Dust RGB (standard EUMETSAT algorithm).

To allow users to identify regions where the atmospheric dust load is high. However, alongside this use these images are also very useful as a general purpose indicator of the meteorology, especially the location and development of deep convective storms.

Dust RGB imagery is entirely produced using infrared channels. As such it is available 24 hours a day). The red channel is the brightness temperature (BT) difference between the IR12.0 and IR10.8 channels and gives some indication of optical thickness. The green channel is BT difference between the IR10.8 and IR8.7 channels and gives information about cloud phase (ice or water). The Blue channel is the IR10.8 BT and indicates surface temperature/height above the surface. The combination of these three colour channels leads to the ability to identify many dust and cloud features from dustRGB imagery.

As can be seen in the image below the dust RGB imagery highlights where dust has been raised from the surface with pink/magenta. This is generally very easy to identify against the light blue background of a dry airmass over a hot desert surface. However, identifying dust can be more difficult under less favourable conditions. When column water vapour is high and the surface temperature is lower (than the desert) the surface appears darker blue and dust can be masked. Cloudy conditions also make identifying dust more difficult as clouds block the dust below. The colour of clouds in dust RGB imagery is determined by optical thickness, phase and cloud top height. Thin cirrus clouds are indicated by black, deep convective clouds such as mesoscale convective systems are red while deep mid level clouds are orange.

Overshooting tops RGB (standard South African Weather Service; SAWS algorithm).

To help identify overshooting tops within larger convective storm systems. These regions are linked to strong convective updrafts and therefore are closely associated with extreme weather at the surface.

The overshooting top RGB uses Water Vapour and Infrared channels (channel differences) to attempt to highlight overshooting tops. The red channel is the BT difference between the WV6.2 and IR10.8 channels. The green channel is the brightness temperature difference between the IR9.7 and IR10.8 channels. The blue channel is the BT of the WV6.2 channel. As this is a relatively underused method (and information on the creation of images is not easy to find) the channels ranges and gammas for each channel are given below. As this method only relies on IR and WV channels it can be used 24 hours a day.

Beam	Channel	Range	Gamma
Red	WV6.2-IR10.8	-50 to 5K	1.0
Green	IR9.7-IR10.8	-30 to 25K	0.5
Blue	WV6.2	193 to 243K	1.0

The use of the overshooting tops RGB is very simple. Deep convective clouds show up as pink while mid level clouds are darker (red). The surface and very low clouds are almost black. Overshooting tops embedded within cold clouds appear whiter than the surrounding clouds and important structures (such as rings or horse shoe shapes) can be made out too. These can help users to identify the location of the most intense updrafts within a storm and therefore where convection (and extreme weather) are most active. One drawback can be the subjective nature of this method and the fact that overshooting tops in shallower clouds will not appear white (even though these could indicate rapid development).

IR10.8 combined metrics RGB (cloud top roughness, BT and cooling rate of BT; non standard)

To include a variety of useful metrics for indicating the behaviour of mesoscale convective systems into a single RGB image to help to interpret the current and possible future behaviour of storm systems.

The three different metrics being represented within this RGB product are (1) the surface “texture” of the cloud, (2) the cloud top temperature and (3) rate of cooling of the cloud top (from one SEVIRI image to the next). All different metrics are based on the IR10.8 BT. As such there is no dependence on time of day so images can be produced 24 hours a day. The cloud top texture (red channel) is represented by the horizontal BT gradient of the image. Clouds with very bumpy tops will have high gradient values and so the red value will be high in these regions. The cloud top temperature (green channel) is simply represented by the IR10.8 BT value. Cold clouds will have a strong green component and warm clouds have a green value of 0. The rate of cooling (blue channel) is indicated by the difference in IR10.8 BT between the latest image and the image preceding it. Only regions that have cooled have blue values and to limit the impact on cloud motion the range is limited to -10 ℃ to 0 ℃. The details required to replicate these images are given in the table below. Once the RGB values are generated they are masked using a IR10.8 BT threshold of -40 ℃. These are then plotted over a greyscale IR10.8 BT image for context.

Beam	Channel	Range	Gamma
Red	Horizontal   gradient of IR10.8 BT	0 to 7 ℃/pixel	0.5
Green	IR10.8 BT	-90 to -40℃	1
Blue	IR10.8 BT   cooling (since last image)	-10 to 0℃	0.5

The interpretation of these images might require some time for familiarisation with the images. However they are quite intuitive in their use once this barrier has been overcome. Due to the thresholding only cold clouds are shown in colour. The green channel indicates the cloud temperature so if an MCS is mostly bright green then this mean that it has a well developed anvil. However if green is dominant this doesn’t suggest a cloud that is very convectively active. The regions within a storm that are light blue and white colour indicate locations where the cloud top is still very cold but also where there is significant cloud top texture and cooling. This is indicative of active convective updrafts and overshooting tops. These regions are likely to be connected to extreme weather at the surface. Often the same structures that are visible in the convection RGB and overshooting top RGB are also present in these images making the identification of well developed, active, convective storms very easy. Another feature of importance is the colour of the leading edge of the storm. If there is a pink to blue band along the edge of a storm then this storm is moving rapidly and generating fresh convective cells along the leading edge. If the storm is surrounded by these colours then you can infer that the storm is growing in all directions rapidly. However if the leading edge of a storm is darker (magenta/purple) then you can infer that the storm is less vigorous and possibly slower moving. Regions of a storm (or even entire convective storms) that appear darker green fading to browns and red can be expected to be dissipating. Often regions of a storm that take on these colours break apart and only the more active parts of a storm continue. Regions of cold cloud that fade to dark green/brown in all regions are likely to dissipate over the next 2 to 4 hours. Obviously reinvigoration can happen but this will be clear early in the process as the rapidly developing cells will show up as white/light blue regions surrounded by a pink fringe. White overshooting tops and ring and horseshoe structures might also be evident.