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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| Predawn Low Cloud | Morning Low Cloud |
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| This product shows low clouds and fog as white at all times during a twenty-four hour period. During the daytime the product is based on the visible imagery; at night the image is produced by a combination of GOES infrared channels. This combination is based on the 3.9 and 11 micron wavelength channels. Stratus and fog have different emmissive properties in the two wavelengths. This difference is used to produce an image of nighttime low clouds. Visible images are the most ideal images for viewing low clouds. Reflecting solar radiation, low clouds and fog appear as white against darker land and sea surfaces on visible images. However, visible images are only available during the daytime. Infrared images, available at night, are a poor substitute, showing low clouds and fog poorly or not at all. This is because low clouds and fog have a very similar radiating temperature as the underlying land or ocean. Thus, low clouds and fog appear nearly invisible on infrared images. The bispectral nighttime image, on the other hand, highlights low clouds well at night in white gray shades. On loops, therefore, the user can identify low clouds and fog (white gray shades) during both day and night. | |
| The evolution of low clouds and fog can be seen at times during a twenty- four hour period. Traditionally, fog and low clouds have only been easily viewable during the daytime only with visible images. This product extends the ability to view low clouds around the clock. The product is especially useful for unobserved areas like oceans where surface reports of low clouds, fog or poor surface visibilties are rare. On this product low clouds always appear white. High clouds appear as black during the nighttime and white during the daytime. |
| The nighttime product has been known elsewhere as the "fog" product. But it is also a "stratus" product and can not distinguish between low stratus clouds and actual fog. This is because the satellite only observes the top of clouds; it receives no information about cloud bases. Thus, the forecaster may need to use additonal data and knowledge to distinguish between low clouds and fog, such as surface observations of visibility. The nighttime portion of the loop is only useful for stratus, fog, or stratocumulus. It will not perform well over cumulus clouds. During the nighttime high clouds will appear in a variety of appearances, often as black gray shades. At times it will be difficult to distinguish nighttime high clouds from the ocean/land background. This difficulty is alleviated by use of the loop with buttons below. The user can then discern from the movement of clouds which are high vs. low clouds. This product uses the 1 km spatial resolution GOES visible data, available during the daytime. Thus, daytime images have a crisp, highly-defined appearance. During the nighttime, the product uses a combination of two infrared channels, both with a resolution of 4 km. Thus, the nighttime images have less sharpness and definition. Examples |
| Predawn Low Cloud | Corresponding IR |
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| The image on the left shows the nighttime low cloud image just before dawn over the Pacific northwest. Notice the fog in the coastal valleys of Washington and Oregon. On this image high clouds are shown in black and appear to the west of the coast. Notice that on the IR image to the right it is impossible to see the fog in the coastal valleys. This is because the fog has the same temperature as the background. The lowcloud loop buttons show the transition between use of the bispectral products at night to the use of visible images during the daytime. Notice, for example, the dissipation of the fog that occurs under the influence of daytime heating. | |
| Morning Low Cloud | Corresponding IR |
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| The image on the left shows the morning low cloud product which is simply the visible image. It shows low cloud dissipating in the coastal valleys of Washington and Oregon. The corresponding IR image on the right does not show the low cloud in these areas well because the fog and ground are radiating at about the same temperatures. The low cloud loop buttons show the transition between the low cloud products at night and visible images used during the daytime. | |
Author: Tom Lee Last Updated: Tue Dec 10 16:38:13 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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