The evaluation of FY4A's Geostationary Interferometric Infrared Sounder (GIIRS) long‐wave temperature sounding channels using the GRAPES global 4D‐Var. (29th January 2020)

• Record Type: Journal Article
• Authors: Yin, Ruoying; Han, Wei; Gao, Zhiqiu; Di, Di
• Is Part Of: Quarterly journal of the Royal Meteorological Society. Volume 146:Number 728(2020)
• Page Start: 1459
• Page End: 1476
• Publication Date: 2020-01-29
• Subjects: 4D‐Var -- bias characteristics -- bias correction -- Fengyun‐4A -- Geostationary Interferometric Infrared Sounder; Meteorology -- Periodicals; 551.5
• DOI: 10.1002/qj.3746
• Languages: English
• ISSNs: 0035-9009

Abstract

Abstract: The theory of classical variational assimilation assumes that biases are unbiased Gaussian. This article investigates the bias characteristic estimate and the bias correction of Geostationary Interferometric Infrared Sounder (GIIRS) on board the FY‐4A satellite. Quality‐control procedures for GIIRS long‐wave temperature channels brightness temperature include cloud detection based on the Advanced Geosynchronous Radiation Imager (AGRI) and outlier removal. The mean biases for most channels are within ±2 K after quality control except for the contaminated channels. Statistical evaluation of the first‐guess departures of GIIRS observations in GRAPES (Global/Regional Assimilation and Prediction Enhanced System) global 4D‐Var reveal that biases for the long‐wave temperature channels depend on fields of view (FOVs) and latitudinal distribution. Additionally, the diurnal variation of biases is obvious only for the upper tropospheric channels, and the biases for high tropospheric channels are smaller than the biases for low tropospheric channels. Finally, off‐line bias correction that was used in this study accounts for the field‐of‐regard (FOR) dependence and the diurnal variation bias characteristics of GIIRS. After bias correction, the results show that biases of long‐wave temperature sounding channels are reduced to ±0.02 K, and the standard deviations are less than 1 K except for the contaminated channels. The probability density function of the differences between observations and simulations for some common assimilation channels is closer to the unbiased Gaussian distribution. Abstract : Brightness temperatures (black line) and peaks of weighting function (blue line) for (a) 1, 650 GIIRS channels and (b) 120 GIIRS channels calculated by RTTOV for a typical midlatitude summer atmosphere profile. The three red lines in (b) represent the spectral location of channels 6, 27 and 87, respectively. Key findings FY4A' s Geostationary Interferometric Infrared Sounder (GIIRS) is the first hyperspectral infrared sounder on board a geostationary weather satellite. Statistical evaluation of the departures of GIIRS observations from GRAPES (Global/Regional Assimilation and Prediction Enhanced System) global 4D‐Var shows that the mean biases for most channels are within ±2 K after quality control and within ±0.02 K after bias correction except for the contaminated channels. The biases and standard deviations of GIIRS for long‐wave sounding channels have fields‐of‐view (FOVs) dependencies that are smaller near the centre of the observation array and become larger to the north/south ends with maximum values in the 32nd and 96th FOVs. The latitudinal dependences of mean biases and standard deviations are obvious due to the FOVs array observation mode, and the area interval latitude boundaries are 13°N, 20°N, 27.5°N, 35°N, 45°N and 60°N, respectively.