散射角对DSCOVR/EPIC观测的影响

G. Wen, A. Marshak
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摘要

深空气候观测站(DSCOVR)上的地球多色成像仪(EPIC)定期捕获整个地球受阳光照射的近反向反射辐射,以监测地球的变化。该仪器一直正常运行,直到2019年6月27日,由于惯性导航单元退化,航天器被放置在延长的安全舱中。scovr于2020年3月2日恢复全面运作。从那时起,入射太阳光和传感器方向之间的散射角范围比以前更大,最大散射角达到~ 178°,距离完美的后向散射只有2°,证明了在这种极端条件下研究大气顶(TOA)反射率的独特机会。在本文中,我们比较了2021-2016年EPIC全球光谱反射率。我们发现,在2021年,有4次散射角达到178°左右,与之相关的全球日平均光谱反射率增强。2016年最大散射角约为174.5°时,没有发现散射角相关的反射率增强。CERES数据在全球日反射短波通量中没有显示这种情况。因此,这些反射增强的场合主要是由于散射角的变化。散射角的增强与波长的关系很大,这主要是由于云散射相函数的波长依赖性。辐射传输计算表明,散射角的变化对680 nm和780 nm红光通道和近红外通道的反射率影响最大,对388 nm紫外通道的反射率影响最小,与EPIC观测结果一致。全球平均云量的变化对反射率增强也起着重要作用。云效应的影响取决于散射角的变化是否同相。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Effect of scattering angle on DSCOVR/EPIC observations
The Earth Polychromatic Imaging Camera (EPIC) on the Deep Space Climate Observatory (DSCOVR) routinely captures reflected radiation from the whole sunlit side of the Earth in the near backward direction to monitor the changing planet. The instrument had routinely operated until 27 June 2019, when the spacecraft was placed in an extended safe hold due to degradation of an inertial navigation unit. DSCOVR returned to full operations on 2 March 2020. Since then, the range of scattering angles between the incident sunlight and sensor direction has been larger than before and the largest scattering angle reaches ∼178°, only 2° from perfect backscattering, proving a unique opportunity to study the top-of-atmosphere (TOA) reflectance under such extreme conditions. In the paper, we compare EPIC global spectral reflectances in 2021–2016. We found that there are four occasions when the scattering angle reaches about 178° and associated with them enhanced global daily average spectral reflectances in 2021. The scattering angle related reflectance enhancements are not found in 2016 data when the maximum scattering angle is about 174.5°. CERES data do not show such occasions in global daily reflected shortwave flux. As a result, those enhanced reflectance occasions are primarily due to the change in scattering angle. The enhancement due to changes in scattering angle depends strongly on wavelength, primarily because of wavelength dependence of cloud scattering phase function. Radiative transfer calculations show that the change in scattering angles has the largest impact on reflectance in the red and NIR channels at 680 nm and 780 nm and the smallest influence on reflectance in the UV channel at 388 nm, consistent with EPIC observations. The change of global average cloud amount also plays an important role in the reflectance enhancement. The influence of the cloud effect depends on whether the change is in phase or not with the change of scattering angle.
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