The Influence of Anisotropic Surface Reflection on Earth’s Outgoing Shortwave Radiance in the Lunar Direction

Jie Wu,Lei Li,Huadong Guo,Tong Li,Mingyang Lv,Yixing Ding,Haolu Shang

doi:10.3390/rs14040887

Abstract

The variation in the radiation budget at Earth’s top of the atmosphere (TOA) represents the most fundamental metric defining the status of global climate change. The accurate estimation of Earth’s shortwave radiant exitance is of critical importance to study Earth’s radiation budget (ERB) at TOA. Measuring Earth’s outgoing shortwave radiance (OSR) is a key point to estimate Earth’s shortwave radiant exitance. Compared with space-borne satellite systems, Moon-based sensors (MS) could provide large-scale, continuous, and long-term data for Earth radiation observations, bringing a new perspective on ERB. However, the factors affecting the estimation of Earth’s OSR in the lunar direction have not yet been fully explored, for example, anisotropic surface reflection and the effects of clouds and aerosols on radiation budget. In this work, we only focused on the influence of anisotropic surface reflection. To evaluate the extent of this influence, we constructed a model to estimate Earth’s OSR in the lunar direction (EOSRiLD), integrating the variables of anisotropic surface reflection (scene types, solar zenith angles, viewing zenith angles, and relative azimuth angles) and radiant flux in Moon-viewed sunlit regions. Then, we discussed it over three time periods (Earth’s rotation, revolution period, and synodic month cycle) and analyzed the impact of three variables (area of the Moon-viewed sunlit region, scene types, and incident-viewing angular bins) on anisotropic EOSRiLD. Our results indicate that EOSRiLD based on the assumptions of anisotropic and isotropic reflection is different but they all show the same monthly cycle change, which is related to the area of the Moon-viewed sunlit region. At the beginning and end of the lunar month, the differences between anisotropy and isotropy are greatest in each cycle; when it is close to the first half of each cycle, there is a small difference peak. Both anisotropy and isotropy are caused by the relative azimuth angles between the Sun and Moon. In conclusion, even if the Moon-based platform has a wider scope than space-borne satellites, the difference is still large between anisotropy and isotropy. Therefore, we still need to consider the anisotropic surface reflection based on the Moon-based observation.

Highlights

The variation in the radiation budget at the TOA represents the most fundamental metric defining the status of global climate change [1]
We simulated Earth’s OSR in the lunar direction (EOSRiLD) over different periods to explore the regularity of its changes
Due represents to the complex anisotropic reflecttaling metric defining the status of global climate change

Summary

Introduction

Frequent occurrences of extreme climate events in recent years have attracted increasing attention to global climate change among scientists. The variation in the radiation budget at the TOA (top of the atmosphere) represents the most fundamental metric defining the status of global climate change [1]. A positive budget balance reflects an energy accumulation in the Earth system, while a negative budget balance indicates a release. In the long-term balance, about 30% of the incoming solar shortwave radiation is directly reflected and scattered by Earth’s atmosphere, ocean, and land surface. The remainder is converted into different forms, stored in the Earth system, and will eventually be released in the Remote Sens.

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