EUMeTrain: Water Vapour CAL Module - Water Vapour Channels

Radiances in water vapour channels - Topographic presentation of the brightness temperature

Topographic presentation is a simplified model that explains qualitatively the gray shades of a water vapour image.

Threshold level is an altitude, below which the atmosphere is moist so that water vapour absorbs all the radiation coming from the surface. In practise it is the level below which the satellite does not see in water vapour bands.

Assumptions of the topographic presentation:

• Lower troposphere is moist up to a threshold level
• Above threshold level air is relatively dry and moisture is not arranged in layers
• Air temperature decreases with height without significant inversions
• No clouds present

In topographic presentation, as the threshold level goes up, the brightness temperature decreases and grey shade becomes lighter. A water vapour image can be seen as a moisture terrain.

A vertical cross section of a moisture terrain:

Threshold height depends on temperature. In cold areas the threshold is lower than in warm areas (lower and upper troposphere, respectively).

Usually the first assumption is valid in the troposphere. Therefore, the interpretation of images depends on layered moisture, inversions, and clouds. Letīs look closer at the layered moisture.