The image above shows an extended region containing differently shaped cellular cold air cloud over the Atlantic and W. Europe. Large areas with uniform cloud cells with relatively warm tops (grey in the IR image) represent the usual Cold Air Cloudiness (CAC). The extended area with much colder tops (white in IR) called ECAC (Enhanced Cold Air Cloudiness) is typical for an area with a low tropopause leading to higher instability. Mesoscale areas with enhanced cloud cells labelled EC (Enhanced Cumulus) are the topic of this chapter; they are not only associated with enhanced instability but also some other effects causing increased upward motion which are discussed in the following chapters (see Meteorological physical background and Key parameters). Mesoscale convective spirals are Comma features (CO) (see Comma ); for them a similar physical background exists as for the ECs but curvature vorticity is the reason for a distinct spiral structure and further development leads to increased weather activity.

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
• The more grey clusters are not normally surrounded by lower tops and the distance from the Cold Front is greater.
• 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.

The example above from 02 October 2002/12.00 UTC shows a large area of convective cells in the cold air behind a Cold Front over the British Isles. Three meso-scale EC areas can be found: EC1, EC2 and EC3. All of them are rather far away from the Cold Front band. The best developed EC which will be discussed further is EC1. The accumulation of cells which are larger than the cold air cloudiness around are brighter in the IR (which means colder) and in the VIS image (which means thicker) than the surrounding cells. However, they do not reach to very high levels as they cannot be seen clearly in the WV image which only shows a grey region indicating high WV content. The example above from 25 February 2003/06.00 UTC contains an EC area which reaches well above 600 hPa, as they are clearly seen in the WV image. It is not as symmetrical shaped as the one in the first case and it is rather close to the Cold Front band over the western Iberian Peninsula.

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

Typical Developments: From EC to Comma

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

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. 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.) In the images above, the EC cloud patterns east of Iceland are easily identified, with a high propability 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.