Modeling phase-angle dependence of lunar irradiance using long-term lunar measurements by VIRS on TRMM

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Moon reflects sun light and its surface is radiometicly stable, making it an ideal target for calibrating satellite radiometers. Since lunar irradiance depends strongly on lunar phase and differs between waxing and waning phases, an accurate modeling of dependence of lunar irradiance on lunar phase angle is needed and requires long term consistent observations of the moon. Since its operation in 1998, the Visible and Infrared Scanner (VIRS) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite makes regular observations of moon through space view for about 15 years with comprehensive coverage of lunar phases varying from waxing to waning. Two of these VIRS bands are reflected solar bands centered at 0.62 and 1.61um. Lunar measurements through space view of VIRS are not subject to atmospheric effects. Therefore, long term lunar observation by VIRS on TRMM is an invaluable dataset for both verifying and calibrating lunar irradiance models. In this study, analysis of long-term lunar observations using VIRS data are performed and phase-angle dependence of lunar irradiance is modeled. Effects of waxing and waning phases on lunar irradiance for two visible bands of VIRS are quantified. It is found that the lunar disk-integrated intensity of waxing lunar phase is higher than those of waning phase for phase angle >40° for both channels and is consistent with the fact that the waning moon shows more of dark maria. The derived phase angledependences of lunar disk effective reflectance for these two channels are compared with model.