Cold-U shaped convective storm close to Maputo

Cold U-shaped convective storm close to Maputo

16 January 2008 00:00 UTC

Cold-U shaped convective storm close to Maputo
Cold-U shaped convective storm close to Maputo

In the afternoon and evening of 16 January 2008, a series of convective storm cells developed above eastern parts of South Africa, Swaziland and southern Mozambique, close to Maputo.

Last Updated

21 October 2020

Published on

15 January 2008

by Martin Setvak (CHMI ), Estelle de Coning (South African Weather Service ) and HansPeter Roesli (EUMETSAT)

Though there is no ground-based information about severity of these storms, in satellite imagery they displayed several distinct signatures which are typical for severe storms in the United States, Europe, the Middle East or Argentina. Indeed, South Africa is prone to severe convective weather (see hazard analysis for South Africa ). This case also illustrates the importance of using an appropriate image colour enhancement when documenting similar cloud top signatures.

The Meteosat-9 image below (enhanced IR10.8 image) shows a well developed cold U-shaped feature (alternatively known as 'enhanced-V'), which is broadly known as a satellite signature of a possible severe storm. The feature is also closely related to another similar cloud-top phenomenon, cold-ring feature, from which it may evolve.

The original name of the feature, 'enhanced-V', reflects the fact that to reveal the feature the original IR images have to be properly 'enhanced', either in black and white, or in colour. The role of the enhancement is illustrated in this animation . The animation shows how the appearance of the storm changes as the enhancement (showing an interval of 40 K) is being shifted, from warmer to colder cloud-top brightness temperature range (240–200 K to 220–180 K).

Meteosat-9 enhanced IR10.8 image

Met-9, 16 January 2008, 18:00 UTC
Channel 09 (IR10.8)
Full Resolution
MP4 Animation (11:00–23:00 UTC)


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Note: The colours used for the colour enhancement are artificial, having no real physical 'justification' or meaning — they are just tuned to show certain cloud-top features in such a manner which makes revealing these features to a human eye as easy as possible, and linking certain cloud top temperatures to certain colours. Many different colour schemes are used around the world, this 'blue-red' one (see colour table ) is used operationally by some of the weather services in Central Europe. While for Central Europe the optimal enhancement covers the range of about 240–200 K, elsewhere it needs to be shifted or stretched according to local circumstances (in particular, the tropopause temperature).

The evolution of the storms from 16 January 2008 from 11:00 till 23:00 UTC (in the same projection as above, with coastlines and country borders) is shown in the animation , using the enhancement range from 222 to 182 K. Additional comments addressing various cloud-top features of these storms can be found in the following Powerpoint report (ZIP). To get a better overview of the synoptic situation, a larger geographical area has been processed using the same temperature range and enhancement as above (see animation) . A standard black and white scale is also offered for comparison (see animation ). Furthermore, animations showing the same geographical coverage, but different bands or RGB products are shown here below:

Note that the setup parameters of the convective storms RGB product must be modified when compared to those used in Europe, in order to match the overall lower storm-top temperature.

Example of slightly different colour enhancement of the IR10.8 band , with a comparison of the two above used colour enhancement scales (ZIP).

Please note: We do not conclude which of the scales is 'better' — each of these shows slightly different details of various cloud top phenomena, depending on contrast settings. What we stress here is the fact that each case requires tuning of the 'standard' colour scales, since the operational scales may only provide medium quality images, with poor resolution of the features of interest.

Besides the interesting cloud-top features found in the IR10.8 enhanced imagery above, the storm also exhibited several other interesting cloud-top features. The band 12 (HRV) images from 14:30 UTC and 14:45 UTC show a new storm cell developing above south Mozambique, with distinct overshooting tops.

Unfortunately, due to the flexible area coverage of the HRV mode ('following' the Sun), the storm is not covered by the HRV channel after 14:45 UTC. Nevertheless, even in the lower resolution visible imagery (e.g. VIS0.8 band), the overshooting tops located above the strongest updrafts are clearly visible, as shown in this example from 16:15 UTC . Left is the VIS0.8 image, showing besides the nice overshooting top also an above-anvil plume 'streaming' SW from the storm core. Right is the corresponding IR10.8 image (colour enhanced, 222–182 K).

The blended image of these two bands shows that the plume corresponds to the warmer area inside the cold U-shape feature, while the coldest pixels correspond to the summit or upwind side (NE) of the overshooting tops. Finally, this animation shows the transition from band 2 (VIS0.8) to band 9 (IR10.8), and back.

The corresponding radar image from 16:15 UTC shows an indication of a hook shape echo; however to validate if this is indeed an indication of a supercell or just an artifact, more detailed radar images (showing individual PPI or CAPPI levels) would be necessary. The evolution of the storm in radar imagery is shown here (AVI, source: SAWS).

When it comes to observations and nowcasting of deep convective storms, the satellite observations and images are usually only a complementary tool to the radar observations. While radar detects the internal structure of storms and their precipitation, satellite 'sees' the cloud tops only. However, since the cloud tops reflect to a certain degree the internal storm structure and severity, they can replace radar observations in regions where the availability of radar data is limited, e.g. in large parts of Africa. In such cases the awareness of certain cloud-top features known to be closely related to storm severity (e.g. the cold-V features, or cold-rings) is essential for timely severe storm nowcasting.

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