Fog event 9/27 – multi-state, post frontal

MODIS spectral difference and Night-time Microphysics RGB

MODIS spectral difference (11-3.9 micron, left) and Night-time Microphysics RGB (multi-channel composite, right)

Light precipitation on 9/26 and subsequent clearing overnight (post frontal) setup a wide spread fog event that stretched from SE Mississippi to central Ohio. The MODIS spectral difference product (11-3.9 micron) can help identify areas of low clouds and fog.  The MODIS RGB night-time microphysics product (derived from EUMETSAT’s SEVIRI standard RGB suite) uses the same spectral difference, but only for the green channel.  Two additional inputs are provided to create the night-time microphysics RGB: 12.0-11 micron (relating to cloud thickness) for the red channel and 11 micron (related to thermal emission) for the blue channel. The images have similar areas denoted for fog (right click to open in new window and click in new window for full image).  The night-time microphysics image for 0657 UTC shows varying shades of light blue corresponding to fog thickness because it includes the additional channels.  Thicker clouds have more contribution from the red channel, and the warmer temperatures of the low level fog and clouds provides a brighter blue channel (colder temperatures are actually closer to black).  Pixels with thin fog are influenced by some of the thermal emission of the land surface being transmitted through the fog layer; hence the light violet color of the land surface bleeds through the thin fog. This makes it easier for the forecaster to see areas of thick vs. thin fog. The night-time microphysics RGB indicates a slightly wider area of thin fog affecting the central and southwestern portions of the BMX CWA than indicated by the spectral difference product at this time.  Other mid and high level clouds (not fully shown here) are more easily identified with the RGB product in order to not confuse them with other low cloud features. Similar channels will be available on GOES-R’s ABI instrument to provide a geostationary version of this product.

One thought on “Fog event 9/27 – multi-state, post frontal

  1. Great post! We already receive the MODIS spectral difference product but currently do not have the MODIS RGB night-time microphysics product. This would compliment are data void areas and the transitioned low cloud base products nicely.

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