Environmental Engineering Reference
In-Depth Information
1.0
a
0.8
0.6
CH2( 3.9)
CH3( 6.7)
CH4(10.8)
CH5(12.0)
0.4
0.2
0.0
200
220
240
260
280
300
320
Skin temperature (K)
60
b
40
20
0
+ 3.9
6.7
10.8
12.0 micron
-20
0
1
2
3
4
5
6
Precipitable water (cm)
Fig. 19.1 The transmittance ( a ) and temperature deficits ( T s T b )( b ) for the four infrared
channels of the GOES 8-11
temperature. The transmittances of the 11- and 12-
m channels decrease signifi-
cantly at high temperature (285-305 K); most values are below 0.8. This is why
most existing split-window algorithms that use the 11- and 12-
μ
m channels get
larger errors at the warmer temperature range of 285-305 K. The transmittance for
MIR channel 3.9
μ
m is more stable, with less sensitivity to the surface skin
temperature most values being above 0.8. Therefore, the MIR 3.9-
μ
m channel is
a more appropriate window channel for retrieving LST than IR 11- and 12-
μ
m
channels. Moreover, temperature deficit between skin temperature T s and bright-
ness temperature T b ,( T s T b ) as shown in Fig. 19.1b , increases quickly at water
vapor channel 6.7
μ
μ
m, and it can be as large as 60 K. Temperature deficit is
relatively stable at window channels, it increases with water vapor at IR channels,
but it almost doesn't change with water vapor amount at MIR channel. Therefore, it
is best to use the MIR 3.9-
μ
m channels during nighttime, when the MIR channel does not contain solar energy
reflected by surface.
The imagers on board the GOES M (12)-Q series, including the current opera-
tional GOES-13, don't have the 12-
μ
m channel combined with the split-window 11- and 12-
m channel (Fig. 19.2 ), so it would not be
possible to use the brightness temperature difference in the 11- and 12-
μ
m channels
to correct for atmospheric effects. Attempts have been made to use ancillary data
μ
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