Viewing Clouds from Above

A part of eastern Iceland peeks out through an opening in the clouds in this afternoon satellite image recorded on July 28, 1996 by the Advanced Very High Resolution Radiometer on board the U.S. NOAA-14 weather satellite. The white area to the south is the large Vatnajökull glacier, with the bright blue area on its northern edge indicating pyroclastic fields soaked with meltwater, followed by lava fields in grey and vegetated areas in green. (Vatnajökull made headlines in 1996 because of a volcanic eruption under this glacier; see here for more information).

The reason for selecting this image, however, is rather to illustrate how sophisticated data processing can make subtle cloud structures visible from a mass of digital data. On the left, the existence of a low pressure zone is betrayed by a large, elegant counterclockwise spiral. On the right, a collection of stratiform clouds which are typical for polar and subpolar regions in summertime are being pushed and pulled into complex shapes by local atmospheric currents. These clouds strongly influence the polar energy budget and are a good indicator for such dynamics in the lower troposphere as eddies, or wave patterns like those in the cloud wedge north of Iceland. Though long predicted by theory, such large scale structures are hardly discernible from the ground, and it is only thanks to satellite earth observation that it is regularly possible to discover and analyze them.

For a higher resolution image click here. (~890k)

AVHRR measures radiance in five channels from visible red to thermal infrared at a spatial resolution of 1 to 3 km. This image made use of solar spectral channels 1 (visible red) and 2 (near infrared), where reflected solar irradiance dominates, as well as channel 5 (thermal infrared, where the heat radiated from the earth can be measured) to generate a color composite showing both reflection characteristics and cloud surface temperatures. These channels were chosen to make it possible to distinguish relatively high (white) clouds from clouds which are relatively lower and warmer (yellow), in a color composite which looks as "natural" as possible. If the goal had been instead to indicate where precipitation can be expected, then AVHRR channel 3 (mid-infrared) would also have been used, in order to bring out the presence of high level clouds with ice tops, since outside tropical regions significant amounts of precipitation are always correlated with such clouds.

Several processing steps are required at DFD in order to represent the data at this level of detail. A mix of commercial and public domain software was used, as well as the software package APOLLO.

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