Cloud Over Snow

Table describing some of the most widely used RGB products, with a sample image for the cloud over snow RGB

Description:

This RGB, the Cloud / Snow Discriminator product, distinguishes clouds from snow cover, something that is hard to do in most visible images. The product is useful mostly in winter and over mountain ranges.

The current NOAA and Metop AVHRR, Terra and Aqua MODIS, Suomi NPP VIIRS, FY-3 MERSI, and MSG SEVIRI imagers have the needed spectral channels for producing the Cloud / Snow Discriminator product.  The future GOES-R ABI, MTG FCI, and FY-4 AGRI imagers will also have the required visible and near-infrared channels to produce a similar product with the benefit of routine 5 to 15 minute image updates for the full Earth disk.

Coverage: Daytime only, requires solar reflectance information

Channels:

  • Polar-orbiting satellites:
    • NOAA and Metop AVHRR, FY-3 MERSI:
      0.63 µm Vis; 0.865 µm NIR; 1.61 µm SWIR
    • Terra and Aqua MODIS:
      0.645 µm Vis; 0.86 µm NIR; 1.64 µm SWIR (Terra); 2.13 µm SWIR (Aqua)
    • Suomi NPP and future JPSS VIIRS
      0.64 µm Vis; 0.865 µm NIR; 1.61 µm SWIR
  • Geostationary satellites:
    • MSG SEVIRI and future MTG FCI:
      0.64 µm Vis; 0.81 µm NIR; 1.6 µm SWIR
    • Future GOES-R ABI:
      0.64 µm Vis; 0.865 µm NIR; 1.38 µm SWIR (cirrus detection); 1.61 µm SWIR
    • Future FY-4 AGRI:
      0.65 µm Vis; 0.825 µm NIR; 1.375 µm SWIR (cirrus detection); 1.61 µm SWIR

Color scheme:

  • Snow cover is bluish white
  • Clouds are shades of yellow
  • Cloud free land is darker green

Advantages:

  • Simplifies interpretation during winter and over mountainous terrain
  • Avoids the ambiguity between snow cover and clouds seen in visible and true color images

Limitations:

  • Sometimes features do not appear in the expected colors; snow cover can look like cloud and vice versa
  • Can not discriminate between high and low clouds
  • Old snow cover can look like cloud-free land
  • Product works best for fresh snow that’s less than a few days old
  • Does poorly in detection of patchy snow cover or trace amounts

Live data links:

More information:

Reference:

Miller, S. D, T. F. Lee, and R. L. Fennimore, 2005: Satellite-Based imagery techniques for daytime cloud/snow delineation from MODIS. J. Appl. Meteor., 44, 987-997.

Example:

Terra MODIS True color RGB over the Great Lakes Region (U.S), 03 December 2007

In this true color product, we cannot differentiate between snow cover and cloud. It’s particularly difficult to distinguish lake-effect cloud from snow on the ground.

Terra MODIS Cloud Over Snow RGB over the Great Lakes Region (U.S), 03 December 2007

With bluish-white snow cover and yellow clouds, this RGB minimizes the ambiguity.


Exercise:

MODIS Visimage, 4 April 2007