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A new measurement approach for validating satellite-based above-cloud aerosol optical depth
Atmospheric Measurement Techniques ( IF 3.8 ) Pub Date : 2021-02-24 , DOI: 10.5194/amt-14-1405-2021
Charles K. Gatebe , Hiren Jethva , Ritesh Gautam , Rajesh Poudyal , Tamás Várnai

The retrieval of aerosol parameters from passive satellite instruments in cloudy scenes is challenging, partly because clouds and cloud-related processes may significantly modify aerosol optical depth (AOD) and particle size, a problem that is further compounded by 3D radiative processes. Recent advances in retrieval algorithms such as the “color ratio” method, which utilizes the measurements at a shorter (470 nm) and a longer (860 nm) wavelength, have demonstrated the simultaneous derivation of AOD and cloud optical depth (COD) for scenes in which absorbing aerosols are found to overlay low-level cloud decks. This study shows simultaneous retrievals of above-cloud aerosol optical depth (ACAOD) and aerosol-corrected cloud optical depth (COD) from airborne measurements of cloud-reflected and sky radiances using the color ratio method. These airborne measurements were taken over marine stratocumulus clouds with NASA's Cloud Absorption Radiometer (CAR) during the SAFARI 2000 field campaign offshore of Namibia. The ACAOD is partitioned between the AOD below-aircraft (AOD_cloudtop) and above-aircraft AOD (AOD_sky). The results show good agreement between AOD_sky and sun-photometer measurements of the above-aircraft AOD. The results also show that the use of aircraft-based sun-photometer measurements to validate satellite retrievals of the ACAOD is complicated by the lack of information on AOD below aircraft. Specifically, the CAR-retrieved AOD_cloudtop captures this “missing” aerosol layer caught between the aircraft and cloud top, which is required to quantify above-cloud aerosol loading and effectively validate satellite retrievals. In addition, the study finds a strong anticorrelation between the AOD_cloudtop and COD for cases in which COD < 10 and a weaker anticorrelation for COD > 10, which may be associated with the uncertainties in the color ratio method at lower AODs and CODs. The influence of 3D radiative effects on the retrievals is examined, and the results show that at cloud troughs, 3D effects increase retrieved ACAOD by about 3 %–11 % and retrieved COD by about 25 %. The results show that the color ratio method has little sensitivity to 3D effects at overcast stratocumulus cloud decks. These results demonstrate a novel airborne measurement approach for assessing satellite retrievals of aerosols above clouds, thereby filling a major gap in global aerosol observations.

中文翻译:

验证基于卫星的云层上方气溶胶光学深度的新测量方法

在多云的场景中从被动卫星仪器中获取气溶胶参数具有挑战性,部分原因是云和与云有关的过程可能会显着改变气溶胶光学深度(AOD)和颗粒大小,而3D辐射过程进一步加剧了这一问题。检索算法(例如“色比”方法)的最新进展利用了较短波长(470 nm)和较长波长(860 nm)的测量结果,证明了场景的AOD和云光学深度(COD)可同时导出其中发现吸收性气溶胶覆盖低层云层。这项研究显示了使用色比法从云反射和天空辐射的机载测量中同时获取云层上方的气溶胶光学深度(ACAOD)和气溶胶校正的云光学深度(COD)。这些空气传播的测量是在纳米比亚海上SAFARI 2000野战期间使用NASA的云吸收辐射计(CAR)对海洋层积云进行的。ACAOD在飞机下方的AOD(AOD_cloudtop)和飞机上方的AOD(AOD_sky)之间划分。结果表明,AOD_sky与上述飞机AOD的太阳光度计测量值之间具有良好的一致性。结果还表明,由于缺乏有关飞机下方AOD的信息,因此使用基于飞机的太阳光度计测量值来验证ACAOD的卫星检索非常复杂。具体来说,CAR回收的AOD_cloudtop捕获捕获在飞机和云顶之间的“缺失”气溶胶层,这是量化高于云的气溶胶负载并有效验证卫星检索所必需的。此外, <  10且COD >  10的抗相关性较弱,这可能与较低AOD和COD时 色比法的不确定性有关。研究了3D辐射效应对取回的影响,结果表明,在云槽处,3D效应使取回的ACAOD增大约3%–11%,使COD取回约25%。结果表明,颜色比率方法对阴云密布的层积云甲板上的3D效果几乎没有敏感性。这些结果证明了一种新颖的机载测量方法,可用于评估卫星对云层上方气溶胶的回收,从而填补了全球气溶胶观测的主要空白。
更新日期:2021-02-24
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