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Appearance in Satellite Data

In this study 12 cases of Back Bent Occlusions were investigated. In most cases the cloud band related to the occluded front is long and broad, but there are also cases, especially in cold air masses, where the cloud band is short, narrow and not very striking in the satellite image.

In the initial stage there is a quite straight or only slightly cyclonically curved occluded front. The movement of the occluded front stops, and after that the whole front or a part of it begins to move backwards. In most cases the occluded front is bent back by a cold advection originating from another low (usually Upper Level Low; see Upper Level Low ) in a polar airmass.

On the 21st of November 2000 at 12.00 UTC, there is a long, broad occluded front over the Atlantic and an upper level low over England and Scotland:

21 November 2000/12.00 UTC - Meteosat IR image
21 November 2000/12.00 UTC - Meteosat VIS image
21 November 2000/12.00 UTC - Meteosat WV image

The bending of the occluded front can clearly be seen in a sequence of satellite images. Part of the occluded front bends backwards, and the occluded front forms an s-shaped cloud band:

21 November 2000/09.00 UTC - Meteosat IR image
21 November 2000/15.00 UTC - Meteosat IR image
21 November 2000/21.00 - Meteosat IR image
22 November 2000/03.00 UTC - Meteosat IR image

On 18.00 June 2001 at 09.00 UTC there is a short, narrow Occlusion southwest of Iceland and a small upper level low between Iceland and Greenland with only slight cloudiness:

18 June 2001/09.00 UTC - Meteosat IR image
18 June 2001/09.00 UTC - Meteosat VIS image
18 June 2001/09.00 UTC - Meteosat WV image

The movement of the occluded front backwards can be seen in a sequence of satellite images. In this case the bending is not very pronounced:

18 June 2001/09.00 UTC - Meteosat IR image
18 June 2001/18.00 UTC - Meteosat IR image
19 June 2001/06.00 - Meteosat IR image

Meteorological Physical Background

The Back Bent Occlusion can be formed, if the surface pressure gradient is relatively weak near the center of the low. The low elongates in the direction of the occluded front, and the tail of the front remains almost straight or only slightly curved; it does not curl around the low centre like in other types of Occlusions.

 

The bending back of an Occlusion due to cold advection

In this case there is a stationary or slowly moving low, towards which an occluded cyclone moves. They merge into each other to some extent and the cold advection from the cold side of the stationary low forces the occluded front of the cyclone to bend back.

On 18 June 2001 at 00.00 UTC there is a Warm Occlusion moving eastwards over the Atlantic. An upper low, that appears just a weak trough on the lower levels, is located west of Iceland:

18 June 2001/00.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
18 June 2001/00.00 UTC - Meteosat IR image; red: temperature advection 700 hPa

On 18 June 2001 at 12.00 UTC the lows have merged into one and there is cold advection originating from the northern side of the Iceland low affecting on the occluded front:

18 June 2001/12.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
18 June 2001/12.00 UTC - Meteosat IR image; red: temperature advection 700 hPa

On the 19 June 2001 at 06.00 UTC there is only one low left, and the cold advection has turned the occluded front into a Cold Front:

19 June 2001/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
19 June 2001/06.00 UTC - Meteosat IR image; red: temperature advection 700 hPa

If the cold advection is very strong, the occluded front can bend tightly as it turns into a Cold (or even into an Arctic) Front. In that case the strongest winds are connected with the new Cold Front. The area between the two Cold Fronts is sometimes called "the false warm sector":

 

The bending back of an occlusion due to a stationary high pressure area

Sometimes Back Bent Occlusions form when they approach a stationary high pressure area. A moving cyclone slows down in front of a high. The occluded front stops and bends back due to the strengthening of the high pressure centre, while the Warm and the Cold Front continue slowly within a weaker high ridge area. In this case the Occlusion can be neutral, but usually these kind of Back Bent Occlusions occur in connection with cold continental highs during the winter. There is cold advection towards the occluded front from the edge of the high pressure, and the back bent front becomes a Cold Front.

 

Relative streams

From the relative streams it can be seen that the cold air affecting the Occlusion originates from behind a low pressure in the cold airmass.

The development of the Back Bent Occlusion:

  • In the first stage there is a Warm Occlusion; the air within the occluded front is rising near the Occlusion point, and remains on a constant level in area of the tail.
  • The Occlusion turns into a cold one, and finally it becomes a Cold Front. Behind the Cold Occlusion and the Cold Front the air is sinking.

17 June 2001/18.00 UTC there is a Warm Occlusion southwest of Iceland:

17 June 2001/18.00 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
17 June 2001/18.00 UTC - Meteosat IR image; magenta: relative streams 312K

By 18 June 2001/12.00 UTC, the Occlusion has turned into a Cold Occlusion:

18 June 2001/12.00 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
18 June 2001/12.00 UTC - Meteosat IR image; magenta: relative streams 312K
19 June 2001/00.00 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
19 June 2001/00.00 UTC - Meteosat IR image; magenta: relative streams 312K

It should be noted that when a trough in the polar airmass reaches an occluded cyclone from the rear, the situation can resemble a Back Bent Occlusion in the satellite image or even on the surface chart. In case of a trough, however, the coldest air is within the trough, and the cold advection is ahead of the trough, in contrast with the Back Bent Occlusion, where the cold advection is behind the back bent front.

Key Parameters

  • Height contours at 1000 hPa:
    The surface pressure gradient is relatively weak near the centre of the low, and often elongated in the direction of the occluded front.
  • Height contours at 500 hPa:
    In case of bending back of an Occlusion by a cold advection there is usually a low on upper levels in the cold airmass ahead of the Occlusion. In the case of a stationary high there is a pronounced large high pressure ahead of the Occlusion.
  • Temperature advection at 700 hPa:
    In the initial stage there is warm advection ahead of the occluded front. When the front bends back, it turns into a Cold Front and there is cold advection behind it. If the cyclone is in very cold airmass, the temperature advections are even clearer at 850 hPa.
    A Back Bent Occlusion can also be neutral if it is formed by a relatively warm stationary high, but this type is very rare.
  • Vorticity advection at 500 hPa and 300 hPa:
    The field of vorticity advection shows at 300 as well as at 500 hPa a pronounced PVA maximum in connection with the Comma, but overrides the frontal cloud band during the merging process. The PVA maximum mostly is situated within the area of the Comma tail, which typically can be found within the left exit region of a jet streak.
  • Thermal Front Parameter:
    The bending back of an occluded front can be seen from the curvature of TFP field around the Occlusion.

 

Height contours at 1000 hPa

18 June 2001/18.00 UTC - Meteosat IR image; magenta: height contours 1000 hPa, blue: temperature 850 hPa

 

Height contours at 500 hPa

18 June 2001/00.00 UTC - Meteosat IR image; cyan: height contours 500 hPa

The lows are merging and the occluded front becomes a Cold Front; black: absolute topography 500 hPa.

18 June 2001/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa

 

Temperature advection at 700 hPa

18 June 2001/00.00 UTC - Meteosat IR image; red: temperature advection 700 hPa

Temperature advections in a Back Bent Occlusion that turns into a Cold Occlusion:

18 June 2001/18.00 UTC - Meteosat IR image; red: temperature advection 700 hPa

 

Thermal frontal parameter

Surface fronts in the final stage:

18 June 2001/18.00 UTC - Meteosat IR image; red: temperature advection 700 hPa

Typical Appearance In Vertical Cross Sections

Cross sections for different types of occluded fronts are presented for example in the chapter Occlusion: Warm Conveyor Belt Type (see Occlusion: Warm Conveyor Belt Type - Key Parameters ). In this chapter only those parameters that indicate the process of bending back are mentioned.

  • Isentropes:
    In the beginning the isentropes show the structure of an Occlusion. It turns into a structure of a Cold Front during the process.
  • Temperature advection:
    In the beginning the vertical structure of temperature advection shows a distribution typical for an Occlusion. If the Back Bent Occlusion turns into a Cold Front, which is the most usual case, it can be seen in the change of the distribution of temperature advection.

The process in which a warm occluded front turns into a Cold Front can be seen from a sequence of cross sections:

 

A Warm Occlusion:

17 June 2001/18.00 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

 

A Cold Occlusion:

18 June 2001/12.00 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

 

A Cold Front:

19 June 2001/06.00 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

Weather Events

Parameter Description
Precipitation Moderate rain ahead of the warm occluded front near the low, elsewhere moderate to light rain or drizzle. Showers behind the cold occluded front.
Temperature A decrease in the temperature behind the cold occluded front.
Wind (incl. gusts) Possibly strong and gusty winds behind the cold occluded front.

18 June 2001/18.00 UTC - Meteosat IR image; weather events (green: rain and showers, blue: drizzle, cyan: snow, yellow: fog, black: no precipitation)

References

General Meteorology and Basics

  • KOISTINEN J.: Lectures on Synoptic Meteorology in the University of Helsinki, unpublished paper
  • KURZ M.: Synoptic Meteorology; Deutscher Wetterdienst 1998

General Satellite Meteorology

  • BADER M. J., FORBES G. S., GRANT J. R., LILLEY R. B. E. and WATERS A. J. (1995): Images in weather forecasting - A practical guide for interpreting satellite and radar imagery; Cambridge University Press.

Specific Satellite Meteorology

  • The Life Cycles of Extratropical Cyclones (1999)
  • A Planetary-Scale to Mesoscale Perspective of the Life Cycles of Extratropical Cyclones p. 139-161 (Shapiro, Wernli, Bao, Methven, Zou, Doyle, Holt, Donald-Grell, Neiman)
  • Mesoscale Aspects of Extratropical Cyclones: An Observational Perspective p. 265- Browning (University of Reading)