Snow in the Northeast

March 17th, 2010 |

A series of snowstorms this winter, sometimes chronicled here in the CIMSS blog, have left a hefty snowpack over the Mountains of New England and New York. The series of storms has also meant abundant cloudiness, but on March 16th, clear skies prevailed as the Aqua satellite, with a MODIS instrument, moved overhead shortly after noon on an ascending pass. The snow-capped peaks of the Catskills and Adirondacks in New York, and the Berkshires in Massachusetts, and the Green and White Mountains in Vermont and New Hampshire, respectively, are plainly evident in this 1/2-kilometer resolution image.

MODIS detects reflected radiation at a series of wavelengths in the visible part of the electromagnetic spectrum: Band 1 is at 646 nanometers (or 0.646 microns; this is very close to the visible channel on the GOES Imager), Band 2 is at 857 nanometers, Band 3 is at 466 nanometers and Band 4 is at 554 nanometers). An animation of the four channels, below, shows differences in surface detection with the four channels. For example, the reflected radiance in Band 2 is less than in Band 3 over water because water reflects blue light more readily than longer wavelength light.

Each of the single wavelength images in the loop above is presented as a greyscale, with darker values where there are smaller quantities of reflected radiance (that is, where the albedo is smaller). Because the wavelengths are within the visible part of the electromagnetic spectrum, Bands 1 (“Red”), 4 (“Green”) and 3 (“Blue”) can be combined to yield the “true color” image at the top of this post. This type of image combination is done routinely with MODIS imagery at sites like WisconsinView, as shown in this blog post, for example.

The ABI instrument will include sensors at detectors for radiation at 470, 640 and 865 nanometers, but detection of radiation with a wavelength of 550 nanometers will not occur. So-called “False Color” imagery can be derived from the three channels; however, because the “green” and “red” channels are shifted to longer (redder) wavelengths, the derived image has a reddish tinge.

(Added: the MODIS Today website also includes True-Color imagery for each MODIS overpass! Here is the pass from March 16 at 1806 UTC)

One Response to “Snow in the Northeast”

  1. scott.bachmeier says:

    Another source of MODIS true color and false color RGB images is the SSEC MODIS Direct Broadcast site, where you can choose your satellite of interest (Terra, or Aqua):

    http://eosweb.ssec.wisc.edu/cgi-bin/eosdb.cgi

    For example, here is a comparison of the 500-meter resolution Aqua true color and false color RGB images from the 18:06 to 18:17 UTC overpass on 16 March 2010:

    http://cimss.ssec.wisc.edu/goes/blog/wp-content/uploads/2010/03/100316_modis_truecolor_falsecolor_anim.gif

    Snow on the ground (as well as clouds composed of ice crystals) appear as shades of red on the false color RGB image, while supercooled water droplet clouds show up as the brighter white to cyan colored cloud features.

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