Using the Fog Product to detect low clouds and fog, one more look

The utility of the Fog Product has been well established, but it never hurts to show a few more examples. The following is a case which occurred during the early morning hours on 23 November, 1997. As seen in the weather depiction chart at 0814Z, low clouds cover most of Tennessee and northern Alabama, while dense fog blankets most of the Delta Region of southeast Arkansas and northeast Louisiana.

Three satellite images are presented in this case:

• A "conventional" 10.7 micron IR image;
• A 3.9 micron "IR2" image;
• A "Fog Product", formed by subtracting the 3.9 micron image from the 10.7 image, and scaling the result
  (See this previous poster for more information on how the Fog Product is generated)

Compare the depiction of the low clouds and fog by the three image types. In your opinion, which one does the best job of delineating the edge of the low clouds? Which one best reveals the presence of the fog over the Delta Region? I believe the answer will be clear.

Conventional 10.7 micron IR image at 0815 UTC
Where are the low clouds? There are a few mottled areas on the image, suggesting the presence of stratus, but it's hard to differentiate between the cloud edges and the normal variations in surface skin temperature. For example, take a look at northern Mississippi. How much of this area is covered by low clouds? Aside from one patch near Tupelo, not much of it is, though there are several large patches that suggest the presence of clouds. Now, take a look at Tennessee and western Kentucky. Where does the cloud mass stop and the cold ground begin? I don't know about you, but it's hard for me to make that call with any certainty. Oh, one more thing: can this image tell you anything about the areal extent of the fog in the Delta? Forgettaboutit!

3.9 micron "IR2" image at 0815 UTC
The shortwave IR image does a little better job of distinguishing between clouds and ground than did the 10.7 micron IR. In particular, the cloudy/clear edge over northern Alabama shows up more clearly, as does the cloudy patch over western Mississippi. However, the various cloud features are still not as distinct as they could be. Plus, the fog area over the Delta still hasn't been picked up yet.

Fog Product image at 0815 UTC
Ahhhhhhh, that's better! Who would have guessed that the Fog Product, of all things, would show you where fog was (with low clouds thrown in for good measure)? Much more so than the first two, this image clearly depicts the edges of the cloud fields over Tennessee, northern Alabama, and Mississippi. How do we know these are clouds, not fog? Well, (other than looking at the surface obs.), the presence of reds and blues in the enhancement indicates a large difference between the IR and IR2 brightness temperatures. This in turn indicates the presence of a relatively thick condensed layer of moisture - most likely an elevated deck of low clouds. On the other hand, the patch of lighter yellows over southeast Arkansas suggests the presence of a warmer, shallower cloud layer with a temperature much closer to that of the ground - most likely ground fog. The high clouds over the Gulf are the easiest to pick out of all...cold cirrus shows up as bright white on the Fog Product, with very cold tops appearing as multi-colored speckles (This speckling occurs because the 3.9 micron channel is very noisy at cold temperatures).

Final Thoughts...

Wouldn't you agree that the Fog Product did the best job of depicting not only the low clouds over Alabama, but also the shallow, dense ground fog over the Delta? This case illustrates the Fog Product's usefulness as a tool for monitoring the development of stratus and fog during the nighttime hours. Clearly, it outperforms its counterparts in this role.

Remember that mid/high level clouds, even in thin layers, will disrupt the differencing that is occurring between the IR and IR2 images. As a result, low clouds/fog will not appear in areas where high clouds are present. When using the Fog Product, be careful interpreting the development or movement of low clouds that are beneath or in the vicinity of high level cloudiness. Thin wisps of fast moving cirrus can mask the lower clouds in some images of a loop, giving the false appearance of movement or development. Of course, this product should always be used in conjunction with surface observations to get a complete, quantitative picture of the ceilings and visibilities across the forecast area.

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