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Infrared

Infrared Band Differencing:6.5-10.7 micron
Infrared Band Differencing:13.3-10.7 micron

Infrared Band 10.7 micron
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Channel 2 on the current GOES satellites (GOES-8 through 12) senses radiation over a spectral range that is centered at about 3.9 microns which contains both reflected solar energy and emitted terrestrial energy. Since this wavelength contains reflected sloar energy, it cannot be directly related to cloud-top temperature. Therefore, this channel is not very useful if used alone. However, when used in combination with the atmospheric window channel (10.7 micron) it can provide a great deal of useful information. Click here for more information on the 3.9 micron band.

The subtraction of the 10.7 micron brightness temeprature (TB) from the 3.9 micron TB is useful for determining whether a cloud top is composed of liquid water or ice. This product is commonly known as the "fog product" for its ability to detect low liquid water clouds (i.e. fog and stratus), especially at night. A scientific description of this product is provided in the following paragraph.

The imaginary index of refraction for both ice and water is higher at 10.7 microns than at 3.9 microns. However, the difference in this index is greater for ice clouds. Hence, ice clouds will have larger 3.9-10.7 difference values than water clouds. Negative values of this difference are indicitive of clear sky, due to the fact that the earth's surface emits more at 10.7 microns than 3.9 (the values for clear sky will vary depending on the surface type). Values may be close to 0 for water clouds (blue). The magnitude of the difference for ice clouds in this image may also be dependent on the number of particles in the cloud and the effective radius of the particle size distribution. In this image, the areas with the largest differences (red or orange) likely contain a higher concentration of ice particles, with a smaller effective radius in the size distribution. Since the 3.9 micron channel contains a reflected solar energy component, the values of this band subtraction will be different between the day and night. Nevertheless, the greatest temperature difference between these two bands will still represent the presence of high ice clouds. An example of a nocturnal difference field over the Southeast US is provided in the "Advanced Satellite Product Description" section.

This difference is not very useful for nocturnal analysis of convective because of noise in the 3.9 micron channel at low TB's. Monitoring this difference field over time can be quite useful in assessing convective initiation. The transition from a cloud top dominated by liquid water to an ice cloud often signals the onset of precipitation. Therefore, by looking at temporal trends in this band difference, one can identify vertically growing and potentially glaciating cumulus clouds.

As stated above, values of this band difference vary throughout the day because radiation detected in the 3.9 micron channel contains a reflected solar radiation component. Ice clouds with have a higher difference value during the afternoon hours than at night. Although this technique is very useful for characterizing cloud top microphysics, we do not incorporate this technique into our CI assessment algorithm due to the difficulty in accounting for variations in this technique throughout the day. (back to top-->)

Infrared Band Differencing:6.5-10.7 micron
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The 6.5-10.7 µm differencing technique is useful for determining the cloud-top height relative to the tropopause, or to very dry mid- and upper-tropospheric air. Positive values of the water vapor-IR window temperature difference have been shown to correspond with convective cloud tops that are at or above the tropopause (i.e. overshooting tops), or growing into dry upper tropospheric air where the water vapor band "saturates" near the same altitude of the 10.7 µm temperature, in AVHRR and HIRS-2 (Ackerman 1996) and in METEOSAT-7 (Schmetz et al. 1997) data. In clear sky situations, radiation at 6.5 µm is emitted by atmospheric water vapor between approximately 20 and 50 kPa (Soden and Bretherton 1993). The radiation emitted at 6.5 and 6.7 µm by the surface or low clouds is absorbed by atmospheric water vapor in the lower troposphere and is not detected by satellites. On the other hand, absorption by atmospheric gases at 10.7 µm is weak, and therefore, detected radiation at 10.7 µm originates mainly from the surface. Because the surface is normally warmer than the upper troposphere, the difference between the 6.5 and 10.7 µm TB is usually negative. In regions of intense convective updrafts, with cloud tops possibly extending into the lower stratosphere, the 10.7 µm TB is colder than that at 6.5 µm, resulting in a positive TB difference between these bands. For the assessment of pre-CI signatures, convective clouds with positive differences have likely already begun to precipitate, especially in tropical atmospheres that support warm-top convection. Therefore, clouds with moderately negative difference values (-35 to -10 K) represent a useful CI interest field and imply the presence of low to mid-level cloud tops (~85-50 kPa).
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Infrared Band Differencing:13.3-10.7 micron
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The 13.3-10.7 µm differencing technique is another measure used to characterize and delineate cumulus clouds in a pre-CI state from mature precipitating cumulus and cirrus in GOES-12 imagery. Limited documentation of this band difference method is available (Hilger and Clark 2002), and therefore this study may be the first organized use of this technique for convective cloud studies (T. Schmit, NOAA, personal communications). The subtraction of the 10.7-µm from the 13.3 um channel yields similar results to the 6.5-10.7 µm technique for mature cumulus clouds, but is much different for immature cumulus. As noted above, the 6.5 µm channel detects emitted radiation mainly from the upper troposphere. Therefore, large 6.5-10.7 µm differences exist for immature cumulus clouds because the 10.7 µm TB is much warmer than that sensed at 6.5 µm. In contrast, the 13.3 µm channel detects radiation from the middle and lower troposphere. As a result, the 13.3-10.7 µm difference values are much less negative because the 13.3 µm TB is warmer than that of the 6.5 µm channel. For opaque cumulonimbus cloud tops, this band difference yields values near zero because the 13.3 and 10.7 µm bands detect equivalent TB's. The 13.3-µm channel is more sensitive to thin cirrus (i.e. colder TB than the 10.7 µm channel), resulting in slightly negative 13.3-10.7 µm differences. The 13.3-10.7 µm difference would therefore appear to be a better indicator of low cloud development (deepening) as compared to the 6.5/6.7-10.7 µm technique.Cumulus clouds in a pre-CI state exhibit difference values from -25 to -5 K for several convective storm events examined in formulating CI interest field criteria. This range of difference values will be used as a CI interest field within the CI nowcast algorithm. Click here for more information on the 13.3 micron band.
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