Environmental Engineering Reference
In-Depth Information
1.10
N18/M2
N6/7
N7/8
N8/9
N9/10
N10/11
N11/12
N12/14
N14/15
N15/16
N16/17
N17/18
N19/M2
1.08
1.06
1.04
1.02
1.00
0.98
0.96
0.94
0.92
0.90
1.10
1.08
1.06
1.04
1.02
1.00
0.98
0.96
N18/M2
N6/7
N7/8
N8/9
N9/10
N10/11
N11/12
N14/15
N15/16
N16/17
N17/18
N19/M2
N12/14
0.94
0.92
0.90
83
85
87
89
91
93
95
Year (YY)
97
99
01
03
05
07
09
Fig. 2.5
HIRS SNO long-term time series for selected CO2 channels (channels 4 and 6)
An example of long-term SNO time series of HIRS is provided in Fig.
2.5
in the
form of radiance ratios to show the inter-satellite biases. Among the 19 infrared
channels of HIRS, channels 4 and 6 are shown here with a time span of 29 years.
There are several observations that can be made from this figure. First, the inter-
satellite biases for channel 4 vary much greater (as much as 6%) than those for
channel 6. In fact, without comparing the time series of these two channels, the bias
variability in channel 4 would lead us to believe that this may be due to major
performance differences at the HIRS instrument level. On the other hand, analysis
of channel 6 time series suggests that the inter-satellite biases are in fact small
(mostly within 1%) for this channel during the same time period. Further investi-
gation reveals that channel 6 is situated in a relatively flat spectral region of the
atmospheric profile, while channel 4 is on a steeper gradient of this profile and is
therefore more sensitive to spectral differences between satellites for this channel.
Therefore, the large biases for channel 4 are likely due to spectral issues related to
the spectral response functions, or spectrally induced biases (Cao et al.
2008b
). This
means in order to develop consistent long-term time series from HIRS, the spectral
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