The Mueller ringing mask for CZCS is based on the algorithm of Mueller, J. L., Nimbus-7 CZCS: electronic overshoot due to cloud reflectance, Appl. Optics, Vol 27, No 3, 1 Feb, 1988, pp 438 - 440.
The mask does no correction, just a masking distance based on the excess radiance in the previous 50 pixels along the line. The general behavior of the mask is shown for the simple example of a set of n pixels above the threshold having a constant total radiance in the 750 nm band (band 5).
The threshold is at 2.45 mW cm -2 sr-1 um-1. So, a set of 50 pixels must have a radiance of about 2.85 to begin to generate a mask for ringing while 1 bright pixel must have a radiance of about 15. Large bright targets can easily generate a down-scan masking of over 100 pixels.
The mask is applied to a CZCS image for 13 Sep 1984 at 1639 around the Florida peninsula. The ringing mask is placed into the stray light mask / flag. The following image shows the effects of the mask on the central part of the scene. The greyscale portion is the total radiances at 750 nm while the colors are: blue for the cloud mask overlaid by green for the pixels where the Lt_750 is above the ringing threshold overlaid by red for the ringing affected pixels (in operations, the pixels exceeding the threshold will also have the ringing mask applied).
Images of water-leaving radiance, chlorophyll and 670 nm optical thickness are shown for 4 cases: no ringing correction, the standard ringing correction and the ringing correction for the lowest and highest confidence limits on the mask distance equation. Click on the image sub-area to see an image of the entire dataset.
| Product | No ringing mask | Standard ringing mask | Low limit ringing mask | high limit ringing mask |
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| Lwn_443 |
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| Lwn_520 |
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| Lwn_550 |
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| chlor_a |
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| tau_670 |
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The low limit ringing mask appears to be generally effective in removing the effects of ringing. The chlorophyll image shows effects well away from the cloud off the east coast of Florida which requires the amount of masking provided by the standard setting. Also, in the full image of tau_670, the cloudy region in the far east edge shows significant effects of the ringing that the standard mask (at least) should be used on. It almost appears that the ringing mask needs to be applied to further distances in regions having lower signal (such as low tau or chlor_a in open waters). The high limit mask also removes about 25% more pixels than the standard setting, so this must also be considered.
The tau_670 is increased in the neighborhood of clouds, but the effect is not from the ringing. Perhaps, a more strict cloud mask would fix this. However, the chlorophyll is unaffected.
The standard ringing mask should be used for the first tests and improved if required in the future.
