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Appearance in Satellite Data

  • IR10.8: often a little bit warmer cloud tops than those associated with the Occlusion cloud, therefore light grey in the IR image, sometimes fibrous and broken
  • VIS0.8: light grey
  • WV6.2: varying from very bright to light grey
  • Airmass RGB: High reaching and a little darker clouds than the associated Occlusion cloud band.

15 January 2009/00.00 UTC - Meteosat 9 IR10.8 image
15 January 2009/00.00 UTC - Meteosat 9 WV6.2 image
15 January 2009/00.00 UTC - Meteosat 9 Airmass RGB image
20 December 2008/21.00 UTC - Meteosat 9 IR10.8 image
20 December 2008/00.00 UTC - Meteosat 9 IR10.8 image; 20 December 00.00 UTC - 21 December 00.00 UTC 3-hourly image loop
20 December 2008/21.00 UTC - Meteosat 9 WV6.2 image
20 December 2008/00.00 UTC - Meteosat 9 IR10.8 image; 20 December 00.00 UTC - 21 December 00.00 UTC 3-hourly image loop

Meteorological Physical Background

Different types of formation could be observed:

  • North of an Occlusion another frontal system is passing by and a zone of confluence is developing
  • Associated with Rapid Cyclogenesis (or Cold Conveyor Belt Occlusion): double structure at cloud spiral becoming detached

First Type

12 December 2003/00.00 UTC - Meteosat IR image
15 January 2009/00.00 UTC - Meteosat 9 IR 10.8 image

When looking at wind fields, both confluence and convergence where studied. Confluence is seen directly from the wind vectors when the direction of two neighbouring vectors are orientated towards each other, e.g. two air flows are coming together. On the other side convergence as a numerical parameter takes wind direction (confluence) as well as wind speed into account (see basic chapter for more details). In some cases where confluence can be observed, no convergence is found. In this study the contribution of confluence seems to be very important.

The Convergence Cloud develops in area of confluence of a stream from the north and a stream from the south where the confluence can seen best near the surface. The southerly flow is caused by the rotation of the Occlusion round the surface depression whereas the northerly results from the other system passing by. As a result of these two wind regimes interaction convergence is seen near the surface. A secondary convergence maximum is situated at higher levels, varying from case to case, both in level and in intensity. The relationship of cloud top height with convergence was investigated. It was found that Convergence Cloudiness shows as higher cloudiness if there is a thick layer of upper tropospheric convergence OR if the convergence in low levels is very distinct. Both convergence maxima conribute to the development of cloudiness.

Second Type

20 December 2008/06.00 UTC - Meteosat 9 IR10.8 image
20 December 2008/18.00 UTC - Meteosat 9 IR10.8 image

The Convergence Cloudiness develops in an area of confluence of the stream from the north and the stream from the south. Deformation, as well as vorticity associated with Occlusion, can clearly be seen. Convergence is found from the surface up to 700 hPa.

20 December 2008/18.00 UTC - Meteosat 9 IR10.8 image; magenta: wind (blue) and isotachs (red) 1000 hPa
20 December 2008/18.00 UTC - Meteosat 9 IR10.8 image; cyan: wind 850 hPa
20 December 2008/18.00 UTC - Meteosat 9 IR10.8 image; magenta: wind 500 hPa

For a different case the relative streams were investigated. It was found that the area of Convergence Cloudiness shows a significantly slower system velocity than the whole Occlusion. This can be interpreted as an indication that the convergence cloudiness can be regarded as a separate system in calculating relative streams. If only the convergence cloudiness is taken into account, the relative streams show following pattern in the majority of cases:

  • The Convergence Cloud has a different velocity which is an indication that it should be treated as a separate CM. In most cases the displacement is less than that of the whole cloud spiral
  • In the majority of cases an isolated cyclonic circulation could be found
  • This circulation is more pronounced at upper levels
  • The convergence cloudiness lies within (slight) upward motion

19 March 2004/12.00 UTC - Meteosat IR image; magenta: relative streams 298K, yellow: isobars 298K
06 February 2004/12.00 UTC - Meteosat IR image; magenta: relative streams 294K, yellow: isobars 294K
06 February 2004/06.00 UTC - Meteosat IR image; magenta: relative streams 298K, yellow: isobars 298K

Some cases do not show this pattern - then the convergence cloudiness lies within the upward (cyclonic) motion of the Occlusion stream lines.

10 February 2004/12.00 UTC - Meteosat IR image; magenta: relative streams 284K, yellow: isobars 284K
10 February 2004/12.00 UTC - Meteosat IR image; magenta: relative streams 288K, yellow: isobars 288K

The above mentioned findings are valid if the Convergence Cloudiness is treated separately. If the Occlusion cloud spiral or even the whole frontal system is used to calculate relative streams, this pattern vanishes. Upward motion can still be seen but there is no separate circulation.

Is Convergence Cloudiness a special type of Warm Front?

Indeed, there are many common features between Warm Fronts and this type of Convergence Cloudiness, as seen in numerical parameter fields and satellite imagery. The main difference is the formation: According to the polar front theory a baroclinic zone at the transition between warmer and colder air masses forms a Wave which then leads to cyclogenesis. The Occlusion develops after the Warm Front. In the case of the Convergence Cloudiness the Occlusion cloud spiral is already well developed and then the convergence shield forms.

Open questions

It is not possible to forecast whether Convergence Cloudiness will develop at the end of an Occlusion spiral or not. There are some indications that it occurs when another system passes nearby, but the processes are not yet fully understood.

Key Parameters

  • Convergence, especially in low levels
  • Confluent wind vectors, confluence more pronounced than convergence
  • Wind vectors: in lower and middle levels different wind regime between Occlusion and Convergence Cloudiness; at higher levels, no noticeable difference
  • Often within weak WA
  • TFP shows small, but positive values
  • Cloud bands connected to the Occlusion tend to be located within a thickness ridge which is less pronounced than with standard Occlusions

20 December 2008/18.00 UTC - Meteosat 9 IR10.8 image; red: temperature advection 700 hPa, blue: TFP, green: equivalent thickness

20 December 2008/18.00 UTC - Meteosat 9 IR10.8 image; red: divergence 850 hPa, blue: divergence 700 hPa

Typical Appearance In Vertical Cross Sections

  • Isentropes are inclined, but less pronounced than for fronts; sometimes there is a double structure (frontal character at the surface and above)
  • Often within weak WA
  • Convergence in lower levels, sometimes in upper levels, but there are big differences from case to case
  • Isotachs show a low level minimum at the transition between the Occlusion and Convergence Cloudiness

20 December 2008/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values

20 December 2008/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), magenta thick: convergence, magenta thin: divergence, orange thin: IR pixel values, orange thick: WV pixel values

Weather Events

The Convergence Cloudiness CM represents a quasi-frontal situation, in effect, attached to the end of an Occlusion. Therefore weather events are frontal in character.

Parameter Description
Precipitation
  • Widespread precipitation in the form of rain or snowfall, in some cases it can be more than from Occlusion cloudiness.
  • Convective features are unlikely.
Temperature
  • Rises weakly after Convergence Cloudiness
Wind (incl. gusts)
  • Surface winds weak at the transition zone where the cloudiness joins the Occlusion
Other relevant information None

10 February 2004/12.00 UTC - Meteosat IR image; weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm, black: no precipitation)

References