Inter-calibration of VIRR/FY-3A/B split-window channels with AIRS/Aqua and IASI/Metop-A measurements

This work addressed the inter-calibration of the two split-window channels 4 and 5 (IR4, around 10.8 mu m; IR5, around 12.0 mu m) of Visible and Infrared Radiometer (VIRR) aboard FengYun 3 (FY-3) A and B satellites against the hyperspectral channels of Atmospheric Infrared Sounder (AIRS) on Aqua and Infrared Atmospheric Sounding Interferometer (IASI) on Metop-A over the Arctic area (180 degrees W-180 degrees E, 60 degrees N-90 degrees N) using the hyperspectral convolution (HSC) method in June and December of 2010-2013. First, the satellite data were re-sampled into the gridded study area using the area-weighted pixel-aggregation method. Then, besides the collocation condition, the measurements acquired by different instruments were matched with the following criteria: 1) the absolute observation time difference is less than 20 min (|.time|< 20 '), and 2) vertical bar cos theta(1) / cos theta(2) -1 vertical bar< 0.015, where theta(1) and theta(2) are the at-surface view zenith angles (VZAs) of the VIRR/ FY-3 measurement and the hyperspectral measurement, respectively. Finally, linear regression analyses on the matching measurements were carried out to obtain the inter-calibration coefficients, and re-calibrated VIRR/FY-3A/B measurements were achieved. The results showed that the calibrations of AIRS/Aqua and IASI/Metop-A channels were quiet consistent, whereas, against the hyperspectral channels, the calibrations of VIRR/FY-3A/B channels 4 and 5 were biased, and they presented certain seasonal and annual variations, especially for VIRR/FY-3A channel 4. In contrast to VIRR/FY-3A channels, the calibration discrepancies of VIRR/FY-3B channels 4 and 5 were smaller. Moreover, no obvious impact of solar illumination on VIRR/FY-3A/B measurements was observed.