An Example of the Fog Products Usefulness

I. Background

As we all know, the detection of nighttime low cloudiness or fog via IR satellite imagery can be difficult, primarily because the difference in brightness temperature between a low cloud and the earth's surface can often be quite small. Conventional IR images in these cases are often monochromatic, and subtle frame to frame changes have to identified in order to determine the movement of the cloudiness. However, our ability to detect low clouds has been enhanced with the addition of the new 3.9 micron IR channel on the GOES-8 satellite, and the fog product which is derived from it.

The fog product exploits the radiative differences which exist between the 10.7 micron longwave IR channel (the regular IR channel we are all familiar with), and the 3.9 micron channel. Although both IR channels are basically window channels (that is, they sense radiation from either the earth's surface or from clouds, with negligible attenuation from the atmosphere), the following radiative characteristics of the two channels vary considerably:

1. Emissivity and reflection
2. Temperature responsivity
3. Sub-pixel response
4. Diffraction
5. Noise

Fortunately for our nighttime forecasting purposes, the fog product is now available on the SAC. Although the default enhancement curve for this imagery type is currently sabir, I'd highly recommend using the rams_fog enhancement, as it is specifically designed for this product. With this curve, low clouds and fog appear yellow, and transition to reds and blues as the opaqueness of the cloudiness increases. Colder mid and high level clouds, on the other hand, appear white. Alternatively, you can choose the invert function from within the enhancement menu of GARP, which will make the low clouds appear grayish-white, and the cirrus yellow-green. Either way, the fog product greatly enhances our ability to detect the presence of low clouds and fog, as demonstrated in the example below.

II. Example of 11 November, 1996

The morning of 11 November provides a good example of the fog product s utility in low cloud detection. During the pre-dawn hours, an area of scattered to broken cloudiness developed over Mississippi, Arkansas and Louisiana in a region of warm air advection ahead of an approaching 700 mb shortwave. As noted on the regional plot at 11Z , ceilings on the order of 3000 to 6000 feet were noted throughout the three state area shortly before daybreak. In addition, 7000 to 8000 foot ceilings were present over the Tennessee Valley of north Alabama.

Examining the conventional 10.7 micron IR imagery (NOTE: this is an animated GIF size approx. 360 kb containing 10 images) for the period between 07Z to 12Z, we see only small variations in the brightness temperatures over Mississippi, making it very difficult to determine the eastern extent of the cloud field. On the other hand, the low clouds are clearly delineated in the corresponding sequence of fog product images (NOTE: this is an animated GIF size approx. 500 kb containing 8 images). Denoted by bright yellow and red pixels, the cloud mass forms along the Mississippi River around 07Z, then expands in coverage and moves eastward to near Mobile and Meridian by daybreak. A second, slightly higher deck of cloudiness is also evident over northern Alabama, moving south with time. Unlike the 10.7 micron imagery, the fog product clearly reveals the locations of the cloud borders, making it easy to track the movement of these features.

This is a simple illustration of the fog product's ability to detect nighttime low cloudiness. At other offices with longer-term access to digital satellite data, this product has proven to be a highly useful aid in aviation forecasting. You are encouraged to try it out this fall and winter and determine its applicability for yourself.

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