Assessment of the Radiometric Response Stability of Fengyun-4A Lightning Mapping Imager Using Deep Convective Cloud Targets

The radiometric response stability of Fengyun-4A Lightning Mapping Imager (LMI), a narrow-band reflective spectral imager, was studied by using deep convective cloud (DCC) calibration method. Based on the distribution of the measured DCC radiance, the sample size, and the intrinsic characteristics of DCCs, the parameters in the DCC method were optimized for extracting LMI DCC targets. The optimized parameters mainly include a brightness temperature threshold of 205 K, a core size of 3 by 3 for LMI pixels, a LMI spatial standard deviation (STD) threshold of 0.03 and an infrared spatial STD threshold of 1.0 K, and the solar zenith angle (SZA), view zenith angle, and relative azimuth angle in the range of 30°–45°, 15°–30°, and 90°–180°, respectively. The resulting LMI DCC radiance shows a degradation of 0.38% per year throughout the 6 years’ period within the uncertainty of the DCC method, excluding the early stage of its launch. The degradation of the left focal plane of LMI is 1.24 times as much as that of the right one. Moreover, it is highly likely that the LMI radiometric response changed significantly in the first months after launch. A Monte Carlo simulation result infers that the measured radiance of LMI increases by 32% after launch, resulting in a reduction in the number of detected lightning events to 1/10 since the prelaunch calibration results are still utilized. These findings will help explain the changes in the detection efficiency of LMI and promote the improvement of its radiometric calibration and lightning extraction processing.