MODIS and VIIRS Products for Fog Detection in the TN Valley

I didn’t have a chance to make this post last week when the imagery were more time-relevant.  Nevertheless, I wanted to point out another example of the usefulness of MODIS and VIIRS imagery over current GOES imagery and show the usefulness of exciting products and imagery to come!  First, let’s take a look at the color-enhanced GOES-IR image below from the morning (0715 UTC) of June 20th.

Color-enhanced GOES-IR (11um) image valid 0715 UTC 20 June 2014

Image 1.  Color-enhanced GOES-IR (11 µm) image valid 0715 UTC 20 June 2014


I’ve placed the yellow circles in the image for a reason, which you’ll see below.  Further down, I’m going to show areas of fog displayed in the MODIS and VIIRS imagery, and granted, this is not the standard GOES channel difference (11-3.9 µm) typically used for making fog assessments.   However, this post is meant to show current (MODIS / VIIRS) and future capabilities (GOES-R / JPSS) that will make fog detection and cloud differentiation much more easy for the operational forecaster.  So, in the image above, fog is nearly unidentifiable as it was in the 11-3.9 µm channel difference image that morning (not shown).  Mainly high cirrus clouds can be observed scattered across the region.  Now, let’s take a look at the MODIS “fog” product, or channel difference (11-3.9 um) product valid at about the same time (Image 2).

Color-enhanced MODIS 11-3.9 u m product valid 0718 UTC 20 June 2014

Image 2.  Color-enhanced MODIS 11-3.9 µm image valid 0718 UTC 20 June 2014

Notice that in the same areas we can now begin to see low clouds (indicated by yellow colors) scattered around the valleys of the southern Appalachian region.  While the GOES-East imager is capable of detecting larger scale fog often in the valleys in the eastern circle, fog in the valleys in the western circle present challenges for the current GOES-East instrument, and is often not shown very well (even in the standard 11-3.9 µm channel difference).    Next, let’s take a look at a VIIRS Nighttime Microphysics RGB valid at about the same time.

VIIRS Nighttime Microphysics RGB valid 0723 UTC 20 June 2014

Image 3.  VIIRS Nighttime Microphysics RGB valid 0723 UTC 20 June 2014

In the RGB imagery it is much easier to detect the extent of the fog, making the operational forecast process much more effective.  Notice also that it is possible to see the fog through the higher clouds around the TN/GA/NC border region.  Not only does the resolution of the VIIRS and MODIS instruments allow for superior fog detection, but the RGBs in particular offer tremendous operational advantages.  As a user of RGBs for about 2 years now, I am convinced that this type of imagery has a relevant and needed place in future operational forecasting.  Of course, it will take time for forecasters to become accustomed and adjust to the new imagery, but it will happen.


2 thoughts on “MODIS and VIIRS Products for Fog Detection in the TN Valley

  1. Thanks for sharing, Kris. I notice the fog is colored more violet than gray in your region, than it does in our area further north. This makes sense given the longwave IR contribution in the blue channel, and that the temperature of fog in your region is warmer than ours in Montana (in general). This doesn’t reduce the utility of the product, but does introduce some regional (and seasonal) variation that forecasters would eventually learn to interpret.

    Paul Nutter (GTF)

  2. Hi Paul, and thanks so much for the comment. Yes, the IR long wave contribution will often be higher here, thus providing more blue component per the RGB recipe. So, there certainly will be regional differences in the appearance of fog between the Tennessee Valley and western Montana, for example. There is a seasonal component to this as well and the differences between regions will often be most extreme then. Thanks for bringing up this issue!

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