• In satellite images a developing Spanish Plume event is characterized by a south/southwest - north/northeast oriented Cold Front cloud band moving eastwards towards the Iberian peninsula and western Europe. As the Cold Front approaches France and the Iberian Peninsula, clusters of Cumulonimbus Clouds (Cb) develop on the mainland. As the clusters grow, they acquire an oval MCS - like appearance with a larger horizontal extent.
• In IR, WV and VIS images, the Plume is characterized by rapidly increasing pixel values, indicating strong vertical development of convective cells, which soon extend through the whole troposphere.
• On the WV images, a smooth, grey zone extends from the Spanish plateau in a northerly direction towards France, the Benelux and the British Isles. Within this zone the first signs of developing Cb cells are easy to detect as tiny white spots emerging from the greyer environment.
• The contrast between the clouds and cloud-free area is distinct, especially at the upstream edges of cloud clusters. In the case of strong upper level winds, high level cloudiness is transported downstream leading to an extended white cloud shield (the anvil) in IR but a fibrous grey texture in VIS. The brightest grey shades can be found in the active part upstream.
• In the mature and subsequent stages, numerous MCS's may develop along the "Spanish Plume".

In WV imagery the pre-convection environment often shows a moist mid-tropospheric warm conveyor belt extending from the Spanish plateau northwards. The first small-scale convective cells are easily detected in either IR or WV images. The Pyrenees - mountains sometimes cause some Lee Cloudiness, which can partly hide convective development in its earliest stage. The Lee Cloudiness can, however, also be used as an indicator of the existing moist southerly air flow.

The water vapour images below clearly show the Cold Front cloudiness over and south of the British Isles, and the broad moist upper tropospheric flow known as the "Spanish Plume" lying ahead of it over the Iberian peninsula and western parts of continental Europe. Individual Cb cells can be found embedded within this moist zone.

The rapidly growing convective cells often form a long chain. The chain is oriented parallel to the Cold Front, but the distance between the chain and the front may vary. Local topographic forcing also determines the precise location at which the storms will form. Favoured locations for new development seem to be in northern Spain, near the coast of France and in the French Massif, from where the cells move north or northeastwards with the upper level flow.

During the earliest hours of development small-scale convective cells quickly grow both horizontally and vertically. This can be seen as rapid increase in brightness in all channels. Most strikingly, this increase in brightness can be seen in WV images, as the contrast between the individual developing cumulonimbus tops (bright white) and the environment (greyer shades) is well marked.

Development images (see Introduction To Development Images ) can show the development of convective clusters very clearly. The new development is seen as semi-continuous bright line along the edges of the clouds. This is a sign of rapid new development and growth of the cloud.

While the convective cells along the Spanish Plume grow, the Cold Front cloudiness gradually becomes less distinct. The relatively dry upper layer between the Cold Front and the Spanish Plume becomes narrower, as the front overtakes the convective plume area. At later stages the Cold Front cloudiness may either merge with the convective cloudiness or gradually weaken with no interaction.

Appearance in AVHRR imagery

• AVHRR imagery can help visualizing the initial appearance of a Spanish Plume. However, the poorer temporal resolution of this imnagery at 40-50°N does not allow regular monitoring over time. The growth and merging of small-scale Cb cells to larger Cb clusters and/or MCS's take place in the time between two consecutive passes of the polar orbiting satellite.
• Visible channel 2 images show high and thick convective clouds as bright clusters. The presence of relatively bright lower level cloudiness may at times make it difficult to distinguish upper clouds from lower clouds. The shadows cast by individual convective clouds, however, create some useful contrast (see, for example, the clouds over Spain in Ch2 image).
• Infrared channel 4 images show the distinct convective cells as bright spots or clusters against dark (warm) ground.
• The best channel combination is NOAA 124, as it gives a very good contrast between the upper thinner cirrus clouds and thick convective cores. The cores of the developing convective cells are seen as bright white, the bluish anvils not being very large at this stage. Later, as the development reaches the MCS stage, the extending cirrus anvils become more pronounced, and distinct from the bright inner parts of the convective clouds. The usually moderate south - southwesterly upper wind soon carries the anvils downstream and the MCSs appear as SE - NE orientes oval - shaped cloud masses.
• Daytime NOAA 345 combinations show less contrast than NOAA 124 images. The sensitivity of these channels to cirrus clouds, however, makes the detection of Lee Cloudiness north of the Pyrenees easier, especially if the cirrus is very thin. However, the Lee Clouds are often optically thick enough to be detected with NOAA 124 images.