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Snow and ice play an important role in the balance of energy between the earth's surface and the atmosphere. Visible imagery is useful for measuring snow/ice cover, since snow and ice have a high reflectivity for sunlight and provide a good contrast in visible imagery compared with other objects on the earth's surface (except for clouds). At the MSC, the Snow-ice Index (SI) is estimated once a day from MTSAT-1R visible imagery in the area from 60° N to 20° N and from 80° E to 160° W in 0.10-degree square meshes.
Sunlight is reflects off the earth's surface and is observed by satellites
through the atmosphere. The reflected sunlight is observed as radiation
at 0.55 – 0.90µm of MTSAT-1R's visible channel. The intensity
of the observed visible imagery depends on the albedo (i.e., reflectivity of the surface), the atmospheric conditions and the
incident angle of sunlight at each grid point of the mesh.
In order to obtain a precise albedo value, the radiation observed by satellites should be corrected with the solar zenith angle (SZA) and atmospheric efficiency data. At the MSC, the SI for MTSAT-1R is defined as
SI=As - Amin
where As is the albedo corrected by SZA at the targeted location (i.e., divided
by the cosine of SZA), and Amin is the clear-sky albedo corrected by SZA. Amin is prepared in advance from visible images for previous years. As is derived from visible images during the daytime at 00, 03, 06, 09 and
The intensity of visible images covered by clouds is higher than that of clear-sky images. In order to reduce the influence of clouds and the atmospheric efficiencies, two visible images with smaller SZAs are selected from the five images of each day for the past fifteen days – thirty images for fifteen days. As is then given by the minimum albedo of the thirty images for each grid point.
If the influence of clouds is reduced enough and the earth's surface is covered with snow/ice, As should be greater than zero, except for some places where the earth's surface is covered by snow/ice through the year.