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

氧气 A 波段高分辨率测量的光子路径长度分布

• 批准号: 9973701
• 负责人: Qilong Min
• 金额: $ 17.2万
• 依托单位: SUNY at Albany
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1999
• 资助国家: 美国
• 起止时间: 1999-07-01 至 2002-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9973701&HistoricalAwards=false
• 关键词: Photon; Pathlength; Distributions; Resolution; Measurements

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.

太阳辐射穿过多云大气层时会经历许多散射事件。 因此，光子到达地面的总路径长度可能比视线路径长，在给定时刻接收到的光子中，一些通常会经历比其他更多的散射事件。 描述传播介质效应的方法之一是通过光子路径长度的概率分布。 根据辐射传输理论，原则上可以从强氧吸收线附近的太阳光谱测量中推断出光程分布。 实际上，该过程是反演光谱测量以确定（或“检索”）未知路径长度分布的过程之一。 计算的分布的准确性取决于光谱的准确性和分辨率以及特定的反演过程。 这笔赠款支持开发高分辨率，灵敏的光谱仪，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，可以测量的路径长度分布具有高的时间分辨率，使云移动开销的快速变化的路径长度分布的观察。 蒙特卡罗模拟表明，参数反演过程是能够恢复的路径长度分布与可接受的精度。 将对结果进行分析，以确定多重散射是否可以解释异常云吸收，这是指观察到云底出现的太阳辐照度低于散射计算的预测值。 路径长度越长，通过散射和吸收的辐射消光越大。 如果路径长度比假设的长，那么计算将低估实际吸收。 该项目的另一个目标是研究将路径长度分布引入气候和大气环流模型的方法，以更准确地描述云对辐射转移的影响。