Training Module on METOP AVHRR RGB Images

• Chapter IV: AVHRR Natural Colour RGB


•  
• Aim
•  
• Physical Basis
•  
• How to create Natural Colour RGB images
•  
• Typical Colors
•  
• Examples of Interpretation
•  
• Benefits and limitations
•  
• Comparison of AVHRR and SEVIRI Natural Colour RGB images
•  
• Exercises

Aim

Natural Colour RGB images are created by combining data from three different channels. The 1.6 micrometer channel (NIR1.61) data is visualized in red, the 0.87 micrometer channel (NIR0.87) in green and the 0.63 micrometer channel (VIS0.63) in blue. As a daytime RGB it uses shortwave channels.

The purpose of this RGB type is to provide general cloud analysis. It provides information on cloud top phase and cloud optical thickness in cloudy areas, and on the green vegetation fraction ('chlorophyll content') in cloud-free areas. In most cases water clouds can be distinguished from ice clouds. Snow-covered land is easy to distinguish from snow-free land and also from water clouds and fog. Unfortunately, ice clouds and snow have similar colors.

The colors are (relatively) close to the natural colors of the depicted features, which makes these images easy to understand and convenient for the public.