Appearance in Satellite Data

13 January 2008/12.00 UTC, Meteosat 9 IR10.8 image

The image above shows a large system over the Atlantic and the western and northwestern Europe, consisting of Occlusion, Warm Front and Cold Front. South of Ireland there is a Wave on the front and behind it a large area, containing differently shaped cellular cold air clouds, can be observed. The cloud tops, white in the IR image, are the tops of the clouds which are the topic of this chapter - EC or Enhanced Cumulus. They are associated not only with the enhanced instability but also with some other effects causing increased upward motion, which will be discussed in the following chapters (see Meteorological physical background and Key parameters).

For EC (Enhanced Cumulus) areas the following satellite image features exist:

  • The satellite images show a meso-scale area of cellular, sharp edged cloud clusters, with some variety in shape and size. Usually these cloud clusters are surrounded by cloud cells with warmer tops, i.e. typical cellular Cold Air Cloudiness.
  • The enhanced cells of EC are usually white in the IR, VIS and WV images indicating thick, multi-level convective clouds.
  • There are types of ECs which are white in VIS but only light grey to white in IR and grey in WV indicating that the tops do not reach to high levels
  • In well-developed situations, the cirrus shields of single Cb cells merge, leading to a very cold smooth cloud shield in the IR but a transparent appearance in the VIS image.
  • There are different developments associated with ECs:
    • Under certain circumstances Cold Air Cloudiness (CAC) grows into EC
    • EC can grow further into a Comma, and
    • A Comma can grow into Cold Air Development (CAD).

The diagrams below show different kinds of convective cloud as well as the difference between Cb Clusters and ECs.

13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image (zoom)
13 January 2008/12.00 UTC - Meteosat 9 IR10.8 image
13 January 2008/12.00 UTC - Meteosat 9 HRVIS image (zoom)
13 January 2008/12.00 UTC - Meteosat 9 HRVIS image
13 January 2008/12.00 UTC - Meteosat 9 WV 6.2 image (zoom)
13 January 2008/12.00 UTC - Meteosat 9 WV 6.2 image

In the images above, one can take a closer look at the cloud structures introduced at the beginning of the chapter. ECs usually show up behind the Cold Front. They can be recognized as the accumulation of cells larger than the cold air cloudiness around, brighter than the surrounding cells in the IR (which means colder) and in the VIS (HRVIS) image (which means thicker). The cells are here well developed and easily detectable also in the WV channel, which is not often the case.

High spatial resolution of 1km in SSP (Sub-Satellite-Point), provided by Meteosat HRVIS images, makes small-scale cloud structures more distinct. In the above images the meso-scale area with cellular cloud clusters is nicely visible.

Typical Developments: From Cb Cluster to EC

The case of 02 April 2003, below, shows a development over the North Sea immediately east of England over a period of 3 hours. At the beginning (left image, 02.00 UTC) there are only single cells while at 05.00 UTC a more compact area of cells has developed which has all the features of an EC.

02 April 2003/02.00 UTC - Meteosat IR image
02 April 2003/05.00 UTC - Meteosat IR image

The following loop shows with half-hourly time steps the development from a Cb Cluster to an EC.

02 April 2003/02.00 - 05.00 UTC - Meteosat enhanced IR image; half-hourly image loop

Typical Developments: From EC to Comma

The following 3-hourly loop shows the transition of an EC configuration to a Comma feature.

22 November 2002/21.00 UTC - Meteosat IR image; EC and Comma indicated; 22 November 21.00 - 24 November 00.00 UTC 3-hourly image loop

First there is a rather rapid increase in the extent of the EC from which a Comma spiral then develops over a longer period of time. Looking only at the satellite images the EC moves rather quickly from an area within the Cold Air Cloudiness, which is far away from the front across the cold air region to an area much closer to the front. During this movement the cloud area develops a spiral structure and finally becomes a very well developed Comma.

Appearance in Meteosat RGB composite imagery

To take into account and combine different types of information retrieved from SEVIRI imagery, such as optical thickness of clouds, particle size and phase, upper and mid level moisture and cloud top temperature, combinations of channels are constructed.

As the name itself says, the Dust RGB is more suited for the detection of dust in the atmosphere but it can also be used for convection monitoring. The icing in the clouds makes the cells appear red, so when the clouds change their colours from orange to the reddish ones we can conclude that there is vertical motion going on which is building up cumuli-like clouds.

13 January 2008/12.00 UTC -Meteosat 9 Dust RGB image (IR12.0-IR10.8, IR10.8-IR8.7, IR10.8) (zoom)
EC as it appears in Dust RGB

One of the RGBs frequently used for convection detection and monitoring is so called Convective Storms RGB (WV6.2 -WV7.3, NIR3.9 -IR10.8 and NIR1.6 -VIS0.6). In this type of composite cold Cb tops with small ice particles appear in yellowish colours and well developed Cbs with large ice particles appear red.

In the Convective Storms RGB shown below ECs appear in different shades of pink. The most striking feature of clouds over the Atlantic is again their cellular shape.

13 January 2008/12.00 UTC -Meteosat 9 Convective Storms RGB image (WV6.2 -WV7.3, NIR3.9 -IR10.8 and NIR1.6 -VIS0.6) (zoom)
EC as it appears in Convective Storms RGB

The Airmass RGB (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i) is very useful for detection of the Enhanced Cumuli clouds for several reasons. ECs are expected to be present in the cold air behind the front, so they will be usually seen in the dark bluish areas. Moreover, reddish bands behind the front locate the regions favorable for convection because they point out the areas where the dry stratospheric air, which enhances the instability, is intruded. Although EC is thick and reaches high levels and as such would appear white, it is in fact pinkish due to the cold environment with high PV values in which it is most frequently found.

13 January 2008/12.00 UTC -Meteosat 9 Airmass RGB image (WV6.2-WV7.3, IR9.7-IR10.8 and WV6.2i) (zoom)
EC as it appears in Airmass RGB

Appearance in AVHRR imagery

  • In the observed areas to the east and southeast of Iceland, AVHRR channel imagery show more detailed structures than Meteosat imagery.
  • Comma clouds to the NNE and N. of Iceland (see below) have developed from the Enhanced Cumulus (EC). Note: these areas are too far to the north to be seen by Meteosat.
  • RGB-combination of channels (below left, second row) provides a quick overview of high and middle level cloudiness.
  • Channel manipulation (third row, right) highlights cloud patterns and physical features.

17 February 2000/04.35 UTC - NOAA RGB image (channel 3, 4 and 5)
17 February 2000/04.35 UTC - NOAA CH5 image; EC in Atlantic: SE and E. of Iceland (approx. 63N/15W)

In the images above, cold air is flowing from Greenland to the Norwegian Sea. Cold Air Cloudiness develops downstream, and there are Enhanced Cumulus areas southeast of Iceland (northeast of Iceland a "Comma" can also be seen). In the EC area the white spots are associated with high cold cloudiness (clustering Cb). At the edges of these white spots thin high and middle level clouds can be recognized.

17 February 2000/04.35 UTC - NOAA RGB image (channel 3, 4 and 5)
17 February 2000/04.35 UTC - NOAA RGB image (channel 1, 2 and 4); EC in Atlantic: SE and E. of Iceland (approx. 64N/10W)

In the images above, cold air is flowing to the south of Iceland from Greenland to the Norwegian Sea. Enhanced Cumulus areas east of Iceland are located near the left exit of a jet streak. At the east side of this cloudiness, large shadows can be seen since sun elevation is low and Scandinavia already dark. (At this latitude (+/- 65N) Meteosat is not very useful any more because of the low resolution of a geostationairy satellite at that latitude.)

17 February 2000/04.35 UTC - NOAA CH2 image
17 February 2000/04.35 UTC - NOAA CH1 minus CH3B- image; EC in Atlantic: SE and E. of Iceland (approx. 64N/10W)

In the images above, the EC cloud patterns east of Iceland are easily identified, with a high probability of precipitation. The thin, lower level cloud, seen in the image above left (and also in the second row of images), is filtered out of the above right image.