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FY-4 series comprises China's second-generation geostationary meteorological satellites. As the first flight unit of the FY-4 series, FY-4A was successfully launched into space on 11 December 2016, carrying the Advanced Geosynchronous Radiation Imager (AGRI), Geostationary Interferometric Infrared Sounder (GIIRS), and the Lightning Mapping Imager (LMI) (Yang et al., 2017). FY-4A’s GIIRS is the first high-spectral-resolution advanced IR sounder onboard a geostationary weather satellite, complementing the advanced IR sounders in polar orbit and providing nearly continuous temporal, horizontal, and vertical observations.
The GIIRS is a Michelson Fourier transform infrared interferometer that measures atmospheric infrared radiation, covering the range of the long-wavelength IR (LWIR) band (700−1130 cm−1) and the mid-wavelength IR (MWIR) band (1650−2250 cm−1) at a spectral resolution of 0.625 cm−1. It has 1650 spectral channels, of which 689 channels are for the LWIR band and 961 channels are for the MWIR band. As FY-4A moves, the GIIRS observes a total of 128 fields of view (FOVs) arranged in a 32 × 4 array, corresponding to an FOV with a 16-km diameter at nadir. The specific GIIRS instrument characteristics are given in Table 1 (Yu et al., 2020).
Parameter Performance Spectral bandwidth Longwave: 700−1130 cm−1
Mid wave: 1650−2250 cm−1Spectral channels Longwave: 689
Mid wave: 961Spectral resolution Longwave: 0.625 cm−1
Mid wave: 0.625 cm−1Sensitivity Longwave: 0.5–1.1 mW m−2 sr–1 cm–2
Mid wave: 0.1–0.14 mW m−2 sr–1 cm–2Operational model China area: 5000 × 5000 km2
Mesoscale area: 2000 × 2000 km2Spatial resolution 16 km Temporal resolution China area: 67 min
Mesoscale area: 35 minCalibration accuracy 1.5 K (3σ) radiation
10 ppm (3σ) spectrumTable 1. Specification for GIIRS onboard FY-4A.
In this study, GIIRS level-1 (L1) observed data and level-2 (L2) operational products from December 2019 to January 2020 (winter season) and from July 2020 to August 2020 (summer season) were collected twice daily, at 0000 to 0100 and 1200 to 1300 Coordinated Universal Time (UTC). The GIIRS L1 observed data can provide information such as the measured radiation values of the 1650 channels, noise equivalent spectral radiation values, and the longitude and latitude of the observation points. The GIIRS L2 operational products include the temperature profile of the GIIRS, cloud mask, land/sea mask, and surface parameters. The GIIRS L1 and L2 datasets can both be downloaded from the Chinese National Satellite Meteorological Center (NSMC).
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The radiosonde data of specific synoptic hours from upper-air stations in China are used to estimate the performance of both the retrieved profiles and the GIIRS profile products from the NSMC. The radiosonde data are received twice daily, at 0000 and 1200 UTC, from the China Meteorological Data Service Center (CMDC). Sounding observations from December 2019 to January 2020 and from July 2020 to August 2020 from 89 upper-air stations in the China area were used in this paper. The data include observational information such as geopotential height, temperature, dew point temperature, wind direction, wind speed at all specific isobaric levels, and pressure-temperature-humidity layers.
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In 2018, the European Centre for Medium-Range Weather Forecasts (ECMWF) released its fifth-generation global climate reanalysis dataset, called ERA5, which is produced using 4D-Var data assimilation in CY41R2 of ECMWF’s Integrated Forecast System (IFS), with 137 hybrid sigma/pressure (model) vertical levels, with the top-level at 0.01 hPa. Atmospheric data are available on these levels and are also interpolated to 37 pressure, 16 potential temperature and 1 potential vorticity level(s). The ERA5 dataset has a horizontal resolution of 0.25° × 0.25° (± 31 km at the equator) and a temporal resolution of 1 h (Hersbach et al., 2020). In this paper, atmospheric parameters at 37 pressure levels, including temperature, specific humidity, and ozone mass mixing ratio, were collected at 0000 to 0100 UTC and again at 1200 to 1300 UTC as initial guesses for the 1D-Var retrieval. Surface level parameters, including surface pressure, geopotential, and skin temperature, were used at the same time. To address the pressure level mismatch, a regression matrix was applied to map the data from the ERA5 37 pressure levels to the 101 levels required by the retrieval, which is consistent with the levels of the U.S. standard profile.
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The forward model is one of the most critical components of the retrieval algorithm. It computes radiances in a clear-sky corresponding to given atmospheric and surface states as well as Jacobian radiances with respect to atmospheric and surface parameters for use by the retrieval module. In this study, the Community Radiative Transfer Model (CRTM) developed by the United States Joint Center for Satellite Data Assimilation (Weng et al., 2005) was applied. In the 1D-Var retrieval algorithm, the initial profiles generated from the ERA5 datasets were used as inputs for CRTM. The number of levels in the CRTM model was set to 101, which is consistent with the input profiles for the retrievals.
Parameter | Performance |
Spectral bandwidth | Longwave: 700−1130 cm−1 Mid wave: 1650−2250 cm−1 |
Spectral channels | Longwave: 689 Mid wave: 961 |
Spectral resolution | Longwave: 0.625 cm−1 Mid wave: 0.625 cm−1 |
Sensitivity | Longwave: 0.5–1.1 mW m−2 sr–1 cm–2 Mid wave: 0.1–0.14 mW m−2 sr–1 cm–2 |
Operational model | China area: 5000 × 5000 km2 Mesoscale area: 2000 × 2000 km2 |
Spatial resolution | 16 km |
Temporal resolution | China area: 67 min Mesoscale area: 35 min |
Calibration accuracy | 1.5 K (3σ) radiation 10 ppm (3σ) spectrum |