INSTANT OCCLUSION - KEY PARAMETERS
by ZAMG
- Absolute topography at 500 and 1000 hPa:
In the initial stages the absolute topography at 500 and 1000 hPa is characterized by two troughs. In case of a west to east propagating system the eastern trough, which represents a bigger scale, is connected to the Cold Front. The western, smaller trough is connected to the faster propagating Comma cloudiness behind the Cold Front. In the maturing stage the absolute topography at 1000 hPa is characterized by a low upstream of the merging zone between the former Comma cloudiness and the cloud band of the Cold Front. The absolute topography at 500 hPa shows a broad trough within the cold air mass behind the Cold Front. - Equivalent thickness and thermal front parameter (TFP):
In the initial stages the Comma cloud can be found in the thickness trough while the frontal cloud band is accompanied by usually a thickness gradient with a TFP at the leading edge. In the mature stage of the Instant Occlusion, equivalent thickness and TFP develop a ridge in the former Comma area. But both fields show rather small ridges compared to the classical Occlusion types (see Occlusion: Warm Conveyor Belt Type and Occlusion: Cold Conveyor Belt Type ). Maybe this indicates a different process of development. - Temperature advection:
The field of temperature advection is dominated by two phenomena:- The CA moving within and behind the Comma.
- A WA maximum extending from the Wave area in the frontal cloud band westward to the Comma head area. This WA may intensify during the merging process; but the maximum of WA remains, in the Wave area of the frontal cloud, which again is in contrast to classical Occlusion types (see Occlusion: Warm Conveyor Belt Type and Occlusion: Cold Conveyor Belt Type ).
- 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. - Isotachs at 300 hPa:
In the inital stages the Comma cloudiness is situated within the area of the left exit region of a jet streak, but in any case curvature plays an additional role. In the maturing stage the jet axis is situated parallel to the rear cloud edge of the Cold Front. Therefore it crosses the cloudiness in the area of the Instant Occlusion and leads to dark grey shades in the WV image there (see Cloud structure in satellite image ). In this stage there might be differences between a Cold Conveyor Belt Occlusion and an Instant Occlusion development. In the case of a Cold Conveyor Belt Type Occlusion the cloud spiral is situated within the left exit region of a jet streak; this however is not apparent for an Instant Occlusion development. On the contrary the PVA maximum is very close to the jet streak centre which is a sign for the existance of strong curvature vorticity.
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12 January 1997/12.00 UTC - Meteosat IR image; green: equivalent thickness 500/850 hPa, blue: thermal front parameter 500/850 hPa
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13 January 1997/00.00 UTC - Meteosat IR image; green: equivalent thickness 500/850 hPa, blue: thermal front parameter 500/850 hPa
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12 January 1997/12.00 UTC - Meteosat IR image; green: equivalent thickness 500/850 hPa, red: temperature advection 500/1000 hPa
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13 January 1997/00.00 UTC - Meteosat IR image; green: equivalent thickness 500/850 hPa, red: temperature advection 500/1000 hPa
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12 January 1997/12.00 UTC - Meteosat IR image; green: vorticity advection 500 hPa
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13 January 1997/00.00 UTC - Meteosat IR image; green: vorticity advection 500 hPa
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12 January 1997/12.00 UTC - Meteosat IR image; yellow: isotachs 300 hPa, red: vorticity advection 300 hPa
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12 January 1997/12.00 UTC - Meteosat WV image; yellow: isotachs 300 hPa, red: vorticity advection 300 hPa
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13 January 1997/00.00 UTC - Meteosat IR image; yellow: isotachs 300 hPa, red: vorticity advection 300 hPa
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13 January 1997/00.00 UTC - Meteosat WV image; yellow: isotachs 300 hPa, red: vorticity advection 300 hPa
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