The image shown is the MODIS “fog and low cloud ” product generated from differencing the 11 and 3.9 µ m imagery While this MODIS spectral difference image is sometimes referred to as the “fog” product, it really identifies both fog near the ground and low clouds, whose bases may be near the ground but whose vertical extent is usually much greater than that of fog. Much of the image shown here is comprised of low clouds as opposed to fog. The channel differencing technique used in this product relies on the emissivity differences between the land/water surface (generally close to zero) and clouds (general much larger than zero) at these wavelengths to detect opacity in the line of sight emission of energy from the surface to the satellite. The positive differences in the fog and low cloud product are mostly due to the 3.9um wavelength observing features (fog and water clouds) having slightly lower emissivity than the 11um (hence colder 3.9um brightness temperature). The spatial extent and consistency of the fog and low clouds within a given pixel seen by the satellite may cause the emissivity of these two wavelengths to be nearly the same, especially in the coarse resolution GOES fog and low cloud product For example, only a small portion of the pixel maybe filled with fog/low clouds, or these clouds may be scattered in nature across the pixel. However, the use of the higher resolution MODIS may alleviate some of this problem. Negative values in the difference image product occur with non-constant emissivity variations between the channels resulting from the presence of middle and high clouds. The emissivity of fog and water clouds is similar to each other at these wavelengths so a significant difference between fog and low clouds can not be detected with this product alone. However, because of the greater vertical thickness of clouds, the tops of low clouds are likely to occur farther from the surface than the top of the fog bank, and therefore the emission signature from these features may be different. Small positive and negative differences occur from clear regions. Cutoffs between these image features are not fixed as instrument noise and real surface emissivity variations may produce larger than expected channel differences.
Since the vertical extent of low clouds is usually greater than that of low level fog, the 11 µm channel image can be used to estimate the cloud or fog top temperature (height) to help differentiate fog from low clouds in the difference image. When this difference image is viewed in AWIPS, the 11um imagery can be loaded along with this image product but kept transparent. The AWIPS sampling tool can be used to display 11 µm values to help distinguish fog from low clouds. The fog will likely have warmer temperature values compared to the low clouds in the surrounding regions. This difference in cloud or fog top temperature can be a few or as many as 7-8C or more as shown in the image.
A short, 6-minute web-based training module on this product is available at: