Rapid Cyclogenesis (RaCy) describes the development of a cloud head located on the pole ward side of a frontal zone into a cyclonic cloud spiral. The life cycle of this mechanism ranges from 6 to 48 hours.

Appearance in METEOSAT SECOND GENERATION imagery

IR10.8 imagery:

• During the initial stages of Rapid Cyclogenesis cloudiness with warmer cloud tops can be seen forming an increasingly dense shield with fibrous edges.
  This cloudiness develops on the pole ward side of usually west-east oriented frontal cloud bands. The developing cloud head varies between grey and light grey, mostly with higher tops on the pole ward side.
• Advection of dry sinking air originating from the lower levels of the stratosphere at the cyclonic side of a jet stream is advected downstream which leads to cloud dissipation between the cloud band of the Cold Front and the cloud head, thereby creating a V - structure (often also called a dry tongue).
• Due to the potentially unstable stratification of the troposphere (see Meteorological physical background) the inner edge of the cloud head is often characterized by convective cells seen as white spots.
• During the maturing stages the cloud head grows to a cyclonically curved cloud band with a broad dark area between the spiral and the frontal cloud band; additionally, the rear part of the frontal cloudiness frequently dissolves as a consequence of sinking dry air.

WV6.2 imagery:

• As the cloud head becomes more distinct the typical V - pattern, a black stripe, is formed caused by the advection of dry sinking air originating from the lower levels of the stratosphere along the cyclonic side of a jet stream.
• During the mature stage the cloud head grows into a cyclonically curved cloud band with a broad black area between the spiral and the frontal cloud band.
• Often a further Dark Stripe is present along the pole ward side of the cloud head, indicating a second jet streak.

VIS0.6 imagery:

• During the initial stages the cloud head forms an increasingly dense shield with fibrous edges and develops on the pole ward edge of the frontal cloud band. This cloudiness is characterized by high pixel values (white) with distinct shadows between the different high cloud systems.

Airmass RGB imagery:

• The Airmass-RGB is an artificial composite consisting of the WV 6.2 - WV 7.3 brightness temperature difference (BTD), on red, the IR 9.7 - IR 10.8 BTD on green and the WV 6.2 channel on blue. Typical features seen on the Airmass RGBs include dry intrusions, deformation zones and jet streaks. Especially the stratospheric air which is protruding down in the cold air behind the frontal zone therefore appears as a reddish colour.

The following examples, showing typical structures within the life cycle of Rapid Cyclogenesis, are composed of three cases:

• The case of 31 January 2008 shows a typical rapid development into a pronounced cloud spiral;
• The case of 09 March 2008 shows the dissipation of frontal cloudiness by dry stratospheric air;
• The case of 25 December 2002 is a good example of convective development in the area of the V - pattern.

The two loops above show a very rapid development of a depression over 12 hours from 1005 hPa on the 30th of January 2008/06.00 UTC to the extreme low value of 965 hPa on 31 January 2008/06.00 UTC. The evolution from an emerging cloud head, as the most predominant feature in the initial state, towards the V - pattern of the dry tongue, in the advanced stage and, eventually, the spiral in the mature stage are well shown in both loops. All the strong features that the two previous loops have shown are combined in the Airmass RGB loop above. The dry sinking air, or to be specific the protruding stratospheric air is pictured in reddish colours. This in combination with the IR information, to observe the high clouds, makes this RGB highly suitable to track Rapid Cyclogenesis. In all channels the images above show a strong dissipation of cloudiness, not only in the dry tongue but also in the associated Cold Front in the transition from advanced to mature stages of the cyclone. Around the V - pattern of the dry tongue NW of Iberian Peninsula (shown above), distinct convective activity is shown, not only in the Cold Front but also on the south side of the cloud head.

Appearance in AVHRR imagery

• AVHRR images (below right and third row left) show more detail of the cloud structures when compared to Meteosat images.
• A disadvantage is the lack of water vapour information.
• Pronounced substructures (in all channels) can be seen near the cyclonic and the anticyclonic regions.
• RGB-combination of channels (below left; second row) provides a quick overview of thin or thick, layered, low, middle or high cloudiness.
• Quite often low clouds form under the dry air intrusion, seen as a dark red (night) or yellow (day) area in RGB124.
• Image manipulation (e.g. subtraction with tresholding; third row right) helps to highlight the cloud pattern and some physical features.

Rapid Cyclogenesis over the Atlantic Ocean (approx. 53N/27W)

The area of Rapid Cyclogenesis is located in the left exit of a SW-NE (42N/35W - 53N/25W) jet streak and in the right entrance of another jet streak (56N/25W - 55N/12W). The jet, from SW to NE (wind max at 50N/30W) is seen as a line of fibrous clouds. The E - W oriented jet streak cloud is more fuzzy and less well defined. The initial formation of an occlusion can already be observed.

Rapid Cyclogenesis over the Atlantic Ocean (approx. 57N/22W)

The core of the SW - NE - orientated jet can be found as a narrow line of high clouds. Below the dry intrusion low clouds are seen (above left: dark red, above right: yellow). In this stage of development the Occlusion is more pronounced. In the warm sector (approx. 51N/16W) parallel lines of alternating high and low clouds indicate changes of wind speed at different levels.

Rapid Cyclogenesis over the Atlantic Ocean (approx. 57N/28W)

The core of the SW - NE - orientated jet is quite well defined in both images (above). The image above right also shows the line structures in the Occlusion and the fuzzy parallel lines in the warm sector. A wave in the Cold Front (approx. 50N/22W) is developing under the right entrance of the jet streak.