Cloud Structure In Satellite Images

Appearance in METEOSAT Second Generation imagery

IR10.8 imagery:

  • Two main cloud configurations are seen during the initial stages of a Rapid Cyclogenesis: a frontal cloud band usually oriented east to west and clouds with warmer tops that form an increasingly dense shield, a so-called cloud head, on the poleward side of the frontal cloud band and protruding from below it. The developing cloud head varies between grey and light grey in the IR channels, mostly with higher tops on the poleward side.
  • Dry sinking air originating from the lower levels of the stratosphere on the cyclonic side of a jet stream is advected downstream. This leads to a dark, cloud-free area between the cloud band of the cold front and the cloud head, thereby creating a V-pattern (often also called a dry slot).
  • During the advanced stages the cloud head grows to a cyclonically curved cloud spiral with a broad dark area between the spiral and the frontal cloud band; additionally, the rear part of the cold frontal cloudiness frequently dissolves as a consequence of sinking dry air.
  • Further development leading to the mature stage of the cyclone often includes a cloud spiral around the low center.

WV6.2 imagery:

  • WV images have an important role in identifying the possibility of rapid cyclogenesis initiation.
  • In the initial stage of the development a dark grey stripe can be seen along the northward edge of the white frontal cloud band. This stripe represents dry air sinking from the stratosphere along the cyclonic side of a jet stream.
  • Before the development starts, the dark grey stripe is more distinct upstream of the cloud head.
  • In the next stage the dark grey stripe approaches the cloud head while becoming darker and darker (indicating dry air moving downwards in the upper troposphere) and finally forming a typical V-pattern together with the cloud head.
  • During the advanced stages the cloud head grows into a cyclonically curved cloud spiral with a broad black area between the spiral and the frontal cloud band.
  • A further dark stripe often develops along the north-northeastern boundary of the cloud head in the cold air within the cold airmass, indicating sinking movement in connection with a second jet streak which often develops there.

VIS0.6 imagery:

  • During their initial stages both the west-to-east oriented frontal cloud band as well as the cloud head appear white in the VIS image, indicating fairly thick cloudiness, but with an irregular structure and sometimes fibrous edges on the poleward edge of the frontal cloud band.
  • As the top of the frontal cloud band is higher than that of the cloud head, the frontal clouds can cast a distinct shadow on the cloud head, which is visible in VIS images.

Airmass RGB imagery:

  • The Airmass-RGB (see Basics: Artificial and Combination Channels) is appropriate for differentiating between cold, warm and dry air masses.
  • The dry intrusion and jet streak axis take the distinctive forms of a reddish stripe along the frontal cloudiness and a reddish hole between the cloud head and frontal clouds. This makes RGB images a good tool for the identification of Rapid Cyclogenesis developments.

Dust RGB imagery:

  • The Dust RGB (see Basics: Artificial and Combination Channels) is appropriate for differentiating between clouds of different thickness, especially for identifying high cirrus cloud.
  • Its abilities help to identify the different elements of frontal and cloud head elements, as well as the cirrus fibers connected to the jet axis.

The following schematics show the typical features in Airmass RGB images at three important development stages of a Rapid Cyclogenesis:

Fig. 1: Initial stage Fig. 2: Development stage
Fig. 3: Advanced stage

The typical features of a Rapid Cyclogenesis seen in satellite images and image loops are:

  1. A (south)west to (north)east oriented cold front-warm front cloud band together with a cloud head on the poleward side
  2. An approaching dark grey stripe in the WV image. As soon as the leading edge of this dark grey strip becomes darker and wider close to the area between the cloud head and frontal cloud band, the development described in this chapter will take place in just a few hours.

The example used throughout this conceptual model took place over the eastern Atlantic in 7-9 January 2015 and displays structures and developments typical of rapid cyclogenesis.

The next images show four illustrative channels (IR 10.8 μm, WV 6.2 μm, Airmass RGB, Dust RGB) for the three stages of development in the schematic above: Initial stage, Development stage and Advanced stage.

Initial stage:

Main characteristics: Frontal cloud band, cloud head and dry tongue

Fig. 4a: 8 January 2015, 06 UTC; Meteosat 10, IR 10.8; Initial stage: frontal cloud band, cloud head. Fig. 4b: 8 January 2015, 06 UTC, Meteosat 10, WV 6.3; Initial stage: frontal cloud band, cloud head, dark grey stripes of dry air - V-pattern
Fig. 4c: 8 January 2015, 06 UTC; Meteosat 10, Airmass RGB; Intitial stage: frontal cloud band, cloud head, brownish stripes of dry air. Fig. 4d: 8 January 2015, 06 UTC, Meteosat 10, Dust RGB; Intitial stage: frontal cloud band, cloud head, black cirrus fibers.

All channels show the west-to-east oriented frontal cloud band as well as the separate cloud head area. The WV and Airmass images indicate with dark grey and brownish stripes the dry tongue at the rear side of the frontal cloud band, which has protruded further in the direction of the cloud head area. The IR and Dust images indicate the lower cloud tops with darker shades of grey and lighter shades of brown compared to the frontal cloud band.

Development stage:

Frontal cloud band, intensified dry intrusion, spiral structure of cloud head

Fig. 5a: 8 January 2015, 12 UTC; Meteosat 10, IR 10.8; Development stage: frontal cloud band, cloud head, intensified dry intrusion. Fig. 5b: 8 January 2015, 12 UTC, Meteosat 10, WV 6.3; Development stage: frontal cloud band, cloud head, intensified dry intrusion.
Fig. 5c: 8 January 2015, 12 UTC; Meteosat 10, Airmass RGB; Development stage: frontal cloud band, cloud head, brownish stripes of dry air, beginning dissolution of cold front cloudiness in dark blue. Fig. 5d: 8 January 2015, 12 UTC, Meteosat 10, Dust RGB; Development stage: frontal cloud band, cloud head, black cirrus fibers.

All channels show the west-to-east oriented frontal cloud band as well as a beginning spiral structure in the cloud head area accompanied by an intensified dark grey (WV) and brown (Airmass) area between the cloud head and cloud band. As a consequence of cloud evaporation, frontal cloudiness in this area has started to become thinner with a high cirrus stripe on the rearward side (blue in Airmass and black in Dust RGB).

The following image shows the VIS 0.6 μm and high resolution visible channels for 8 January 2015, 12 UTC:

Fig. 6a: 8 January 2015, 12 UTC; Meteosat 10, Vis 0.6. Development stage: Frontal cloud, cloud head, shadow of higher frontal cloud on the lower cloud head; low cold air cloudiness. Fig. 6b: 8 January 2015, 12 UTC; Meteosat 10, HRVis. Blue: high cloud; yellow: low cloud. Development stage: frontal cloud, cloud head, shadow of higher frontal cloud on the lower cloud head; low cold air cloudiness.

Convection in winter cases like this one does not reach as high as in summertime Rapid Cyclogenesis events.

The shadows of high and thick frontal clouds over the cloud head show as black and dark blue stripes in VIS images.

Advanced stage:

Fig. 7a: 8 January 2015, 18 UTC; Meteosat 10, IR 10.8; Advanced stage: frontal cloud band, dissolution of cold front clouds, cloud head changes to spiral form. Fig. 7b: 8 January 2015, 18 UTC, Meteosat 10, WV 6.3; Advanced stage: frontal cloud band, cloud head changes to spiral form, dry area in the center grows.
Fig. 7c: 8 January 2015, 18 UTC; Meteosat 10, Airmass RGB. Advanced stage: frontal cloud band, cloud head spiral, brownish stripes of dry air,dissolution of cold front cloudiness. Fig. 7d: 8 January 2015, 18 UTC; Meteosat 10, Dust RGB. Advanced stage: frontal cloud band, cloud head spiral, black cirrus fibres.

The most indicative structures for this advanced development stage of a Rapid Cyclogenesis are the more distinct cloud spiral with a large dry area in the spiral center, and the large cloud-free (dark) areas within the cold front cloud band. Both features are very well developed in this case. Convective cells develop in the center of the spiral; they show as white spots in the IR channel and Airmass RGBs and as brown spots in Dust RGBs.

The convectivity is often stronger in the warmer seasons.

Cloud dissolution shows in IR images and Airmass RGBs as a darkening of the clouds. In Dust RGBs it is signaled by bluish areas and black cirrus stripes.

IR channels as well as Airmass RGB images from 9 January at 00 and 06 UTC show the development from advanced to mature stage in the form of a cloud spiral that can circle the cyclone center several times.

Fig. 8a: 9 January 2015, 00 UTC; Meteosat 10, IR 10.8; Mature cloud spiral. Fig. 8b: 9 January 2015, 06 UTC; Meteosat 10, IR 10.8; Mature cloud spiral.
Fig. 8c: 9 January 2015, 00 UTC; Meteosat 10, Airmass RG; Mature cloud spiral. Fig. 8d: 9 January 2015, 06 UTC; Meteosat 10, Airmass RGB; Mature cloud spiral.

In the advanced and mature stages, when the cloud spiral is well developed and circling the cyclone's center, parallel cloud lines can appear close to the innermost part of the cloud spiral. These cloud bands are parallel rain bands. Although rain bands are by their nature more visible on radar than in satellite images, they are mentioned here because they can appear together with a very dangerous phenomenon, a sting jet. The sting jet will be described in more detail in the chapter on weather phenomena.

Fig. 9: 9 January 2015, 00 UTC - Meteosat 10, Airmass RGB. The black line indicates an area of banded cloud.

The following loops show a very rapid development of the depression over 12 hours from 990 hPa on 8 January 2015, 06 UTC to the extreme low value of 965 hPa on 8 Januar 2015, 18 UTC.

Fig. 10: 9 7 January 2015, 18 UTC - 9 January 2015, 06 UTC, 15 minute image loop; IR 10.8.

Appearance in AVHRR imagery

As usual, the advantage of AVHRR images is the high resolution which is most useful for observing the development of convection, especially in high latitudes.