1. Viju John, Rob Roebeling, Tim Hewison
Spectral band adjustment factors (SBAF) for recalibration for IR channels
Viju John, Rob Roebeling, Tim Hewison
xxxxxxxxx, 08 September 2014

2. Objectives
To develop SBAFs for re/inter-calibration of METEOSAT IR and WV channel radiances;
Ultimate aim is to produce a fundamental climate data record of these channels from 1982 to present for climate monitoring;
Before hyper-spectral era, we have to rely on broadband infrared radiometers such as HIRS as reference instruments;
We also need SBAFs to homogenise the METEOSAT time series (to remove inter-satellite biases arising from known spectral band differences);
We use IASI data to estimate SBAF and their uncertainties;
We also use IASI data to estimate uncertainties arising from gaps in AIRS spectra;

3. Meteosat and HIRS Response Spectral Functions

4. Choosing Inter-Calibration References
Relationship of HIRS/2 like radiances from IASI and AIRS.
AIRS and IASI spectra with HIRS SRF in blue.
AIRS gaps introduces negligible errors

5. Spectral Band Adjustment Factors
-0.183;
0.024
-0.088;
0.064
More than IASI spectra are used to obtain radiances
These spectra were taken from randomly selected one overpasses – one for each month of 2013
0.0483; 1.029
0.0337
-0.149;
0.0229
Numbers in each subpanel:
First line – SBAFs
Second line – Uncertainty (mW/m2/Sr/cm-1)
IASI M_7 WV - M7 MV radiances produced by convolving IASI spectra with M7 WV SRF
IASI N_14 H_12 - NOAA-14 HIRS Channel12 radiances produced by convolving IASI spectra with NOAA-14 HIRS Channel12 SRF
AIRS M_7 MV - M7 MV radiances produced by convolving IASI spectra sub-sampled to AIRS resolution/grid with M7 WV SRF
AIRS N_14 H_12 - NOAA-14 HIRS Channel12 radiances produced by convolving IASI spectra sub-sampled to AIRS resolution with NOAA-14 HIRS Channel12 SRF

6. Future work
We will test the robustness of these results by using another set of IASI spectra (e.g., difference year);
We will extend this work for other IR sensors: GOES, Kalpana, MTSAT, ...;
Write up the results.

7. Thank You
Any Questions?

8. Choosing an inter-calibration reference
We planned to use HIRS/2, HIRS/3, HIRS/4 and IASI to inter-calibrate the MFG and MSG heritage channels (WV and IR)
HIRS/3 and HIRS/4 are not suited as reference. Due to shift and narrowing of spectral response function from HIRS/2 to HIRS/3 and HIRS/4 the Spectral Band Adjustment Factor (SBAF) uncertainty of the WV channel increased from ~0.2 K to ~0.7 K.
To replace HIRS/3 and HIRS/4 we intend to use observations from the NASA hyper spectral instrument: AIRS
We have also tested combination of HIRS channels as reference, but found that more uncertainty is introduced die to this.
HIRS/2
HIRS/2
LARGER!
HIRS/3
The figure shows match–up data for Metosat-7 WV channel with HIRS-2 and HIRS-3 over 0° coverage for January 2003.
HIRS/3

9. Testing HIRS-2 radiances simulated from AIRS
We collocated AIRS with MET7 for January 2003 and converted AIRS spectra to HIRS12 radiances;
RMSD for this is similar to match-up data for HIRS 12 on NOAA14;
This confirms the use of AIRS as a reference instruments for MFG/MSG despite gaps in HIRS12 band;
Comparable to Met7-HIRS/2
HIRS/2
The figure shows match–up data for Metosat-7 WV channel with HIRS-2 radiances simulated from AIRS over 0° coverage for January 2003.
Availability of reference instruments
HIRS/2
AIRS
IASI
HIRS/3