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Appearance in Satellite Data

Usually Cold Fronts are associated with cold advection behind the frontal zone, but there are cases, where there is warm advection also within and behind it, albeit usually weak. These are called Cold Fronts in Warm Advection, abbreviated CF in WA.

35 cases of CF in WA have been investigated here.

In satellite imagery the cloud bands of CF in WA look generally similar to Cold Fronts, but often they are weaker: Roughly one third of cloud bands consist only of low clouds, and almost half of them are more fibrous and broken than Cold Fronts.

As with Cold Front in Cold Advection, CF in WA can be a whole front, or a part of it (the rest of the front being either classical Cold Front or CF in CA).

CF in WA forms, when a Cold Front encounters warm air advection related to an another approaching system. Most often this happens, when a new wave forms within a Cold Front.

The appearance of CF in WA in different channels:

  • In VIS images a light grey cloud band with some broken structure
  • In IR images grey, broken cloud band, which is clearly darker than other frontal parts of the system
  • In WV images a grey cloud band
  • In the combination (HRVis;HRVis;IR10.8i) greyish yellow cloud band with possibly 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 3rd of October 2006 at 12.00 UTC, there is an occlusion over Russia, with a west-east oriented Cold Front. There is a WF Shield over Germany, and the warm advection ahead of it covers the area of the Cold Front.

As a result, a Cold Front in Warm Advection is formed over Poland and Belarus.

03 October 2006/12.00 UTC - Meteosat 8 VIS 0.6 image
03 October 2006/12.00 UTC - Meteosat 8 IR 10.8 image
03 October 2006/12.00 UTC - Meteosat 8 WV 6.2 image
03 October 2006/12.00 UTC - Meteosat 8 RGB (HRVis;HRVis;IR10.8i) image
03 October 2006/12.00 UTC - Meteosat 8 RGB (WV6.2-WV7.3; NIR3.9-IR10.8; NIR1.6-VIS0.8) image

Meteorological Physical Background

Cold Fronts in Warm Advection, abbreviated CF in WA, are principally like normal Cold Fronts, but they are wholly within warm advection. The warm advection is generally weak, and it is weaker behind the frontal zone than ahead of it.

In summer the Cold Fronts in Warm Advection are nearly as common as Cold Fronts in Cold Advection, but in winter they are rare.

CF in WA forms, when a Cold Front encounters an area of warm advection connected to another, approaching system. Usually this system is a Warm Front, either a wave forming within the Cold Front, or another cyclone catching up the Cold Front. It can also be Detached Warm Front, Warm Conveyor Belt, Baroclinic Boundary, or an Occlusion with warm advection ahead of it.

CF in WA is often a relatively old, weakening front. This stage does not last long (typically less than 12 hours), and it soon turns into some other type of Cold Front or a Wave, or just disappears.

According to studies at ZAMG, about half of CF in WA cases are weakening, and only about 10 % are intensifying.

The main feature of a CF in WA is a Warm Conveyor Belt. It tends to either intensify with height or to exist only at higher levels. In about half of the cases the Warm Conveyor Belt overruns the cloud band to the north. An upper level stream occurs only in the minority of cases. In several cases a closed cyclonic circulation can be found behind the Warm Conveyor Belt, but with little vertical displacement.

As there is only one conveyor belt involved, the temperature and humidity contrasts are rather weak in the area of the frontal cloud band.

03 October 2006/12.00 UTC - Meteosat 8 IR10.8 image; position of vertical cross section indicated
03 October 2006/12.00 UTC - Meteosat 8 IR10.8 image; magenta: relative streams 318K, yellow: isobars 318K
03 October 2006/12.00 UTC - Vertical cross section; black: thetaE, red: warm advection

Key Parameters

In this chapter the key parameters of Cold Front in Warm Advection (over Belarus and Poland) 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 WA there is a maximum of PVA over the rear edge of the cloud band.
  • In a CF the maximum of PVA is equally distributed, but remarkably stronger.
  • In a CF in WA the cloud band is wholly under weak warm advection; some warm advection is also ahead of the front. Behind the front there is usually very little temperature advection.
  • 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.
03 October 2006/12.00 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 WA it is weaker.
  • Equivalent thickness shows a crowding zone near the rear edge of the cloud band, and a ridge ahead of it.
03 October 2006/12.00 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 and coincides the jet stream.
03 October 2006/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: 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

The vertical structure of CF in WA shows most clearly, that the front is indeed a Cold Front, despite being within warm advection.

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

  • TA: in CF warm advection ahead of the frontal zone and strong cold advection behind it, in CF in WA only weak warm advection is present
  • Omega: in CF strong ascend ahead of the frontal zone and weak descend behind it, in CF in WA mostly only weak ascend on the middle levels
  • PVA: In a CF the frontal zone is within PVA, which increases with height. The maximum is just below the tropopause. In a CF in WA there is only weak PVA on the upper levels.
  • Divergence: convergence within the frontal zone, divergence above it. Features are similar in both types, but in CF they are stronger.
  • Relative humidity: Humid air ahead of the frontal zone, dry air intruding into it from behind. In CF in WA the dry air does not penetrate through the whole frontal zone.

Locations of the cross section for both cases:

Cold Front in Warm Advection: 03 October 2006/12.00 UTC - Meteosat 8 10.8 image; position of vertical cross section indicated
Cold Front: 19 September 2005/12.00 UTC - Meteosat 8 10.8 image; position of vertical cross section indicated

Temperature advection:

  • In CF in WA the whole system is under weak warm advection.
  • In CF there is warm advection above and ahead of the frontal zone and pronounced cold advection behind it, and partly within it.
03 October 2006/12.00 UTC - Vertical cross section; black: isentropes, red: warm advection
19 September 2005/12.00 UTC - Vertical cross section; black: isentropes, red: warm advection (no cold advection present)

Vertical motion:

  • In CF in WA there is weak ascent ahead of the frontal zone on the middle levels, and even weaker descent behind it.
  • In CF there is strong ascent ahead of the frontal zone and descent behind it.
03 October 2006/12.00 UTC - Vertical cross section; black: isentropes, cyan thick: upward motion, cyan thin: downward motion
19 September 2005/12.00 UTC - Vertical cross section; black: isentropes, cyan thick: upward motion, cyan thin: downward motion

Vorticity advection:

  • In CF in WA there is weak PVA on the upper levels.
  • In CF the frontal zone is within PVA, which increases with height. The maximum is just below the tropopause.
03 October 2006/12.00 UTC - Vertical cross section; black: isentropes, green: PVA
19 September 2005/12.00 UTC - Vertical cross section; black: isentropes, green thick: PVA, green thin: NVA

Divergence:

  • In connection with fronts there is convergence within the frontal zone and divergence above it.
03 October 2006/12.00 UTC - Vertical cross section; black: isentropes, magenta thick: convergence, magenta thin: divergence
19 September 2005/12.00 UTC - Vertical cross section; black: isentropes, magenta thick: convergence, magenta thin: divergence

Relative humidity:

  • The relative humidity is high ahead of the frontal zone and low behind it.
03 October 2006/12.00 UTC - Vertical cross section; black: isentropes, blue solid: relative humidity > 60%, blue dashed: relative humidity < 70%
19 September 2005/12.00 UTC - Vertical cross section; black: isentropes, blue solid: relative humidity

Weather Events

As Cold Fronts in WA are usually weaker than other Cold Fronts, they give also less precipitation. In about a third of the cases there is no precipitation at all. Severe weather like thunderstorm is very unlikely.

Parameter Description
Precipitation
  • Mostly slight, sometimes moderate precipitation
  • In about 1/3 of the fronts no precipitation
Temperature
  • Falls slowly after the passage of the surface front
Wind (incl. gusts)
  • Veering of the wind at the front passage
Other relevant information
  • Less precipitation than in a classical Cold Front, often no precipitation at all
  • Risk of moderate icing
  • Very small risk of thunderstorms
  • Front consists mainly of dissipating low and middle cloudiness, sometimes almost cloudless.
03 October 2006/12.00 UTC - Meteosat 8 10.8 image; weather events (green: rain and showers, blue: drizzle, cyan: snow, yellow: fog, black: no precipitation)

References

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