Preliminary assessment of the Advanced Himawari Imager (AHI) measurement onboard Himawari-8 geostationary satellite
The Japan Himawari-8 geostationary satellite was successfully launched into the geosynchronous orbit around 140°E on 17 October 2014. The Advanced Himawari Imager (AHI) onboard the Himawari-8 has its channels 7–16 covering from the short- to the thermal-infrared bands, of which observations can be assimilated into a data assimilation system to improve the atmospheric analysis. Before conducting any AHI data assimilation experiments, it is the first and critical step to correctly quantify AHI bias and error variance, since these two variables are required in a data assimilation system. This study investigates the bias and the error variance of the AHI infrared measurements under clear-sky conditions by using a numerical weather prediction model and a radiative transfer model. Overall, AHI observations agree favourably with the model simulations. It is noted that channels 7–14 has a cold bias of approximately 1.0 K while the cold bias reaches around 2.0 ~ 3.0 K for the longwave Channel 15 and Channel 16. The sums of the observation and model error are about 1.5 K for three water vapour channels 8, 9, 10, and approximately 1.0 K for the remaining channels. The dependence of the bias on the scene temperature and scan angle is further investigated. It shows that a warm bias is negatively correlated with the scene temperature when the scene temperature is low for those surface-sensitive channels. Besides, linear regression tests between the bias and the sensor zenith angle shows that the fitting coefficient is less than 0.015 K/degree for all ten channels, indicating no obvious scan-angle-dependent bias.