Interpretation of Cloud in Visible Infrared Imageries

The cloud configuration seen in satellite imagery represents a visible manifestation of all types of atmospheric processes. The complete interpretation of cloud structure must make use of both satellite imagery and other available observational data. One should remember that the satellite sensors view only tops of clouds while in surface observations their bases are seen. Basically following six characteristic features of satellite pictures are helpful in extracting information for weather forecasting:

i) Brightness - The brightness depends strongly upon the albedo of the underlying surface in visible pictures. Highly reflecting surfaces like cumulonimbus (Cb) tops and snow appear pure white whereas the sea looks black. Other clouds and land appear in varying degrees of gray. In IR images warm land surfaces appear very dark while cold ones are white (e.g. Cb tops, thick cirrus, snow etc.). Lower level clouds and thin cirrus appear gray.

ii) Pattern - Cloud elements are seen to be organised into identifiable patterns like lines, bands, waves etc.

iii) Structure - In a VIS image, shadows of taller clouds fall on lower surfaces. Shadows and highlights thus give an idea of the cloud structure. In IR

imagery this information is provided by the cloud top temperature (CTT) more directly.

iv) Texture - The cloud surfaces when viewed by the satellite vary in degree of apparent smoothness. Some clouds appear smooth while some may look ragged.

v) Shape - Clouds assume a variety of shapes - rounded, straight, serrated, scalloped, diffused or curved.

vi) Size - The size of a pattern or the size of individual elements in a pattern are useful indicators of the scale of weather systems.

While interpreting satellite pictures continuity in time has to be maintained. The pictures should not be viewed in isolation but must be interpreted with reference to past weather and earlier imageries. It is necessary to keep in mind the time of the day, season and local peculiarities while interpreting satellite imageries. In VIS pictures illumination depends on the position of the sun which will vary the brightness of clouds. Similarly in different seasons (e.g. summer and winter) the image disc of the northern and southern hemisphere will have different brightness. Local features like mountains and valleys introduce their own effects. In IR imagery elevated land like Tibetan Plateau will be cold at night and would appear very bright whereas tropical oceans would maintain about the same gray shade throughout the diurnal cycle.

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