HIRS Pathfinder Intercalibration
After construction of the HIRS Pathfinder clear-sky data set, we began
the intersatellite calibration by first looking at the observed Tb
differences for all the IR channels. The table below presents time
series for each channel and various latitude bands of the monthly mean
clear-sky TB from the 27th (center) scan position.
TB Observed time series
The following tables provide values for the intersatellite TB
differences between the various satellites and channels. Tables
1 and 2 give the modeled TB difference with NOAA-10 using
MODTRAN 3.7 for simulating the HIRS TBs and using TIGR-3
tropical profiles. The figure shows the bias between all the
satellites. This figure excludes data for N15 channel 16 and
N11-N15 channel 17 data since these channels were moved to a new
central frequency. Channel 10 also had changes to the central
frequency for N11, N14 and N15 but experienced little change in
mean Tb value. Changes in the central frequency for N15 HIRS
chn 12 are shown here.
Simulated TB means Table
Simulated TB differences with N10 Table
Simulated TB differences with N10 Figure
The following figures provide values for observed intersatellite
bias from the Pathfinder clear-sky data. The observed bias was
determined from the monthly mean brightness temperature data for
each satellite. The satellite climatology helps remove natural
interannual variations from the bias calculation. However,
satellites with short lifespans, such as NOAA-8, may only have
one monthly mean data point for its climatology. For the
following figures, only scan position 27 (nadir) is averaged.
January and July climatologies are compared here. The shapes of
the bias curves are similar for most channels. Channels 1-6
show negative trends for both months with very similar bias
amplitudes for the individual satellites. Channel 7 does show
differences for N9,N10,N11 between the two months. It is unknown
why Channel 16 shows such a large bias in N9. The observed TBs
are 40K warmer, yet the central frequency for the channel is
reported in Kidwell to be the same as the other satellites.
Channels 8, 18 and 19 have a periodic curve because the daytime
observations cause the afternoon satellites to have warmer
observations. Channels 18 and 19 are larger in amplitude
because the daytime observations have enhanced TBs due to a
small contribution from the visible spectrum. Subtropical
latitude bands are shown to investigate latitudinal dependencies
of the bias. The largest difference from the 10N-10S results
is the amplitude of the warm TB signal from the afternoon passes
for the surface channels.
Observed Nadir view
10N-10S January TB differences with N10 Figure
Observed Nadir
view 10N-10S July TB differences with N10 Figure
Observed Nadir view
10N-30N January TB differences with N10 Figure
Observed Nadir view
10S-30S January TB differences with N10 Figure
So how well do the observed and simulated biases compare? The
following histograms compare the data given in the above
figures. Each histogram compares one channel for all the
satellites. Remember that the observed biases include not only
inter-satellite bias due to filter differences but also can
include natural variations seen in the atmosphere.
Observed versus Simulated intersatellite
bias histograms
A multiple linear regression was developed to predict the Tb
differences between satellites. The results below show the how
linear regression method can both improve and worsen the
intersatellite bias. Results are shown only for channel 1 to 12
since both day and night observations are included here. The
linear regression method is not valid for daytime observations
for the near IR channels (13-19).
TB Linear Regress correction time series
Given the mixed results from the linear regression method, we decided
to remove the intersatellite bias using the EDF method used in
Bates et al. (1996). Final results for the EDF adjustment and bias
correction are given below.
TB EDF correction time series
Last modified: Tue May 15 19:43:57 MDT 2001