Snow-ice Index (SI)

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Outline

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.

Data Processing Method

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=A_s - A_min

where A_s is the albedo corrected by SZA at the targeted location (i.e., divided by the cosine of SZA), and A_min is the clear-sky albedo corrected by SZA. A_min is prepared in advance from visible images for previous years. A_s is derived from visible images during the daytime at 00, 03, 06, 09 and 21 UTC.
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. A_s 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, A_s should be greater than zero, except for some places where the earth's surface is covered by snow/ice through the year.

Remarks

• It should be noted that the GMS-5 data archive is used as a temporary substitute for A_min until more than a year's worth of MTSAT-1R data is in storage. The index value may also be larger than with clear-sky conditions if clouds remain at a location for more than 15 days.
• This product is included on the "Monthly Report of Meteorological Satellite Center (CD-ROM)".