Nighttime Fog Monitoring
Satellite fog monitoring during the nighttime can be a challenge since geostationary datasets are limited to infrared imagery. However, with the new GOES-16/17 and JPSS datasets users can employ polar-orbiting and geostationary imagery to identify and monitor areas of fog and low stratus (a.k.a liquid water clouds). As meteorologists, we know that fog can significantly reduce ‘near-surface’ visibilities affecting aviation and shipping industries along with the general public. Below is a static comparison over eastern Kansas and western Missouri highlighting the SNPP – VIIRS NCC, the GOES-16 Night Fog Difference and Nighttime Microphysics RGB products; all imagery has hourly METAR surface observations overlaid (i.e. shown in green). Note the hourly surface observations are at 0800Z, 24 April 2019, while satellite observations are at ~0747Z, 24 April 2019.
SNPP VIIRS Near-Constant Contrast (NCC) at 0747Z, 24 April 2019
NCC, a derived product of the Day/Night Band (DNB), illuminates atmospheric features and can sense emitted light sources (i.e. city lights) and reflected light sources (i.e cloud cover) during the nighttime. NCC is known as ‘nighttime visible’ imagery that appears similar to 0.64μm daytime visible imagery. In the imagery below, NCC observes emitted lights from cities and towns that reside along the interstates and in rural areas of Kansas and Missouri. Various levels of cloud cover encompass south-central and eastern Kansas along with western Missouri, where areas of fog are not conspicuous without the assistance of surface observations (i.e. fog indicated by parallel, horizontal green lines). In contrast, in northwestern Kansas, NCC observes fairly clear skies.
GOES-16 Night Fog Difference (10.3μm- 3.9μm) at 0746Z, 24 April 2019
Using an approximate time stamp, the GOES-16 Night Fog Difference is utilized. The Night Fog Difference product employs a channel difference of the 10.3μm Brightness Temperatures (BT) minus the 3.9μm BT to identify the fog and low stratus. Liquid water clouds are depicted as positive Brightness Temperature Differences (BTD) (i.e. seen in blue in the imagery below) since liquid water droplets do not emit radiation at 3.9μm but do at 10.3μm; employing the channel difference computes to a positive BTD. Conversely, ice crystals that are embedded in high clouds exhibit a negative BTD (i.e. in grey, refer to the bottom-right of the image).
Note there is an ellipse in western Kansas that observes positive BTD indicating fog. But is it really fog or low stratus we are seeing? The answer is ‘No’. Remember, the NCC product (above) observed clear skies in this area, where the surface observations validate the NCC. This is a false alarm that is produced by the Night Fog Difference product, and it is critical for users to validate this product with surface observations.
GOES-16 Nighttime Microphysics RGB at 0746Z, 24 April 2019
For further differentiation between fog and other types of clouds, look to the Nighttime Microphysics RGB (seen below) that employs the 10.3μm-3.9μm BTD and the 12.4μm-10.4μm BTD. Notice within the same ellipse in western Kansas the RGB observes clear skies (light pink) similarly to NCC and the surface observations.
