 

Positive Vorticity Advection

20 January 2007/06.00 UTC - Vertical cross section; black: isentropes, green: PVA
10 November 2006/06.00 UTC - Vertical cross section; black: isentropes, green: PVA

 

Divergence

20 January 2007/06.00 UTC - Vertical cross section; black: isentropes, magenta thick: convergence, magenta thin: divergence
10 November 2006/06.00 UTC - Vertical cross section; black: isentropes, red thick: warm advection, red thin: cold advection

Weather Events

Parameter Description
Precipitation
  • Mostly slight to moderate precipitation at the rear side of the cloud band
  • Sometimes no rain
  • In summer a small risk of embedded Cbs with thunderstorms and heavy rain
Temperature
  • Falls slowly after the passage of the front.
Wind (incl. gusts)
  • Veering of the wind at the front passage
  • In summertime strong gusts possible after the front passage.
Other relevant information
  • Less precipitation than in a classical Cold Front, often no precipitation at all
  • Risk of moderate icing and turbulence
  • Very small risk of thunderstorms
  • Front consists mainly of dissipating low and middle cloudiness, sometimes almost cloudless.

 

15 January 2007/18.00 UTC - Meteosat 8 10.8 image; weather observations

References

References concerning the "Cold Front in Cold Advection" are similar to the ones for "Cold Front". Compare Cold Front - References