Abstract This global study analyzes how the total column water vapor (in terms of precipitable water, PW) is related to the attenuation of shortwave solar radiation (SWR) using data from the MERRA-2 reanalysis during 2000–2014 and SWR simulations obtained with the REST2 clear-sky radiation model. The water vapor radiative effect (WVRE) on SWR attenuation is investigated by comparing the clear-sky SWR predictions under humid and ideally dry atmospheres, whereas the quantitative effect of PW on WVRE is determined using the concept of precipitable water efficiency (PWE). At global scale, WVRE exhibits a distinct seasonal pattern ranging from −80.2 to 0 W m−2, with lower values in humid regions where PW is typically high (>4 cm), making the water vapor contribution relatively small. This is attributed to the saturation effect in water vapor absorption. Over arid regions, the solar irradiance attenuation is mainly controlled by the high aerosol loads, and less negative WVRE values are found. Over high-altitude regions, the low PW reduces the atmospheric absorption, resulting in low absolute WVRE magnitudes. PWE is found to extend globally between −133.7 and 0 W m−2 cm−1 on a monthly scale. This range is smaller on a long-term mean monthly or annual basis. On a long-term basis, the monthly PWE results are mainly concentrated between −20 and 0 W m−2 cm−1, generating leptokurtic and left-skewed distributions peaking between −10 and −8 W m−2 cm−1, depending on month. PWE is inversely related to PW on a long-term annual scale, indicating that PWE tends to zero under very humid conditions. A general nonlinear model is proposed to evaluate the hourly PWE at global scale on a monthly basis, separately for the two hemispheres. A PW trend analysis reveals that the Middle East, equatorial regions, central Europe, and Australia exhibit significant temporal trends, further inducing significant trends in WVRE. For those areas, WVRE trends are calculated between −3.3 and 1.5 W m−2 over the entire 15-year period. In addition, the global WVRE over landmasses is classified in terms of the Koppen-Geiger (KG) climate classification system. The most pronounced radiative effects are found over the equatorial regions, with an average WVRE of −63 W m−2. The Arid KG class provides a complex WVRE pattern ranging from −66 to −24 W m−2. Further segregation of this class shows strongly variable WVRE, induced by local climate differentiation. Over Warm Temperate climates, WVRE is found to vary widely according to the various KG subclasses. For the Cold climate class, PW is mainly concentrated around 1 cm and WVRE is low. The Polar regions include high mountains where WVRE is low, with a median close to −28 W m−2.

中文翻译：

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晴空下大气水汽对短波辐射的影响：全球时空分析

摘要 这项全球研究使用 2000-2014 年 MERRA-2 再分析的数据和通过 SWR 模拟获得的数据，分析了总柱状水蒸气（以可降水量，PW 计）与短波太阳辐射 (SWR) 衰减的关系。 REST2晴空辐射模型。通过比较潮湿和理想干燥大气下的晴空 SWR 预测，研究了水蒸气辐射效应 (WVRE) 对 SWR 衰减的影响，而 PW 对 WVRE 的定量影响是使用可降水效率 (PWE) 的概念确定的。在全球范围内，WVRE 表现出明显的季节性模式，范围从 -80.2 到 0 W m-2，在 PW 通常较高（> 4 cm）的潮湿地区具有较低的值，使得水蒸气贡献相对较小。这归因于水蒸气吸收的饱和效应。在干旱地区，太阳辐照度衰减主要受高气溶胶负荷控制，发现较少的负 WVRE 值。在高海拔地区，低 PW 减少了大气吸收，导致 WVRE 绝对值较低。发现 PWE 在全球范围内按月在 -133.7 和 0 W m-2 cm-1 之间扩展。这个范围在长期平均每月或每年的基础上较小。从长期来看，每月的 PWE 结果主要集中在 -20 和 0 W m-2 cm-1 之间，产生尖峰态分布和左偏分布，峰值在 -10 和 -8 W m-2 cm-1 之间，具体取决于月。在长期的年度尺度上，PWE 与 PW 呈负相关，表明在非常潮湿的条件下 PWE 趋于零。提出了一个通用的非线性模型，以每月在全球范围内评估两个半球的每小时 PWE。PW 趋势分析表明，中东、赤道地区、中欧和澳大利亚表现出显着的时间趋势，进一步引发了 WVRE 的显着趋势。对于这些地区，在整个 15 年期间计算的 WVRE 趋势介于 -3.3 和 1.5 W m-2 之间。此外，陆地上的全球 WVRE 根据 Koppen-Geiger (KG) 气候分类系统进行分类。在赤道地区发现了最明显的辐射效应，平均 WVRE 为 -63 W m-2。Arid KG 类提供了一个复杂的 WVRE 模式，范围从 -66 到 -24 W m-2。这一类的进一步隔离显示出强烈可变的 WVRE，这是由当地气候分化引起的。在暖温带气候中，根据各种 KG 亚类，发现 WVRE 变化很大。对于寒冷气候等级，PW 主要集中在 1 cm 左右，W​​VRE 较低。极地地区包括 WVRE 较低的高山，中值接近 -28 W m-2。