Fog & Low Clouds, MSG

Example of a MSG Fog and Stratus RGB

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:

Additional information:

Example:

MSG Fog and Stratus RGB Animation Over Southern Africa, 05 July 2003, 0000 to 0600 UTC

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:

MSG Fog and stratus RGB product over South America, 0800 UTC 30 Jul 2001

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

MSG Fog and stratus RGB product over South America, 0800 UTC 30 Jul 2001, with the main features identified

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.