The 'sandwich' product is created by a visualisation technique developed especially for storm top analyses. The method was invented by Martin Setvák (Czech Hydrometeorological Institute, CHMI). The product is also named as a 'blended' image.

The sandwich product is a combination of two layers. With this method one can visualise both a black and white (or nearly black and white) image as a 'background' (bottom layer) and a coloured image as an 'upper layer'. The method blends two images providing different types of information on cloud tops. A colour-enhanced infrared window channel image (as upper layer) is most frequently blended with a visible image (as background). It is practical to state in the filename of specific sandwich product which layers the product is blended from, for example, 'SEVIRI HRV/IR10.8 blended image' means that it is created from SEVIRI HRV (High Resolution Visible) and SEVIRI IR10.8 (10.8 μm infrared) channel data.

These products help to detect and analyse various cloud top features of storms (storm systems) in their mature phase. It eases detection of specific cloud-top features related to storm dynamics and microphysics, structure, and possible storm severity - such as overshooting tops, cold-U/V (enhanced-V) or cold-ring features, embedded warm spots/areas, gravity waves, above-anvil ice plumes, areas composed of very small ice particles, etc. These products directly support monitoring and nowcasting of convective storms. In areas with no, or poor, weather radar and surface observation coverage, this product is essential for proper storm detection.

In the following we will discuss the steps of creating the 'sandwich' image:

  • Selection of the input images
  • Increasing the contrast of the background image
  • Doing a proper colour enhancement for the upper layer image
  • Combining them in a specific way