Fog event of 22 May 2018 in the midwest – focus on nighttime microphysics RGB

During the overnight and morning hours of May 22, 2018 there was widespread fog over the upper midwest region centered around Wisconsin.  We’ll start by looking at the familiar fog product (10.3 – 3.9 micron) overlaid with ceiling (top left: hundreds of feet AGL) and visibility (bottom in miles):

Recall in the fog product, high clouds composed of ice are negative, while low cloud OR fog is positive with increasing confidence in magnitude (darker blue).  Note the positive values cover a rather large area, however it’s not uniform and there appear to be other cloud layers also present.  Note the higher cirrus (indicated in black colors) moving in from Nebraska into southwest Iowa (likely associated with prior convection).  Also note the regions of gray in central and northern Wisconsin, these can be interpreted as clouds that are less likely to be low clouds or fog, however keep in mind this cloud layer may be obscuring low clouds or fog underneath therefore there is a need to check surface observations.

In the GOES-R series, we now have additional RGB products that can provide more detailed and potentially additional information compared to the fog product.  For example, the nighttime microphysics RGB:

If you’re unfamiliar with this product, information can be found on the quick guide:

What additional information can be observed in the nighttime microphysics RGB that we were unable to observe in the fog product?  There are additional colors over regions that were displayed as various shades in blue (fog or low cloud) in the fog product, also, there are additional colors in regions that were depicted as gray in the fog product.  One of the primary advantages to the nighttime microphysics RGB is that it discriminates between low cloud and fog as observed by GOES.

Let’s first focus on eastern South Dakota to extreme southwest Minnesota into northwest Iowa.  The fog product showed positive brightness temperature difference values which would correspond to low cloud or fog.  The nighttime microphysics product over the same region is dull (darker) aqua.  Our assessment of dull aqua is based on a comparison to other aqua colored regions, for example, look further east in Minnesota where the aqua color is brighter which is more likely to be low cloud.  This agrees with surface observations which indicate 1/4 mile visibility during the period when the dull aqua increases.

Now let’s focus on central Wisconsin.  In the fog product, this was a mix of blue and gray colors, the gray represents less confidence in low cloud / fog while the blue is more confidence in low cloud / fog.  In the nighttime microphysics RGB we see a mixture of shades of green, along with some purple and localized red.  This provides information on the different cloud top heights.  Observations indicated occasional light rain in these regions, corresponding to what you may expect with thicker clouds.

Next, let’s focus in on the region indicated by the red box below:

During the loop, note a small region of green passes through the observation site in the red box, after passage we observe a dull aqua color.  The various shades of green correspond to low to mid level clouds composed of water droplets.  After the passage of this cloud we observe dull aqua (fog) and the observation indicates that fog has formed.  Presumably after the cloud goes by, radiation fog formed in this case.

Finally, we will conclude with a challenging scene for interpretation in northern Illinois, southern Wisconsin and the eastern half of Iowa.  The most obvious feature is a region of high clouds as indicated by near black or dark blue colors in the RGB product advecting eastward.  It’s important to note that fog may exist but simply be obscured by high clouds above it and indeed we observe this over our region of interest.  Now let’s focus in on north central Illinois near the Wisconsin border where interpretation is particularly challenging, there is a variety of colors that are changing and advecting through during the loop.  At the start of the loop, observations indicate widespread fog with low visibility, however the dull aqua may not appear as widespread as you may expect relative to the observations.  There is quite a bit of aqua and light green at this time.  This would indicate low clouds are present so that the fog is being obscured by a cloud deck over the fog.  A bit later in the loop, after the passage of the small patch of high clouds, we observe more regions of dull aqua (fog) before later becoming obscured by more widespread high clouds.  Note that during the loop, the observations do not change appreciably in northern Illinois / southern Wisconsin meaning that most of this area remains in fog despite the changes we see in the nighttime microphysics RGB.

In summary, we analyzed a challenging scene where fog was widespread but so were different types of clouds.  The nighttime microphysics RGB showed its utility in identifying some areas of fog but it also showed us that fog can be obscured by cloud layers above it.  When this is the case, using both surface observations and satellite imagery in tandem is critical to understanding the event.  In other words, where we see the dull aqua in the nighttime microphysics RGB our confidence in fog should be high, but the absence of this color does not necessarily mean fog is not present.

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