Night-time cloud monitoring over the Atlantic

By Jochen Kerkmann (EUMETSAT)

This case from 18 March 2005 demonstrates well that the recommended night-time RGB for the monitoring of fog/low stratus is also very useful for the detection of thin cirrus clouds (including the detection of contrails from aircrafts). The upper left image below shows the RGB composite, which uses the following inputs:

• Red: IR 12.0–IR10.8 (range from -4 K (intensity 0) to + 2 K (intensity 255))
• Green: IR 10.8–IR3.9 (range from 0 K (intensity 0) to +10 K (intensity 255))
• Blue: IR10.8 (range from -50°C (intensity 0) to +20°C (intensity 255))

The low-level stratus/stratocumuls clouds over the eastern Atlantic (around the Canary Islands) appear with a light green colour, while the high-level ice clouds to the west of the Canary Islands appear with dark red (thick cloud) or dark blue colours (thin cloud). In the upper-left corner of the image a nice cut-off low can be seen. For the colour interpretation, please click on the interpretation link below the image.

The upper right image, which is the recommended RGB composite for the monitoring of dust storms (day and night) over desert surfaces, is very similar. It uses the following inputs:

• Red: IR 12.0–IR10.8 (range from -4 K (intensity 0) to + 2 K (intensity 255))
• Green: IR 10.8–IR8.7 (range from 0 K (intensity 0) to +15 K (intensity 255), with a Gamma enhancement of 2.5)
• Blue: IR10.8 (range from -12°C (intensity 0) to +16°C (intensity 255))

The interpretation of the colours is very similar to the 'fog/low stratus' RGB: high-level ice clouds appear with dark red (thick cloud) or dark blue to black colours (thin cloud). However, the low-level clouds are not well visible in this RGB composite. In order to better visualise the low-level water clouds, one would have to narrow to range for the IR10.8–IR8.7 difference to about 0 K (intensity 0) to + 4 K (intensity 255). The effect of this can be seen in this image.

It will be noticed that in the infrared image (bottom right) it is quite difficult to monitor the evolution of low-level clouds at night, while in the RGB loop (top left), the formation and advection of the low-level clouds is clearly visible.