Training Module on METOP AVHRR RGB Images

• Chapter II: AVHRR Night Microphysics RGB


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

Aim

The Night Microphysics RGB images are created by combining data from the 12.0, 10.8 and 3.74 micrometer channels (noted by IR12.0, IR10.8 and IR3.74). Brightness temperature differences (BTD) are displayed in red and green, while the IR10.8 channel is shown in blue. As a nighttime RGB it uses only thermal infrared channels.

The main purpose of this RGB type is to distinguish fog and low clouds from cloud-free areas at night. This is very important for traffic security. Fog or stratus usually forms at night by radiation cooling, and since in-situ observations are rare, satellite information is vital.

Although the main goal is the detection of fog and low clouds, the Night Microphysics RGB also reveals other types of clouds. These RGB images contain information on cloud top temperature, cloud optical thickness and cloud top phase. Cloud-free areas contain information on low-level moisture and surface temperature. The color contrast between low and mid-level clouds, thin high level clouds and cloud-free areas is good.

Note that the IR3.74 brightness temperature field is rather noisy in very cold regions. IR3.74 data is not useful for determining the temperature of very cold features like thunderstorm tops. Therefore, the Night Microphysics RGB is not recommended for analyzing nighttime convection. The IR10.8 single channel is more appropriate for this purpose.

Note that the Night Microphysics RGB can be created from NOAA AVHRR data as well. Using the same recipe (see Table 1 in the section 'How to create Night Microphysics RGB images') it will have almost the same appearance.