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GSICS Himawari-8/AHI infrared inter-calibration guide


Inter-calibration between Himawari-8/AHI infrared bands and high-spectral-resolution sounders

The Meteorological Satellite Center (MSC) examines way of improving inter-calibration between Himawari-8/AHI (referred to here as AHI) infrared bands and high-spectral-resolution sounders (hyper sounders). Data from the three hyper sounders detailed below are used for this work.

Inter-calibration is conducted once a day.The hyper-sounder data used in this work are collected via the Internet. Inter-calibration computation may be canceled if network conditions are poor.

As of July 2015, AHI GSICS Corrections are under development. The products will be reviewed within GSICS to enter Demonstration-phase.

Outcome

The results of this inter-calibration work have three statistical parameters. It should be noted that the results contain a certain level of uncertainty caused due to variations in instrument accuracy, differences in observation conditions and spectral compensation residuals.

Coefficients of regression between the radiance of hyper sounders and AHI

Linear regression coefficients (C0 and C1) and their standard uncertainties are computed to allow association of Himawari Standard Data (HSD) radiance with hyper sounder radiance. The radiance is in wavenumber space, and its unit is mW.m-2.sr-1.cm.

Radiance (AHI) = C0 + C1 × Radiance (hyper sounder)

GSICS Correction

GSICS Correction is the initial core product of GSICS. It is a dataset that allows users to determine corrected satellite radiances based on the results of inter-calibration, and consists of the above linear regression coefficients (C0 and C1). Corrected satellite radiances are calculated using the following equation:

Corrected radiance (AHI) = HSD radiance (AHI) / C1 − C0 / C1

GSICS Correction is computed for every day. To reduce the random component of uncertainty, correction is derived from data over 29- and 15-day time periods for Re-Analysis Correction (RAC) and Near Real Time Correction (NRTC), respectively. The smoothing period for RAC is t - 14 days to t + 14 days, and that for NRTC is t - 14 days to t + 0 days (where t is the date of validity).

TB difference between hyper sounders and AHI

The brightness temperature (TB) difference (AHI value minus hyper sounder value) and its standard uncertainties associated with AHI and hyper sounder radiance are computed at reference temperatures of standard radiance, 290 K, 250 K and 220 K. A standard radiance from GSICS is defined to allow comparison and convenient expression of instrument inter-calibration bias in units that are understandable to users.

The standard radiance of AHI was calculated for each channel by RTTOV-11.2 in a 1976 US Standard Atmosphere at nadir, at night, in clear sky, and over the sea with an SST of 288.15K and a wind speed of 7m/s.

d Tb = Tb (AHI) − Tb (hyper sounder)

AHI band
(µm)
Band7
(3.9)
Band8
(6.2)
Band9
(6.9)
Band10
(7.3)
Band11
(8.6)
Band12
(9.6)
Band13
(10.4)
Band14
(11.2)
Band15
(12.4)
Band16
(13.3)
Standard
radiance [K]
285.95234.65243.85254.59283.82259.45286.18286.10283.78269.73

Conversion between brightness temperature and radiance

The Planck function and sensor spectral response functions are used to compute brightness temperature [K] from radiance [mW.m-2.sr-1.cm] and vice-versa. In general, approximation equations called sensor Planck functions, which are generated for AHI infrared bands, are used to facilitate computation.

Brightness temperature to radiance Radiance to brightness temperature
sensor Planck equation sensor Planck equation

AHI band Wavenumber Band correction coefficients
ν (cm-1) a1 a2 b1 b2 b3
Band 7
(3.9 µm)
2575.7670.4646738020.999341618-0.4797571.000766-1.860569e-07
Band 8
(6.2 µm)
1609.2411.6468447990.996401237-1.6626161.003694-1.732716e-07
Band 9
(6.9 µm)
1442.0790.308135370.999259063-0.33570361.000974-4.847962e-07
Band 10
(7.3 µm)
1361.3870.0573694680.999854346-0.063060131.000195-1.069833e-07
Band 11
(8.6 µm)
1164.4430.1351275410.999615566-0.16051051.000589-4.019762e-07
Band 12
(9.6 µm)
1038.1080.0936304240.999703302-0.11435071.000473-3.67168e-07
Band 13
(10.4 µm)
961.3330.0896549150.999700114-0.11921151.000539-4.680314e-07
Band 14
(11.2 µm)
890.7410.1800931310.999356159-0.25304231.001233-1.153788e-06
Band 15
(12.4 µm)
809.2420.2439071940.999046134-0.37664591.002025-2.096994e-06
Band 16
(13.3 µm)
753.3690.0623563540.999737103-0.097731971.000564-6.266746e-07

Algorithm

To allow comparison of data between AHI and hyper sounders, information from simultaneous observations is first collocated. Radiances observed via hyper sounder channels are then accumulated in line with the spectral responses of the AHI infrared bands to enable estimation of AHI radiances.

The footprint size of the hyper sounders is about 12 km at nadir, whereas that of the AHI infrared bands is 2 km. To offset this difference, sounder radiance is compared with an average value for AHI radiances over a box of 6 × 6 pixels (FOV_BOX) corresponding to the sounder footprint.

The collocation and spectral compensation methods are briefly described below. Further details are provided in Tahara and Kato (2009).

Collocation method

The collocation algorithms used in inter-calibration are determined by the GSICS Research Working Group (Hewison et al., 2013). AHI and hyper sounder data meeting the criteria outlined below are selected.

  • Observation time difference check

    | tahi − tsounder | < dtmax

  • Satellite zenith angle difference check

    | cos( SZAsounder ) / cos( SZAahi ) − 1 | < MaxRate_OptPathDiff

  • Environment uniformity check
    To mitigate discrepancies between the observation conditions of the two satellites caused by variables such as time difference, optical path difference and navigation error, only measurements taken over uniform scenes are selected and compared. In this checking, the uniformity of AHI radiance data over a box of 18 × 18 pixels (ENV_BOX) is tested using

    STDV(AHI radiances in ENV_BOX) < MaxSTDV

  • Normality check
    Hyper sounder radiance is compared to average AHI radiance over the corresponding FOV_BOX. The AHI radiance data in the FOV_BOX should therefore represent those in the ENV_BOX as evaluated by the environment uniformity check. The normality of the AHI radiance data in the FOV_BOX is check using

    | MEAN(FOV_BOX) − MEAN(ENV_BOX) | × 18 / STDV(ENV_BOX) < Gaussian .

The table below shows the criteria used in inter-calibration between AHI and AIRS/IASI/CrIS. The values differ according to weather conditions. In this work, if the brightness temperature of Band13 (10.4 μm) exceeds 275 K, the scene condition is categorized as clear, otherwise, it is categorized as cloudy.

AHI bandConditiondtmax
(minutes)
MaxRate
OptPathDiff
MaxSTDV
(mW.m-2.sr-1.cm)
Gaussian
Band07
(3.9 µm)
Clear50.010.02383
Cloudy50.030.0476
Band08
(6.2 µm)
All50.010.3712
Band09
(6.9 µm)
All50.010.5612
Band10
(7.3 µm)
Clear50.010.6612
Band11
(8.6 µm)
Clear50.011.183
Cloudy50.032.36
Band12
(9.6 µm)
Clear50.011.463
Cloudy50.032.92
Band13
(10.4 µm)
Clear50.011.623
Cloudy50.033.24
Band14
(11.2 µm)
Clear50.011.773
Cloudy50.033.54
Band15
(12.4 µm)
Clear50.011.913
Cloudy50.033.82
Band16
(13.3 µm)
Clear50.012.033
Cloudy50.034.06

Spectral response compensation method

The collocated AHI and hyper sounder data cannot be compared without consideration of the sensors' spectral response difference. In this work, the average AHI radiance over the FOV_BOX is compared with the super channel radiance Isuper, which is generated from an accumulation of the hyper sounder radiances Ijsounder.

Iisuper   =   Σj  wij  Ijsounder,

where i is the AHI band number and j is the hyper sounder channel number.

wij represents constants computed in advance to precisely fit the spectral response of the super channel to that of the AHI infrared band i.

If the spectral range of the hyper sounder does not fully cover the AHI spectral range and/or any hyper sounder channel measurements are missing, spectral compensation is applied to the super channel radiance. This is done by evaluating the valid radiance measurements of the hyper sounder and radiances simulated beforehand for eight model profiles including clear and cloudy conditions. For further information on the super channel and the spectral compensation methods, refer to Tahara (2008) and Tahara and Kato (2009).


References

  • Hewison, T. J., X. Wu, F. Yu, Y. Tahara, X. Hu, D. Kim and M. Koenig, 2013: GSICS Inter-Calibration of Infrared Channels of Geostationary Imagers Using Metop/IASI, IEEE Transactions on Geoscience and Remote Sensing, Vol. 51, No. 3, 1160-1170.
  • Tahara, Y., 2008: New Approach to Inter-calibration Using High Spectral Resolution Sounder, Meteorological Satellite Center Technical Note, No. 50, 1-14. (PDF, 746 KB)
  • Tahara, Y. and K. Kato, 2009: New Spectral Compensation Method for Inter-calibration Using High Spectral Resolution Sounder, Meteorological Satellite Center Technical Note, No. 52, 1-37.(PDF, 5.91 MB)
  • WMO, 2006: Implementation Plan for a Global Space-Based Inter-Calibration System (GSICS), version 1, April 2006. World Meteorological Organization.