An investigation of the implications of lunar illumination spectral changes for Day/Night Band‐based cloud property retrieval due to lunar phase transition

The Moon reflects sunlight like a huge mirror hanging in the sky at night, which presents the obviously periodical changes in its luminance or irradiance due to Sun-Earth-Moon geometry variation. The potential effect of the periodical changes in lunar phase angle on nighttime Day/Night Band (DNB) radiative transfer simulation in the presence of cloud has seldom been reported thus far. In this study, a radiative transfer model is developed by coupling the lunar light source with various Sun-Earth-Moon geometries. To elucidate the stability of DNB-averaged cloud bulk scattering properties, we simulate nighttime reflectance and radiances under four typical lunar phase angles (0°, 45°, 90°, and 135°) from 7 April 2016 to 8 May 2016 (e.g., two lunar cycles). Explicit simulation analyses indicated that DNB-averaged cloud bulk scattering properties exhibit weak sensitivity to lunar phase angles. The maximum DNB reflectance differences between any and 90° lunar phase angles are less than 0.05% (0.01%) in the presence of water (ice) clouds, indicating a negligible effect of periodically changes on lunar spectral irradiances. Our findings suggest that the differences of reflectance at lunar phase angle = 90° are less than approximately 0.05% (water cloud)/0.01% (ice cloud), much smaller than 11% radiometric calibration uncertainties of DNB. This means that these differences could be ignored in both nighttime cloud property retrieval and DNB radiative transfer modeling.