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Principal Component Image (PCI) transformation of GOES imager bands - night case

The Principal Component Image (PCI) transformation of the 4 GOES infrared bands available at night is in some respects similar and in some respects different than the PCI transformation of all 5 GOES bands during the daytime. The differences are due to the fact that the visible band (band-1) and the reflected solar radiation component in the shortwave infrared band (band-2) are missing. Below is an example of a set of nighttime GOES imagery transformed into GOES PCIs along with an interpretation of the individual component images.

GOES imager bands - night case

The following four images are typical of nighttime imagery in the 4 GOES infrared bands. The scene contains a variety of surfaces (land and water) and clouds at multiple levels. References to these images will be made when discussing the resulting component images to follow. Band-2 is a shortwave infrared image (containing mainly emitted terrestrial radiation at night, with no reflected solar contribution. However the low surface emissivity in this band is a significant contributor to the component images. Band-3 is an infrared water vapor image (with significant atmospheric absorption and emission), and bands 4 and 5 are longwave infrared images (containing emitted terrestrial radiation). Some of the bands use enhanced gray scales to bring out features in the imagery.

GOES band-2 (shortwave IR) image GOES band-3 (water vapor) image GOES band-4 (longwave IR) image GOES band-5 (longwave IR) image

GOES PCI transform - night case

The following plot contains the transformation vectors used to convert the 4 GOES infrared bands above into 4 component images. These transformations were generated for this data set and in the strictest sense are unique to the data set. However, the resulting transformation is typical of that for nighttime imagery when performed on all 4 GOES infrared bands. The contribution to each component image (1-4 on the vertical axis) is read across the graph. Each component image contains a contribution from each of the 4 GOES infrared bands (lines 2-5), although that contribution may be positive, negative, or zero. The magnitude of each band's contribution is indicated by the horizontal axis. Positive contributions are on the right, negative contributions on the left, and smaller, near-zero contributions are in the middle. Each band is represented by a colored line and the contribution of each band to each component image is connected merely to help the user see where the original bands contribute their information. Bands contributing a significant amount to a particular component image are those bands with either large positive or negative values.

PCI transform for GOES infrared bands 2-5 at night

The right-hand vertical axis gives the explained variance in the original images that is associated with each of the component images. Together these variances add up to 100%, with the first component explaining the majority, 95.7%, of the variance in the original images. Successive component images explain smaller amounts of variance until the last component image explains only 0.1% of the total variance. Generally all component images, even though the explained variance in some is quite small, contain important information that may be hidden in the original imagery. Some of the details are revealed in the individual component images as explained below.

GOES Principal Component Images (PCIs) - night case

The following four images are the Principal Component Image (PCI) transformation of the 4 GOES infrared bands above. Near the bottom of each image, the GOES bands contributing to that component image are designated as numbers over the gray bar. The positions of the band numbers is on a plus/minus scale similar to that in the transformation graph above, but the positions may be inverted left-to-right due to the fact that some of the component images are inverted to display cloudy areas as white. Positive contributions on the right, negative contributions on the left, and smaller, near-zero contributions are in the middle. Little plus and minus signs are at each end of the gray bar, and a vertical line is the zero point at the center of the gray bar. Bands contributing a significant amount to a particular component image are those bands with their number at either a large positive or negative position.

The first component (PCI-1) contains the signal which is most common among all the 4 GOES infrared bands. As in the daytime all the infrared bands contribute negatively to this component, but with the largest contribution from bands 4 and 5. As a result, this image looks similar to the two longwave infrared bands (bands 4 and 5) and emphasis in this PCI is on the coldest cloud tops and the warmest land surfaces. This component image is similar to the first component for the daytime 5-band GOES PCI case. This is due to the fact that the visible band (band-1) contributed little to the first component during the daytime.

PCI-1 from GOES bands 2-5 at night

The second component (PCI-2) has by far its largest contribution from the GOES water vapor (WV) band (band-3) and much smaller contributions from the other bands. Therefore this component image looks similar to the enhanced WV band (band-3) with emphasis on upper-level water vapor features. But other bands contribute smaller amounts to this component as well resulting in other variations in the scene. This component is similar to the third component for the daytime 5-band GOES PCI case.

PCI-2 from GOES bands 2-5 at night

The third component (PCI-3) has its largest contributions from GOES band-2 (negative) and band-5 (positive). This image combination is similar to the GOES fog product generated by subtracting band-4 (similar to band-5) from band-2. In this component image the different layers of cloud in the northeastern part of the image are distinguished. Higher, colder (ice) clouds are darker than lower, warmer (water) clouds.

PCI-3 from GOES bands 2-5 at night

The fourth component (PCI-4) has its largest contributions from GOES bands 4 and 5, but with opposite signs. This component emphasizes thin cirrus clouds (shown as white in this image). These thin clouds show up in the band difference due to differential transmittance (different optical thickness) between the two bands. Also seen are slight variations in surface emissivity between these two bands. During the summer when the atmosphere contains more moisture, variations in the amount of water vapor absorption in the lower atmosphere can also be detected.

PCI-4 from GOES bands 2-5 at night

Summary

This set of component images shows the transformation of the 4 GOES infrared bands at night into 4 component images. Similar to the daytime PCI case, this component analysis separates the redundant information in multi-spectral imagery to reveal differing cloud and surface features not as readily viewable in the original band images. The transformation plot and the numbers over the gray bar on the component images help explain the combination of bands making each component image.

The differences between the component images from day to night are due to the fact that the visible band (band-1) and the reflected component of the shortwave infrared (band-2) are missing. This effectively eliminates the component image which focuses on the visible band during the daytime (PCI-2). With little or no contribution from the visible band (band-1) PCIs 2 and 4 at night look similar to PCIs 3 and 5 during the day. There is however a significant difference between PCI-3 at night and PCI-4 during the day due the daytime contribution from the visible band (band-1).

Back to RAMM Branch GOES image display page.