Low Cloud Product - San Francisco Bay Area Tutorial

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

Low Cloud Product - San Francisco Bay Area Tutorial

Click thumbnails to view original full-sized images.

Introduction

Later Afternoon Visible Image	Early Evening (After "dark") Image

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.

Later Afternoon Visible Image

Early Evening (After "dark") Image

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.

Advantages

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.

Limits

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 images are only useful for stratus, fog, or stratocumulus. They 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 loops which combine the daytime and nighttime products. 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

1A Late Afternoon Image	1B Early Evening Image (After Dark)

Image 1A, a visible image, shows low stratus off northern California. Unfortunately, visible images are only available during the daytime. During the nighttime infrared images simply do not "see" stratus well. Therefore, the low cloud product substitutes a bispectral image based on the difference of two GOES channels (Image 1B). This second image also shows low cloud as white. However, since its spatial resolution is not as good (4 km vs 1 km for visible daytime images), the image has a "blockey" appearance with individual pixels apparent. Since 1B uses 3.9 micron data, the image shows "shiptracks" or "anomalous cloud lines". These are the long streaks paralleling the coast. These can not be seen in visible images.

1A Late Afternoon Image

1B Early Evening Image (After Dark)

Image 1A, a visible image, shows low stratus off northern California. Unfortunately, visible images are only available during the daytime. During the nighttime infrared images simply do not "see" stratus well. Therefore, the low cloud product substitutes a bispectral image based on the difference of two GOES channels (Image 1B). This second image also shows low cloud as white. However, since its spatial resolution is not as good (4 km vs 1 km for visible daytime images), the image has a "blockey" appearance with individual pixels apparent.

Since 1B uses 3.9 micron data, the image shows "shiptracks" or "anomalous cloud lines". These are the long streaks paralleling the coast. These can not be seen in visible images.

2A Late Afternoon Image -- Cirrus and Low Clouds Mixed	2B Early-Evening Image Stratus is White Cirrus is Dark

Fig 2A shows a late afternoon scene of cirrus and stratus mixed over the San Francisco Bay Area. It is hard to distinguish the two. The low cloud appears over the southern part of the Bay Area and to the east. The high clouds appear mostly as streaks oriented east to west over the region. A few hours later after dark, Fig. 2B shows low cloud and cirrus in distinct gray shades. The low cloud is white; the cirus is black. Thus, at night it is easy to distinguish high clouds from low clouds using this technique.

2A Late Afternoon Image -- Cirrus and Low Clouds Mixed

2B Early-Evening Image Stratus is White Cirrus is Dark

Fig 2A shows a late afternoon scene of cirrus and stratus mixed over the San Francisco Bay Area. It is hard to distinguish the two. The low cloud appears over the southern part of the Bay Area and to the east. The high clouds appear mostly as streaks oriented east to west over the region. A few hours later after dark, Fig. 2B shows low cloud and cirrus in distinct gray shades. The low cloud is white; the cirus is black. Thus, at night it is easy to distinguish high clouds from low clouds using this technique.

Author: Tom Lee
Last Updated: Tue Dec 10 16:37:26 2002
Produced by: The Composer (Ver: 1.1.2 )

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