Abstract
The Chinese Ocean Color and Temperature Scanner (COCTS) on board the Haiyang-1B (HY-1B) satellite has two thermal infrared channels (9 and 10) centred near 11 μm and 12 μm, respectively, which are intended for sea surface temperature (SST) observations. To improve the accuracy of COCTS SSTs, inter-calibration of COCTS thermal infrared radiance is carried out. The Infrared Atmospheric Sounding Interferometer (IASI) on board MetOp-A satellite is used as inter-calibration reference owing to its hyperspectral nature and high-quality measurements. The inter-calibration of HY-1B COCTS thermal infrared radiances with IASI is undertaken for data from the Period 2009–2011 located in the northwest Pacific. Collocations of COCTS radiance with IASI are identified within a temporal window of 30 min, a spatial window of 0.12° and an atmospheric path tolerance of 3%. Matched IASI spectra are convolved with the COCTS spectral response functions, while COCTS pixels within the footprint of each IASI pixel are spatially averaged, thus creating matched IASI-COCTS radiance pairs that should agree well in the absence of satellite biases. The radiances of COCTS 11 and 12 μm channel are lower than IASI with relatively large biases, and a strong dependence of difference on radiance in the case of 11 μm channel. We used linear robust regression for four different detectors of COCTS separately to obtain the inter-calibration coefficients to correct the COCTS radiance. After correction, the mean values of COCTS 11 and 12 μm channel minus IASI radiance are −0.02 mW m−2 cm sr−1 and −0.01 mW m−2 cm sr−1, respectively, with corresponding standard deviations of 0.51 mW m−2 cm sr−1 and 0.57 mW m−2 cm sr−1. Striped noise is present in COCTS original radiance imagery associated with inconsistency among the four detectors, and inter-calibration is shown to reduce, although not eliminate, the striping. The calibration accuracy of COCTS is improved after inter-calibration, which is potentially useful for improving COCTS SST accuracy in the future.
Highlights
Satellite measurements are critical for quantifying Earth’s environmental parameters, monitoring climate change and informing numerical weather prediction
Infrared Atmospheric Sounding Interferometer (IASI) data have been used in other inter-calibration studies of the sensors on board the polar-orbital satellites such as Advanced Very High Resolution Radiometer (AVHRR) [15,16], Advanced Along-Track Scanning Radiometer (AATSR) [17] and Infrared Atmospheric Sounder (IRAS) [18]
We have provided the same statistics in brightness temperature (BT) space for ease of comprehension
Summary
Satellite measurements are critical for quantifying Earth’s environmental parameters, monitoring climate change and informing numerical weather prediction. Wang et al (2011) compared the radiance from IASI and Atmospheric Infrared Sounder (AIRS) on board Aqua using double-difference and simultaneous-nadir overpass methods. IASI data have been used in other inter-calibration studies of the sensors on board the polar-orbital satellites such as Advanced Very High Resolution Radiometer (AVHRR) [15,16], Advanced Along-Track Scanning Radiometer (AATSR) [17] and Infrared Atmospheric Sounder (IRAS) [18]. Due to the effect of atmospheric path on radiance measurements, the satellite zenith angle of COCTS and IASI should be matched when generating the matchup datasets. We set the threshold of difference in the secant of the satellite zenith angles as 0.03, constraining the difference of COCTS and IASI atmospheric paths to be less than 3% to ensure the view conditions are close. Considering the inconsistency between COCTS four parallel detectors, COCTS pixels inside each IASI IFOV measured by per detector are averaged separately later, to estimate the difference between COCTS detectors and remove the striped noise
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