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Appearance in Satellite Data

The cloud feature called Instant Occlusion forms when a Comma cloud merges with a frontal cloud band. Comma is situated within an cold air mass behind a Cold Front, so it is in unstable environment, which leads to the formation of convective cloudiness. The process of forming an Instant Occlussion is one where stratified frontal cloudiness and convective cloudiness merge.

The appearance of Instant Occlusion in different channels:

  • VIS and IR: In the initial, separate Comma stage, the frontal cloud band is white and the Comma cloud and the surrounding convective cloudiness are white to light grey. In the final, merged stage, the whole cloud system is white.
  • WV: In the initial stage the Comma cloud is initially light grey to grey, and the frontal cloudiness is white with some light grey structure. In the final stage the whole cloud system is white with some light grey structure.
  • Air mass combination WV6.2-WV7.3,IR9.7- IR10.8,WV6.2: In the initial stage the frontal cloudiness is white, while the Comma cloud appears light orange/yellow, being in the reddish very cold and dry air mass descending from the stratosphere behind the Cold Front. In the final stage the whole cloud system is white.

In the following pictures the initial stage is on the left, the final stage on the right side.

On the 1st of December a Comma cloud was located over Ireland, and a Cold Front extends from Norway to England and further southwest. The Instant Occlusion process took place during the next 12 hours.
In the following images the initial and final stages are shown in different channels.

01 December 2006/12.00 UTC - Meteosat 8 IR10.8 image
02 December 2006/12.00 UTC - Meteosat 8 IR10.8 image
01 December 2006/12.00 UTC - Meteosat 8 WV6.2 image
02 December 2006/12.00 UTC - Meteosat 8 WV6.2 image
01 December 2006/12.00 UTC - Meteosat 8 WV6.2-WV7.3,IR9.7- IR10.8,WV6.2
02 December 2006/12.00 UTC - Meteosat 8 WV6.2-WV7.3,IR9.7- IR10.8,WV6.2

Meteorological Physical Background

In the Instant Occlusion process two conceptual models interact, namely a Comma and a Cold Front. The location of the instant occlusion is often the left exit area of a diffluent upper trough.

The Cold Front is connected with a large upper trough. In this trough there is a smaller secondary trough moving faster than the large trough. In the secondary trough there is convective cloud development, a Comma. The development involves the expansion of a cloud cluster into a baroclinic leaf with cyclonic rotation.

Because the smaller trough is moving faster than the large one, Comma overtakes the frontal cloudiness and the two cloud systems merge. Often there is simultaneously going on a wave development within the polar front. The result is a cloud system that resembles an occlusion, but without the actual occlusion process. If the Comma merges with the frontal cloudiness in the area of a Wave, the Occlusion-like feature is very pronounced.

Initial (pre-merging) stage:

  • A Cold Front is associated with an eastward moving upper trough.
  • In the cold air mass behind the Cold Front, but clearly separated from it, there is a Comma cloud related to a small, sharp trough moving along the edge of the large trough (see Key Parameters).
  • There is weak baroclinity related to the Comma; as the cyclonic motion increases, regions of warm and cold advection form ahead and behind the Comma, respectively (see Key Parameters).

Merging stage:

  • Positive vorticity advection and warm advection associated to the Comma produce upward motion. This leads to a rapid cloud development, and as the two cloud features simultaneously get nearer each other, they merge.
  • The warm advection ahead of the Comma creates a short nook in the temperature field of the frontal zone, but it is remarkably smaller than the ridge that is formed in an occlusion leading to the typical appearance of an occlusion cloud spiral (see Key Parameters).

 

Discussion: The relative stream point of view

The Initial Stage:

Within and ahead of the Comma an ascending stream of warm and moist air is orientated from southern to northern directions in lower layers. Within the middle and upper levels of the troposphere, dry air moves around the upper level trough overrunning the Comma tail and turning to northern directions parallel to the rear cloud edge of the Cold Front. The vertical stratification of relative stream with warm, moist air and dry air above causes a potential unstable stratification in the shallow moist zone between the two cloud features.

 

The Merging Stage:

The potential unstable stratification within the area of the shallow moist zone will now be released due to the developing ascending motion. The ascending motion is caused by the combination of WA and PVA which contributes to upward motion; in particular WA is connected with the Wave feature at the Cold Front. As a consequence of this process rapid cloud development can be observed in the shallow moist zone. Due to WA ahead of the Comma a thermal gradient is generated on its northern side.

Features of relative streams:

  • A low level dry outflow from eastern directions appears
  • The wet low level stream from south-west no longer exists
  • A rising warm conveyor belt accompanying the frontal cloud band
  • A relative stream from behind strongly rising in the area of the front and the Comma in accordance with the area of cloud development
  • A splitting of the relative stream from behind close to the Wave point

 

Final Stage:

In the end of the development the cloud configuration may resemble Cold Conveyor Belt occlusion (see Occlusion: Cold Conveyor Belt Type ), or the part of the cloudiness that resembles an occluded front, can dissolve.

Case studies, however, have shown that the relative stream structure described above can not always be found. More investigations with high resolution model would be needed.

Key Parameters

  • Absolute topography at 500 and 1000 hPa:
    • Initially a slowly moving large-scale and a faster moving small-scale trough within it.
    • At final stage the small-scale trough coincides with a surface low.
  • Equivalent thickness and thermal front parameter (TFP):
    • The frontal cloud band is associated with high gradient of thickness.
    • Initially the Comma is within a weak thickness ridge.
    • At the final stage there is a thickness gradient with a small nook in the location of the merging of Comma and frontal cloudiness.
  • Temperature advection at 700 hPa:
    • There is CA behind the Cold Front, and WA ahead of the Warm front. Weaker temperature advections are related to Comma (WA in the leading edge, CA in the tail area).
  • Vorticity advection at 500 hPa:
    • A small, but distinct PVA maximum is in the Comma area, mostly within the comma tail.
    • PVA maxima can also be found near the rear edge of the Cold Front cloud band indicating the big upper trough and/or the approach of a jet streak (see Cold Front ).

Height contours at 1000 and 500 hPa

Initial stage
01 December 2006/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height 500 hPa; magenta: height 1000 hPa
Final stage
02 December 2006/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height 500 hPa; magenta: height 1000 hPa

 

Equivalent thickness and TFP

Initial stage
01 December 2006/12.00 UTC - Meteosat 8 IR 10.8 image; green: equivalent thickness 500/850 hPa, blue: thermal front parameter 500/850 hPa
Final stage
02 December 2006/12.00 UTC - Meteosat 8 IR 10.8 image; green: equivalent thickness 500/850 hPa, blue: thermal front parameter 500/850 hPa

 

Temperature Advection at 700 hPa

Initial stage
01 December 2006/12.00 UTC - Meteosat 8 IR 10.8 image; red solid: warm advection at 700 hPa, red dashed: cold advection 700 hPa
Final stage
02 December 2006/12.00 UTC - Meteosat 8 IR 10.8 image; red solid: warm advection at 700 hPa, red dashed: cold advection 700 hPa

 

Vorticity advection at 500 hPa

Initial stage
01 December 2006/12.00 UTC - Meteosat 8 IR 10.8 image; green: Positive Vorticity Advection 500 hPa
Final stage
02 December 2006/12.00 UTC - Meteosat 8 IR 10.8 image; green: Positive Vorticity Advection 500 hPa

Typical Appearance In Vertical Cross Sections

  • Isentropes:
    The typical downward inclined crowding zone with the Cold Front, potentially unstable stratification in the lower troposphere in the Comma area
  • PVA:
    Maxima in the frontal zone increasing with height, a weaker maximum in the comma area.
  • Divergence:
    In the frontal zone convergence on the lower and middle levels, divergence above. In the Comma area convergence on the low layers, divergence on the middle layers.
  • Omega:
    Upward motion in the frontal zone and in the Comma area in lower and middle troposphere.

12 January 2007/06.00 UTC - Meteosat IR 10.8 image; position of vertical cross section indicated
15 January 2007/06.00 UTC - Vertical cross section; black: isentropes
15 January 2007/06.00 UTC - Vertical cross section; black: isentropes, green: positive vorticity advection
15 January 2007/06.00 UTC - Vertical cross section; black: isentropes, magenta thick: convergence, magenta thin: divergence
15 January 2007/06.00 UTC - Vertical cross section; black: isentropes, cyan: upward motion

Weather Events

Parameter Description
Precipitation
  • Rain, showers and thunderstorms in the area of Comma - like cloudiness.
Temperature
  • No significant change.
Wind (incl. gusts)
  • Moderate to strong winds.

03 March 2008/ 12.00 UTC - Meteosat 8 IR 10.8 image; weather events (green: rain, snow, showers and mist)

References

General Meteorology and Basics

  • BARRY, R. G., CARLETON, A. M. (2001): Synoptic and Dynamic Climatology; Routledge
  • BROWNING K. A. and HILL F. F. (1985): Mesoscale analysis of a polar trough intersecting with a polar front; Quart.J. R. Meteor. Soc., Vol. 111, p. 445 - 462
  • BROWNING K. A. (1985): Conceptual models of precipitation systems; Met. Mag., Vol. 114, p. 293 - 317
  • BROWNING K. A. (1986): Conceptual models of precipitation systems; Weather & Forecasting, Vol. 1, p. 23 - 41
  • BROWNING K. A. (1990): Organization of clouds and precipitation in extratropical cyclones; in: Extratropical Cyclones, The Erik Palmen Memorial Volume, Ed. Chester Newton and Eero O Holopainen, p. 129 - 153
  • CONWAY B. J., GERARD L., LABROUSSE J., LILJAS E., SENESI S., SUNDE J. and ZWATZ-MEISE V. (1996): COST78 Meteorology - Nowcasting, a survey of current knowledge, techniques and practice - Phase 1 report; Office for official publications of the European Communities
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  • LOCATELLI J. D., HOBBS P. V. and WERTH J. A. (1982): Mesoscale structures of vortices in polar air streams; Mon. Wea. Rev., Vol. 110, p. 1417 - 1433

General Satellite Meteorology

  • EVANS M. S., KEYSER D., BOSART L. F. and LACKMANN G. M. (1994): A satellite derived classification scheme for rapid maritime cyclogenesis; Mon. Wea. Rev., Vol. 122, p. 1381 - 1416
  • MCGINNIGLE J. B., YOUNG M. V. and BADER M. J. (1988): The development of instant occlusion in the North Atlantic; Met. Mag., Vol 117, p. 325 - 341
  • MULLEN S. L. (1983): Explosive cyclogenesis associated with cyclones in polar air streams; Mon. Wea. Rev., Vol. 111, p. 1537 - 1552
  • REED R. J. (1979): Cyclogenesis in polar air streams; Mon. Wea. Rev., Vol. 107, p. 38 - 52
  • THEPENIER T. M. and CRUETTE D. (1981): Formation of cloud bands associated with the American subtropical jet stream and their interaction with mid-latitude synoptic disturbances reaching Europe; Mon. Wea. Rev., Vol. 109, p. 2209 - 2220