ABSTRACT
Accurate satellite radiance measurements are significant for data assimilations and quantitative retrieval applications. In the present paper, radiometric calibration accuracy of FungYun-3A (FY-3A) Medium Resolution Spectral Imager (MERSI) thermal infrared (TIR) channel was evaluated based on simultaneous nadir observation (SNO) intercalibration method. Hyperspectral and high-quality measurements of METOP-A/IASI were used as reference. Assessment uncertainty from intercalibration method was also investigated by examining the relation between BT bias against four main collocation factors, i. e. observation time difference, view geometric difference related to zenith angles and azimuth angles, and scene spatial homogeneity. It was indicated that the BT bias is evenly distributed across the collocation variables with no significant linear relationship in MERSI IR channel. Among the four collocation factors, the scene spatial homogeneity may be the most important factor with the uncertainty less than 2% of BT bias. Statistical analysis of monitoring biases during one and a half years indicates that the brightness temperature measured by MERSI is much warmer than that of IASI. The annual mean bias (MERSI-IASI) in 2012 is (3.18±0.34) K. Monthly averaged BT biases show a little seasonal variation character, and fluctuation range is less than 0.8 K. To further verify the reliability, our evaluation result was also compared with the synchronous experiment results at Dunhuang and Qinghai Lake sites, which showed excellent agreement. Preliminary analysis indicates that there are two reasons leading to the warm bias. One is the overestimation of blackbody emissivity, and the other is probably the incorrect spectral respond function which has shifted to window spectral. Considering the variation character of BT biases, SRF error seems to be the dominant factor.
ABSTRACT
Direct measurement of the sky radiation in the direction of a special zenith angle as the average downward radiation is the fastest instantaneous downward radiation measurement. But this method has not been widely used, because there is no accu rate method for determining special zenith angle. The present paper analyzed the special angle with the variation of the aerosol optical thickness and atmospheric temperature and humidity through simulation by radiative transfer model. The results show that the special angles will be affected none by aerosols in clear skies, very little by atmospheric temperature, but significantly by atmospheric water vapor content. Therefore, the special angle could be determined only by atmospheric water vapor content in downward radiance infrared spectroscopy measurements in clear sky conditions. The downward radiation measurements results in western desert region indicate that the special angles determined by local atmospheric water vapor content are the same as experimental results. Besides, the error caused by taking the radiance at the special angle as the integral radiance meets the accuracy requirements. Accordingly, this fast measurement method can be widely used in field experiments while the special angle can be determined by atmospheric water vapor of the study area, since it can measure instantaneous downward radiance conveniently and accurately.
