Photon Pathlength Distributions from High Resolution Measurements of the Oxygen A-band

穿过多云大气的太阳辐射经历了许多散射事件。因此，到达地面的光子的总路径长度可能比视线路径更长。在给定时刻接收到的光子中，有些通常会比其他的经历更多的散射事件。描述传播介质效应的方法之一是用光子路径长度的概率分布来描述。从辐射传输理论出发，通过测量强氧吸收线附近的太阳光谱，原则上可以推断出光程分布。实际上，这个过程就是对光谱测量结果进行倒置，以确定(或“检索”)未知的光程长度分布。计算分布的精确度取决于光谱的精确度和分辨率以及特定的反演过程。这笔赠款支持开发一台高分辨率、高灵敏度的光谱仪，以测量氧气A波段(760 nm波长)附近的太阳光谱和820 nm附近的水蒸气波段。初步实验表明，该仪器足够灵敏，只需要几秒钟的信号积分。因此，可以以高时间分辨率测量路径长度分布，从而能够在云层在头顶移动时观察到快速变化的路径长度分布。蒙特卡罗模拟表明，参数反演法能够以可接受的精度恢复光程分布。将对结果进行分析，以确定多次散射是否可以解释异常的云吸收，异常云吸收是指观察到从云底出现的太阳辐射比散射计算预测的要小。路径长度越长，散射和吸收产生的辐射消光越大。如果路径长度比假设的要长，那么计算将低估实际的吸收。该项目的另一个目标是开发将路径长度分布引入气候和一般环流模式的方法，以此作为更准确地描述云对辐射传输的影响的方法。

Solar radiation penetrating a cloudy atmosphere undergoes many scattering events. The total pathlength of photons reaching the ground may therefore be longer than the line-of-sight path. Of the photons received at a given instant, some will usually have experienced more scattering events than others. One of the ways to describe the effects of the propagation medium is by the probability distribution of photon pathlength. From radiative transfer theory, it is possible in principle to infer the pathlength distribution from measurements of the solar spectrum in the vicinity of strong oxygen absorption lines. In effect, the process is one of inverting the spectroscopic measurements to determine (or "retrieve") the unknown pathlength distribution. The accuracy of the computed distribution depends on the accuracy and resolution of the spectroscopy and on the particular inversion procedure. This grant supports the development of a high-resolution, sensitive spectrometer to measure the solar spectrum near the oxygen A-band (760 nm wavelength) and the water vapor band near 820 nm. Preliminary experiments have shown that the instrument is sensitive enough to require only a few seconds of signal integration. Hence the pathlength distributions can be measured with high time resolution, enabling the observation of rapidly varying pathlength distributions as clouds move overhead. Monte Carlo simulations have shown that a parametric inversion procedure is able to recover pathlength distributions with acceptable accuracy. Results will be analyzed to determine if multiple scattering can account for anomalous cloud absorption, which refers to the observation that the solar irradiance emerging from cloud base is less than predicted from scattering calculations. The longer the pathlength, the greater the extinction of the radiation by scattering and absorption. If the pathlengths are longer than assumed, then the calculations will underestimate the actual absorption. Another objective of the project is to develop ways of introducing pathlength distributions into climate and general circulation models as a more accurate way of characterizing the effects of clouds on radiative transfer.