# Sandwich products - satellite imagery visualisation technique

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## Chapter II: Background image

- Background image

### Background image

Usually a solar channel (with central wavelength less than 1 μm) is used as the background image (layer). The cloud top 'morphology' is nicely seen in these images due to the shadows cast on the anvil (the best at low solar elevation). True Colour RGBs or other RGBs created from solar channels might also serve as a background layer if the clouds appear as almost black and white.

The background image should have a good contrast in the cloud top region. Some ideas how to enhance a single solar channel:

- In the case of operational processing it should be calibrated to a reflectivity value (usually also applying solar zenith angle correction) and then the black and white colour scale can be assigned to a reasonable fixed range, for example to the 10-100 % reflectivity range.
- In the case of interactive processing one can experiment to find the best way to increase the contrast of the background image. For example, in most photo-editing software the background image can be enhanced by some of the standard procedures, for example contrast enhancement, gamma functions, curves, or using the 'Unsharp Mask' filter.

Which features might be seen in the background images?

In the solar channels one can see the cloud top 'morphology' due to the shadows. For example, the overshooting tops (OT, the uppermost parts of updrafts), which are higher than the surrounding anvil top (see Fig. 2.1); the above-anvil ice plumes which also cast shadow at the anvils, or various kinds of the cloud top gravity waves (triggered by pulsing updrafts). These features are better seen if the spatial resolution (pixel size) of the output image is as high as possible, and if the shadows are longer (if the feature is higher compared to the anvil or the sun is lower).

Many distinct, intensively pulsing overshooting tops; long-lived, large, persistent overshooting tops; intense gravity waves or the presence of above-anvil ice plume indicate strong updrafts and, thus, the possible severity of the storm.

Figure 2.1: Overshooting top as viewed from airplane. (Courtesy of Martin Setvák, CHMI)

An above-anvil ice (cirrus) plume (AACP) is a cirrus cloud above the anvil, typically vertically separated from the anvil. It usually originates from almost point-like sources close to the overshooting top (downwind of it), mostly in case of strong updrafts. Fig 2.2 shows a simulated storm with OT and ice plume, while Figs. 2.6-2.9 shows ice plumes in satellite images.

Figure 2.2: Vertical cross section of a simulated supercell (Montana, 1981): relative humidity (colours), wind vectors (black arrows) and equivalent potential temperature (white isolines). The yellow arrow indicates above-anvil plume. (Courtesy of Pao Wang, University of Madison, Wisconsin)

Figs. 2.3-2.8 show storm tops with cloud top features.

Figure 2.3: Overshooting tops in SEVIRI HRV image of storms above southern Germany, taken on 20 June 2013 at 15:37 UTC. (Courtesy of Martin Setvák, CHMI)

Figure 2.4: Overshooting tops (yellow arrows) and several plumes (blue arrows) over Romania and Bulgaria on SEVIRI HRV image, taken on 10 June 2013 at 15:25 UTC.

Figure 2.5: Above-anvil ice plumes in GOES-8 (GOES-East) image of tornadic supercell storms above Colorado and Nebraska, taken on 22 May 1996 at 00:45 UTC. (Courtesy of NOAA, and Martin Setvák, CHMI)

Figure 2.6: Above-anvil ice plumes in Aqua MODIS image of storms above Mexico, taken on 7 May 2007 at 20:08 UTC. (Courtesy of NOAA, and Martin Setvák, CHMI)

Figure 2.7: Above-anvil ice plumes in SEVIRI HRV image over Slovakia, taken on 19 July 2011 at 16:45 UTC.

Figure 2.8: Overshooting tops (yellow arrows) and above-anvil ice plumes (red arrow) and gravity waves (green arrows) in SEVIRI HRV image over Hungary, taken on 9 June 2012 at 17:30 UTC.

Summary

The background image has to be a black and white (or almost black and white) image with good contrast (a solar channel or an RGB created from solar channels).

On the top of mature storms, several cloud top features can be seen. Many, high, intensively pulsing overshooting tops; long-lived, large, persistent overshooting tops; intense gravity waves or the presence of above-anvil ice plumes indicate strong updrafts and, thus, the possible severity of the storms.