21.2. Satellite Channels#
There are sixteen (16) channels that are part of the advanced baseline imager (ABI) that is currently deployed on the geostationary series of satellites currently residing over the United States. Here we’ll cover the most commonly used channels for basic atmospheric analysis, red channel visible, clear IR, and midlevel water vapor. For more detail and information about all available channels can be found here.
Visible Imagery#
Visible satellite imagery is probably the most often thought of and easily understood of the various satellite products. In essence, visible satellite imagery collects light (or radiation) from the visible spectrum that has been reflected back into space. You can think of this type of satellite imagery as a giant photograph of Earth from space.
Visible imagery is very useful for forecasters, especially in convective weather situations. The spatial resolution allows for forecasters to watch for development of fine details in cloud structure, for example, cumulus clouds in regions where thunderstorm development is possible. Thus, visible satellite imagery provides short term monitoring capabilities of thunderstorm and general cloud development, before they are observed on radar. Additionally, the low-level wind field can be determined in severe weather environments but examining the orientation of the cumulus field.
There is one major drawback to using visible imagery – nighttime. Visible imagery uses visible light, so when the sun sets on a particular area, visible imagery quickly becomes useless. Thus we have other channels to use…
“Long-wave” Infrared Radiation Imagery#
As previously mentioned, visible satellite observations are only useful during daylight hours. However, weather doesn’t stop when the sun sets. It is just for this purpose that we have the long-wave infrared radiation channel, or IR, for short. IR satellite observations measure the amount of long-wave radiation that has been emitted by earth, clouds, water, etc. The amount of IR radiation that is emitted by an object is proportional to the temperature of that object (Stefan-Boltzmann Law). The more IR radiation that is emitted by an object, the warmer that object is; the less IR radiation being emitted, the colder an object is. Knowing this, it is possible to determine the temperature of the object emitting the radiation.
Typically, the color scheme used is for warmer temperatures to be represented as darker areas and colder temperatures to be represented as whiter areas on IR imagery. However, you can enhance the satellite image by using color scales.
One caveat that may arise when using IR imagery is that it is based on temperature. Thus, in the winter during severe cold outbreaks, the northern United States and Canada will appear to be completely covered by clouds, when in reality, the ground is so cold that the IR imagery is detecting the cold ground.
Water Vapor Imagery#
One of the crucial ingredients in the development of clouds and various other weather features is water – specifically, midlevel water vapor (WV). This is where water vapor imagery figures into the equation. Water vapor in the middle atmosphere tends to absorb radiation emitted from earth’s surface. Thus, while the name “water vapor” suggests that the satellite is looking for regions of moisture in the atmosphere, standard water vapor imagery is actually looking for regions where the radiation may have been absorbed by moisture in the atmosphere. This channel of the satellite works in the middle atmosphere between 650 mb and 350 mb. Thus, the water vapor image may show the atmosphere is very moist even though the low levels are extremely dry and vice-versa. Water vapor imagery is very useful for determining midlevel flow patterns as well as trough and ridges. The way water vapor imagery is interpreted is as follows:
A dark pixel indicates that radiation was absorbed by the satellite. This is indicative of dry air in the middle portion of the atmosphere.
A bright pixel indicates that radiation was NOT absorbed by the satellite. This means one of two things: the radiation was absorbed by moisture, or that the ground didn’t emit a large amount of radiation.
A band of very dark pixels poleward of a band of very bright pixels may indicate a jet stream. The jet stream or jet streak would be found along the area where the bright and dark bands meet.