Bias and observation error characterization of the Fengyun-2F Stretched Visible and Infrared Spin Scan Radiometer with the aim of its radiance assimilation
Recent studies start to explore the potential benefits of directly assimilating the radiance from the infrared imagers onboard geostationary satellites to the short-range numerical weather prediction both on the synoptic- and meso-scales, where accurate quantification of the bias and observation error arising from the numerical model and the instrument is a key component in such experiments. This paper investigates both the bias and the observation errors of the Stretched Visible and Infrared Spin Scan Radiometer (S-VISSR) onboard Chinese operational geostationary satellite Fengyun-2F, as the first step towards its clear-sky radiance assimilation. In this study, 774,530 observations that passed a five-step quality control procedure during the period of 1–7 July in 2014 are collected to investigate the mentioned two quantities while the synthetic simulated clear radiance is calculated through a numerical weather prediction model and a radiative transfer model. Persistent warm biases are found for FY-2 F S-VISSR channel 3, 4 and 5 during day and night, and they are possibly due to the uncertainty from the instrument calibration process. The observation errors of S-VISSR infrared channels vary with the surface types, generally with larger values over land than over ocean, which could partially be explained by the variation of the land types over land. Besides, whether these biases change with viewing geometry or evolve with time is also examined in our study. Biases from the three surface channels exhibit complex dependence on the scan position and evolve with time, which encourages incorporating both the scan position and temporal corrections into the S-VISSR bias correction scheme in the data assimilation system.