Low cloud / fog over snow covered ground on 25 February 2019

During the overnight hours of 25 February 2019, low clouds and fog developed over portions of northwest Kansas, eastern Colorado and southwest Nebraska.  The low cloud and fog developed over a field of snow on the ground from a recent blizzard.  Low cloud and fog on top of snow on the ground can be difficult to detect in some satellite imagery, while in other satellite imagery it is easy to detect, for example see this 4 panel GOES-16 imagery:


The loop spans the nighttime to daytime hours.

Upper left is the GeoColor product.

Upper right is the Day Cloud Phase Distinction product.

Lower left is the nighttime microphysics product.

Lower right is the 10.3 micron (IR) band.

During daytime hours, note how difficult it is to discriminate between low cloud / fog versus snow on the ground in the GeoColor product, both features appear white.  However, during the daytime we can make the discrimination between low cloud / fog versus snow on the ground in the day cloud phase distinction RGB.  Snow on the ground appears green, why?  There is little contribution from Red (10.3), large contribution from Green (highly reflective at 0.64 microns) and small contribution from Blue (absorptive at 1.6 microns).  The low cloud and fog appears cyan since the contribution from Blue is larger (liquid water clouds reflect much more than snow on the ground at 1.6 microns).

The low cloud and fog during the nighttime hours is observed as bright green in the nighttime microphysics product and light blue in the GeoColor product.  It may be seen in the 10.3 micron band as well, but is much more difficult to detect due to the lack of contrast relative to the other 2 RGB products.  High clouds are also observed moving over the low cloud / fog region in northwest Kansas, these are observed as black or dark red colors in the nighttime microphysics product.  The high clouds acted to seed ice crystals into the low level clouds underneath, leading to snow flurries across the area.

Another product that shows all of this quite well is the CIRA Snow/Cloud-Layers product:

This loop spans daytime hours only since the discrimination between clouds versus snow on the ground can only be made during the daytime in this product.  Snow appears white in color, which may be more intuitive compared to other RGB products.  Bare ground is dark green, low clouds or fog are yellowish-green and high level clouds are pink.

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