Measurements of spectral zenith radiance in the 320–950 nm wavelength range have been carried out since 2021 at the Thule High Arctic Atmospheric Observatory (THAAO, , 76.5° N, 68.8° W, 225 m a.s.l) located in Pituffik, northern Greenland. This study evaluates whether such observations could provide, in principle, scientifically meaningful cloud optical depth (τ) estimates, what would be the limitations, which are the necessary auxiliary measurements, and which radiance wavelengths would be better suited for the goal. Although clouds play a critical role in the Arctic, a climatology of τ in high albedo conditions is particularly difficult to obtain. THAAO might have the instrument capabilities to provide such a long-term dataset. We use a radiative transfer package to simulate visible spectra with different cloud and surface conditions, assuming homogeneous overcast sky and liquid water clouds of fixed geometrical thickness. Simulations are run to reproduce typical conditions encountered at THAAO when measurements are carried out. We find that the assumption of a broadband albedo instead of a spectrally-resolved one is the source of the largest uncertainties. Tests on size and phase of cloud particles showed that a 50% uncertainty in r leads to a ∼10% error in τ, and that a 10% contamination of ice crystals in a low-level liquid water cloud leads to an error in τ estimates of less than 5%. All the tests showed that the most critical τ range is between the thin and thick cloud regimes (τ ∼ 7–15), where the retrievals can be less reliable. Otherwise, tests suggest that in the environmental conditions that characterize late spring and summer at THAAO, and given the observatory measurements capabilities, estimates of τ for low-level clouds could be accurately retrieved both in high and low surface albedo conditions by means of zenith radiance measurements in the UV-Vis-NIR range.

中文翻译：

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从光谱辐射率反演云光学厚度——高反照率表面的敏感性研究


自 2021 年以来，位于格陵兰岛北部皮图菲克的图勒高北极大气观测站（THAAO，北纬 76.5°，西经 68.8°，海拔 225 m）开始对 320-950 nm 波长范围的光谱天顶辐射进行测量。这项研究评估了此类观测原则上是否可以提供具有科学意义的云光学深度（τ）估计，其局限性是什么，哪些是必要的辅助测量，以及哪些辐射波长更适合该目标。尽管云在北极发挥着至关重要的作用，但在高反照率条件下获取 τ 的气候特征特别困难。 THAAO 可能拥有提供此类长期数据集的仪器能力。我们使用辐射传输包来模拟不同云和表面条件下的可见光谱，假设均匀的阴天和固定几何厚度的液态水云。进行模拟以重现进行测量时 THAAO 遇到的典型条件。我们发现宽带反照率而不是光谱分辨假设是最大不确定性的来源。对云粒子大小和相位的测试表明，r 的 50% 不确定性会导致 τ 产生约 10% 的误差，而低层液态水云中 10% 的冰晶污染会导致 τ 估计误差低于5%。所有测试表明，最关键的 τ 范围是在薄云区和厚云区之间 (τ ∼ 7-15)，在该范围内反演的可靠性可能较低。 另外，测试表明，在 THAAO 春末和夏季的环境条件下，并考虑到观测站的测量能力，可以通过天顶辐射率在高和低表面反照率条件下准确地检索低层云的 τ 估计值紫外-可见-近红外范围内的测量。