Description:

Made from infrared channel data, this RGB was originally designed for use with MSG SEVIRI data and tuned for monitoring the evolution of nighttime fog and low-level stratus clouds. Secondary applications include detecting fires and low-level moisture boundaries and classifying clouds in general. Since the product is tuned for nighttime conditions, its use during the day is limited.

Most polar-orbiting and geostationary environmental satellite imagers (with the exception of GOES-12 to -15) have the necessary channels to make this product. Future satellite imagers including JPSS VIIRS, GOES-R ABI, FY-3 and -4, Himawari, and MTG FCI will continue to the needed shortwave and longwave infrared channels for producing a similar nighttime fog and stratus RGB.

Coverage: Nighttime only

Channels:

• Current MSG SEVIRI, NOAA and Metop AVHRR, FY-2 and -3, MODIS
  Future GOES-R ABI, MTG FCI, FY-4 AGRI, Himawari AHI
  
  Red: 12.0 minus 10.35 to 11 µm IR BT difference
  Green: 10.35 to 11 µm IR minus 3.5 to 3.9 µm SWIR BT difference
  Blue: 10.35 to 11 µm IR

Color scheme:

• Low clouds are yellow to light green
• Thick, high clouds are red
• Thin, high clouds are dark blue to black
• Land and sea surfaces appear in various colors

Advantages:

• Fog and stratus often cannot be seen on infrared images at night because they blend in thermally with the background; this RGB enhances the fog/stratus signal
• Is very important for ground and air transportation forecasting

Limitations:

• Thin cirrus may obscure the view of fog and stratus
• May be noisy and difficult to interpret in cold temperature environments (below approximately -10ºC)
• Is difficult to detect thin radiation fog
• The actual area of fog and low cloud is always slightly larger than in the image due the 3.9 µm IR channel’s increased sensitivity to warm pixels around the edges of the cloud cover

Live data links:

• EUMETSAT Available Near-Real-Time Products, http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/
• EUMeTrain, a site providing daily and archived satellite imagery, model output, case studies, and training resources, covering Europe, the Middle East, Africa, and North Polar Regions, http://eumetrain.org/
• GOES-West / GOES-East RAMSDIS Online, http://rammb.cira.colostate.edu/ramsdis/online/goes-west_goes-east.asp

Additional information:

• EUMETSAT training materials:
  
  • MSG Fog RGB Interpretation Guide, http://oiswww.eumetsat.org/IPPS/html/bin/guides/msg_rgb_fog.ppt
  • MSG Channels Interpretation: Guide to weather, surface conditions and atmospheric constituents, http://oiswww.eumetsat.org/WEBOPS/msg_interpretation/index.php

Example:

Loop: The green areas over the southern portion of Africa are either fog or stratus. Notice how the cloud coverage increases throughout the night. When the sun rises toward the end of the loop, the low clouds turn red. That is due to the solar reflection off the water droplets in the fog and stratus as seen by the 3.9 µm shortwave infrared channel, which causes water clouds to appear similar to other features. For this reason, this particular RGB is only useful only during nighttime.

Exercise:

In this RGB over South America, which of the labeled areas are fog or low cloud? (Choose all that apply.)

a) A

b) B

c) C

d) D

e) E

f) F

g) G

The correct answers are C, D, and E.

The areas around C, D, and E are fog or low cloud since they have a light green or cyan color. The reddish cloud is cirrus, which overlies a large area of stratus within the white enclosure.