Fengyun-3C Research Articles

An alternative method for in-flight absolute radiometric calibration of thermal infrared channels of Chinese geostationary meteorological satellites

The aim of this study was to explore the feasibility of an alternative method for in-flight absolute radiometric calibration of the thermal infrared (TIR) channels of the Chinese meteorological satellites FengYun-2B (FY-2B) and FengYun-2C (FY-2C). The alternative method substituted radiosonde atmospheric profiles with those from the National Centers for Environmental Prediction (NCEP) reanalysis and the water surface brightness temperatures from TIR radiometers (CE312) with those from an automated hydrometeorological buoy (AHMB) system over Qinghai Lake (QHL), China. These data were then used to calculate the calibration coefficients and their uncertainty for the TIR channels of FY-2B and FY-2C. The at-sensor radiance (ASR) and at-sensor brightness temperature (ASBT) of the TIR channels of FY-2B and FY-2C were calculated by using 14 atmospheric profiles as measured by radiosonde over QHL in August 2003 and the corresponding NCEP reanalysis data, respectively. In addition, we conducted sensitivity tests to different atmospheric profiles of varying relative humidity and air temperatures on the ASR and ASBT of the TIR channels of FY-2B and FY-2C. Differences in gains between the regular and alternative methods are less than 0.005 mW m–2 sr−1 cm−1 DN−1. The sensitivity tests show that the ASR and ASBT are more sensitive to the relative humidity than the temperature in the atmospheric profile. Our results show that the proposed alternative method, of which the uncertainty is about 1.5 K for the TIR channels of FY-2B and FY-2C, is feasible for the TIR channels of various remote sensors. One of the major benefits of this alternative method is the potential for more frequent, reliable and inexpensive calibrations of the TIR sensors in operational conditions.

Cloudiness characteristics over Southeast Asia from satellite FY‐2C and their comparison to three other cloud data sets

Fengyun‐2C (FY‐2C), launched in October 2004, is the first operational geostationary meteorological satellite in China. It can provide 1‐h interval cloudiness products with a spatial resolution of 0.04° latitude × 0.04° longitude. The main characteristics of the regional‐scale clouds from a 2‐year FY‐2C data set (from July 2005 to June 2007) are presented, including the spatial distribution and the annual and diurnal cycles of cloudiness. The reliability of FY‐2C cloud products over Southeast Asia is investigated through comparisons to cloud cover from the International Satellite Cloud Climatology Project, the Moderate Resolution Imaging Spectroradiometer on board Terra and Aqua satellites, and conventional ground observations. It is shown that the FY‐2C cloud mask performs consistently with other cloud mask products over Southeast Asia. In the boreal winter, the whole domain is dry with little cloudiness. More extensive cloudiness can be observed over the Sichuan Basin, in the East China Sea and the South China Sea, along the northwestern border of China, and around the ITCZ in the Southern Hemisphere. In the boreal summer, the summer monsoon is the dominant system for the studied domain, which is generally humid with extensive cloudiness, corresponding to zones of strong convective activities. Results also reveal considerable discrepancies among different cloud products over extended areas of north China and Mongolia. The Sichuan Basin is another region of large discrepancies among the four cloud products. Diurnal cycles of FY‐2C cloudiness for the four seasons of a year are analyzed. The diurnal range of cloudiness is generally larger over land than over ocean. Remarkable diurnal variation is found over the Tibetan Plateau, the northern part of the Indian Peninsula, and central Asia where there is generally less precipitation. The peaks of diurnal cycle of cloudiness appear around local noon over the subtropical land, in the morning over the Indian Peninsula, and in the afternoon near the equator.

An Improved Cloud Classification Algorithm for China’s FY-2C Multi-Channel Images Using Artificial Neural Network

The crowning objective of this research was to identify a better cloud classification method to upgrade the current window-based clustering algorithm used operationally for China’s first operational geostationary meteorological satellite FengYun-2C (FY-2C) data. First, the capabilities of six widely-used Artificial Neural Network (ANN) methods are analyzed, together with the comparison of two other methods: Principal Component Analysis (PCA) and a Support Vector Machine (SVM), using 2864 cloud samples manually collected by meteorologists in June, July, and August in 2007 from three FY-2C channel (IR1, 10.3–11.3 μm; IR2, 11.5–12.5 μm and WV 6.3–7.6 μm) imagery. The result shows that: (1) ANN approaches, in general, outperformed the PCA and the SVM given sufficient training samples and (2) among the six ANN networks, higher cloud classification accuracy was obtained with the Self-Organizing Map (SOM) and Probabilistic Neural Network (PNN). Second, to compare the ANN methods to the present FY-2C operational algorithm, this study implemented SOM, one of the best ANN network identified from this study, as an automated cloud classification system for the FY-2C multi-channel data. It shows that SOM method has improved the results greatly not only in pixel-level accuracy but also in cloud patch-level classification by more accurately identifying cloud types such as cumulonimbus, cirrus and clouds in high latitude. Findings of this study suggest that the ANN-based classifiers, in particular the SOM, can be potentially used as an improved Automated Cloud Classification Algorithm to upgrade the current window-based clustering method for the FY-2C operational products.

Intercalibration of SVISSR/FY-2C Infrared Channels Against MODIS/Terra and AIRS/Aqua Channels

This paper addresses the intercalibration of the infrared channels 1 (10.9 mum), 2 (11.9 mum), and 4 (3.8 mum) of the Stretched Visible and Infrared Spin Scan Radiometer (SVISSR) onboard China's geostationary satellite FengYun 2C (FY-2C) against the channels of the MODerate resolution Imaging Spectroradiometer (MODIS) onboard Terra and of the Atmospheric InfraRed Sounder (AIRS) onboard Aqua. The effects of the wide spectral range of SVISSR channels 1, 2, and 4 were evaluated. Three methods, including the ray-matching method, the radiative transfer modeling method, and the high spectral convolution method, were developed in a tropical region using the SVISSR/FY-2C, MODIS/Terra, and AIRS/Aqua measurements in December 2006 and 2007. The results reveal that the onboard calibration of SVISSR/FY-2C was stable during that period; however, calibration discrepancies exist between MODIS and SVISSR channels and between AIRS and SVISSR channels. The intercalibration coefficients of SVISSR/FY-2C channels were obtained for each of the three methods.

Onboard Real-Time Absolute Radiometric Calibration for Thermal Infrared Channels of Chinese Geostationary Meteorological Satellites

Abstract Forty-one cloud-free images of Qinghai Lake (QHL) in China and the corresponding digital numbers (DNs) of FengYun-2C (FY-2C) at 0000, 0600, 1200, and 1800 UTC from 1 July to 30 September 2005 are analyzed. The corresponding surface water temperatures of QHL measured by the automated hydrometeorological buoy (AHMB) system and the atmospheric profiles over QHL from the National Centers for Environmental Prediction (NCEP) reanalysis data are inputted into the atmospheric transfer model MODTRAN3.7 to calculate the entrance pupil radiance and brightness temperatures for thermal infrared (TIR) channels of FY-2C. Then, the absolute radiometric calibration coefficients of FY-2C, which are used to calculate the equivalent blackbody (EBB) temperatures TEBB, are calculated by comparing the entrance pupil radiance and brightness temperatures with the corresponding DNs. In addition, the temperatures of onboard blackbody (OBB) TOBB, primary, secondary, refraction, and calibration mirrors on the multichannel scanning radiometer (MSR) of FY-2C are detected remotely. Based on the linear correlation between TEBB − TOBB and temperatures of various mirrors, the transform equations from TOBB to TEBB are developed. Finally, the onboard real-time absolute radiometric calibration for TIR channels of geostationary meteorological satellite FY-2C is implemented with an uncertainty of about 1.5 and 2.1 K for TIR 1 and TIR 2 of FY-2C, respectively.

A method for cross‐calibrating geostationary and polar orbiting satellite sensors in the infrared windows

The forward radiative transfer routine has been used to remove the artefact caused by modulation on meteorological satellite‐sensor responses. This paper describes a direct method where images observed by the infrared channels aboard the Multi‐functional Transport Satellite (MTSAT‐1R) and Fengyun‐2C (FY‐2C) geostationary satellites are compared with those by the Advanced Very High Resolution Radiometer (AVHRR) sensor aboard the National Oceanic and Atmospheric Administration (NOAA) polar‐orbiting satellites. The calibration differences between the polar orbiting and the geostationary satellites are found to be within 1 K of each other.