Let’s analyze the following loop of the synthetic fog product, generated from the 4-km NSSL WRF-ARW model:
In this color table, grey into light blue represents increasing confidence in liquid water clouds. Our example from the 0000 UTC 10 October 2011 NSSL WRF-ARW model run shows a large area of liquid water clouds (most likely stratus) across Texas extending northward through the central US. The darker shades of grey and black correspond to ice clouds (most likely cirrus) forecast by the model.
The region of blue in Arizona and Utah extending southward into northwest Mexico that does not move catches your attention. A quick look at the visible satellite imagery:
shows that the region is mostly cloud free. However, the lighter shade of grey extending northwest to southeast through the four corners of AZ/CO/UT/NM corresponds to the blue in the synthetic fog product; this false cloud signature is a consequence of surface emissivities at the two channels. This is not a model error, rather, an observed feature in GOES-11. Similarly, the other blue region in the northeast quadrant of AZ and southwest AZ into northwest Mexico are also consequences of the surface emissivity.
The easiest way to identify a false signature is to look at the loop, and the areas that don’t move at all throughout the duration of the loop are likely false emissivity signatures.