AVHRR Pseudo True Color Imagery

	NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil

AVHRR Pseudo True Color Imagery - Focus Tutorial

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Introduction

Low Clouds, High Clouds, and Dust Associated with a Frontal Passage Across the Arabian Gulf

Although the Advanced Very High Resolution Radiometer (AVHRR) does not have the red, green, and blue channels required to create the standard true color product, a clever combination of the visible, near-infrared, and thermal infrared channels that are available allows vegetation to appear as green, land as brown, and clouds as various shades of off-white according to their temperatures. This so-called pseudo true color product is not without its interpretive caveats, and should always be cross-referenced with visibile, infrared, and true color imagery whenever available.

Low Clouds, High Clouds, and Dust Associated with a Frontal Passage Across the Arabian Gulf

Although the Advanced Very High Resolution Radiometer (AVHRR) does not have the red, green, and blue channels required to create the standard true color product, a clever combination of the visible, near-infrared, and thermal infrared channels that are available allows vegetation to appear as green, land as brown, and clouds as various shades of off-white according to their temperatures. This so-called pseudo true color product is not without its interpretive caveats, and should always be cross-referenced with visibile, infrared, and true color imagery whenever available.

Background

Dust in the Northern Arabian Gulf, Clouds Crossing the Straights of Hormuz

This enhancement takes advantage of the thermal and spectral (absorption, emission, and reflection) differences between the various constituents of a satellite image (snow, clouds, barren land, vegetation, and water bodies) to produce an image that in some regards looks like true color. AVHRR channel 1 (0.63 micrometers) produces a strong contributions for clouds, snow, and barren land (high reflectance), and small contributions from water and dense vegetation (low reflectance). AVHRR channel 2 (0.8 micrometers) is sensitive to green vegetation (which becomes increasingly reflective in the short-wave infrared), and channel 4 (11.0 micrometers) is a channel sensitive to heat. In this false color enhancement, information from channel 1 is placed in the red color gun, a combination of channels 1 and 2 in the green gun, and a combination of channels 1 and 4 in the blue gun. Since channel 2 is the vegetation-sensitive channel, its presence in the green color gun will produce enhanced green contributions wherever vegetation is present. The temperature information (channel 4) is inverted, such that cold pixels (such as clouds) give the strongest contributions. The scalings on the color guns are chosen such that most clouds will appear white. However, since there is a temperature dependency, the very cold clouds will end up with a relatively stronger contribution in the blue color gun (hence making deep convection and thick cirrus appear light blue) while low clouds in a warmer environment will be depleted of blue (leaving them with a yellowish tinge). One must be careful not to confuse the low clouds with dust, which can also appear as shades of yellow. Water bodies, which in general are relatively warm with low reflection, will have minimal contributions from any of the color guns and will appear black.

Dust in the Northern Arabian Gulf, Clouds Crossing the Straights of Hormuz

This enhancement takes advantage of the thermal and spectral (absorption, emission, and reflection) differences between the various constituents of a satellite image (snow, clouds, barren land, vegetation, and water bodies) to produce an image that in some regards looks like true color. AVHRR channel 1 (0.63 micrometers) produces a strong contributions for clouds, snow, and barren land (high reflectance), and small contributions from water and dense vegetation (low reflectance). AVHRR channel 2 (0.8 micrometers) is sensitive to green vegetation (which becomes increasingly reflective in the short-wave infrared), and channel 4 (11.0 micrometers) is a channel sensitive to heat.

In this false color enhancement, information from channel 1 is placed in the red color gun, a combination of channels 1 and 2 in the green gun, and a combination of channels 1 and 4 in the blue gun. Since channel 2 is the vegetation-sensitive channel, its presence in the green color gun will produce enhanced green contributions wherever vegetation is present. The temperature information (channel 4) is inverted, such that cold pixels (such as clouds) give the strongest contributions. The scalings on the color guns are chosen such that most clouds will appear white. However, since there is a temperature dependency, the very cold clouds will end up with a relatively stronger contribution in the blue color gun (hence making deep convection and thick cirrus appear light blue) while low clouds in a warmer environment will be depleted of blue (leaving them with a yellowish tinge). One must be careful not to confuse the low clouds with dust, which can also appear as shades of yellow. Water bodies, which in general are relatively warm with low reflection, will have minimal contributions from any of the color guns and will appear black.

Advantages

Example of High Clouds, Low Clouds, and Vegetation in the Eastern Mediterranean Region

Despite its pitfalls, the pseudo true color product provides additional information beyond what visible or infrared imagery can provide alone. Familiarity with the behavior of this product enables the user to readily discriminate between low cloud and high cloud while at the same time noting additional details in the topography. The 1km spatial resolution of local area coverage (LAC) data reveals mesoscale cloud structures.

Example of High Clouds, Low Clouds, and Vegetation in the Eastern Mediterranean Region

Despite its pitfalls, the pseudo true color product provides additional information beyond what visible or infrared imagery can provide alone. Familiarity with the behavior of this product enables the user to readily discriminate between low cloud and high cloud while at the same time noting additional details in the topography. The 1km spatial resolution of local area coverage (LAC) data reveals mesoscale cloud structures.

Limits

Utility of Cross Referencing With Other Products

A major benefit of the Satellite Focus webpage the ability to compare and contrast various products that are co-registered (so features line up). In the example above, the pseudo true color product is compared agains the hight/low cloud and snow detection product. Here we see that low clouds and snow assume a similar hue in the pseudo product. The high/low cloud snow product reveals more information on some elements of the scene, but barren land and even water surfaces may appear green. The pseudo true color product provides the complementary surface detail. This is one of many potential applications where additional information can be gleaned simply by combining the strengths of various products.

Utility of Cross Referencing With Other Products

A major benefit of the Satellite Focus webpage the ability to compare and contrast various products that are co-registered (so features line up). In the example above, the pseudo true color product is compared agains the hight/low cloud and snow detection product. Here we see that low clouds and snow assume a similar hue in the pseudo product. The high/low cloud snow product reveals more information on some elements of the scene, but barren land and even water surfaces may appear green. The pseudo true color product provides the complementary surface detail. This is one of many potential applications where additional information can be gleaned simply by combining the strengths of various products.

Author: Steve Miller
Last Updated: Fri Mar 28 14:47:31 2003
Produced by: The Composer (Ver: 1.4 )

NRL Monterey, Marine Meteorology Division http://www.nrlmry.navy.mil