Fog Product Tutorial
The following will explain the different views you will get from the “Fog” channel satellite, which is actually a subtraction product of the shortwave infrared from the long-wave infrared channels. Weather forecasters have been instructed to use this “product” at night to predict ground fog, but as you can see it is a very graphic daytime image for “Ship Tracks” (as the weather people call them). Somehow they don’t “bat an eye” when asked about the size, placement, and often circular (in) direction of these “ship tracks”, they just kind of do that funny twitch thing and quickly change the subject.
The official description of the “Fog Product” follows:
Image data from two GOES or NOAA AVHRR Infrared (IR) channels are combined to create a product for the detection of fog and low stratus clouds at night. The “Fog Image” is obtained by a subtraction of the shortwave IR window channel (3.9 micron wavelength) temperatures from the long-wave IR window channel (10.7 micron) temperatures. An experimental Low Cloud Base (LCB) product helps to identify ceilings <1000 ft for aviation users with help from surface temperatures. The “Fog Depth” image is a special enhancement that estimates the thickness of a low cloud layer, based on the temperature difference between the two IR channels.
In viewing low clouds and fog, visible imagery (except with snow cover) is the most ideal method. Naturally, visible imagery is only available during daylight hours. Infrared imagery is a poor choice for night viewing because low clouds and fog are hard to detect. This is because low clouds and fog have a similar radiating temperature as the underlying land.
The best satellite imagery to use for identifying fog and stratus clouds during the night is commonly called the “fog product.” The identification of fog and stratus at night is an application of the GOES Imagery data (CIRA – Cooperative Institute for Research in the Atmosphere tutorials, 1999). The GOES Imagery data utilizes bi-spectral satellite imagery (11u – 3.9u on AWIPS, i.e. fog product) since low clouds and fog have different emissive properties in the two wavelengths. The imagery is reduced by subtracting the 3.9u brightness temperatures from those at 10.7u . The white “fog” low clouds start to show up on the images when the low clouds are wider than 2 miles, and are thicker than about a hundred feet. The brightness of the image is set up so that low clouds are white, ground is gray, high clouds are black, and very high cold thunderstorm tops are a salt and pepper black and white. This imagery highlights low clouds and fog in white gray shades (a positive difference), clear conditions with little temperature variability shows up as a mid-gray shade, and high clouds in a dark shade (a negative difference). High clouds appear dark in this imagery since much of the sensed energy comes from the ground, and the 3.9u channel response to warm sub-pixel temperatures is greater than that at 10.7u . This is true even though the emissivity of an ice cloud is about the same as at the 3.9u and 10.7u . Using the AWIPS Image Properties the forecaster can colorize the “fog product” making it easier to identify the low clouds and fog. Additionally, putting these images in animation, fog and low clouds would be easily observed since the higher ice clouds would show more rapid movement.
There are limitations in viewing the fog product imagery at night. The “fog product” is also a “stratus product” since the satellite only observes the top of clouds, not the cloud bases. The forecaster must utilize additional data such as surface observations to differentiate between stratus and fog. At times, it will be difficult to distinguish between high clouds and land. This problem is alleviated by animating the product. During the daytime, the fog product uses the 1 km spatial resolution GOES visible data which would give the product a highly defined appearance. During the night, the product utilizes bi-spectral satellite imagery which has a resolution of 4 km. Therefore, fog product imagery at night will have less sharpness and definition.
Images centered off the coast of Southern California and Baja California on 20 May 2008 (below) revealed several ship tracks in the marine stratocumulus cloud deck that was offshore. Once again, such small-scale features showed up with greater clarity on the MODIS fog/stratus product imagery. These comparisons give a bit of a preview of the types of improved products that will be available using the Advanced Baseline Imager (ABI) instrument on the upcoming GOES-R satellite (scheduled to be launched in 2014), which will offer IR imagery and products at a 2-km spatial resolution (at 5-minute intervals on a routine basis).