This study proposed an innovative joint inversion algorithm that synchronized Moderate Resolution Imaging Spectroradiometer (MODIS) near-infrared (NIR) and thermal infrared (TIR) radiance data for accurate estimates of clear-sky precipitable water vapor (PWV). The algorithm consists of three parts: (1) simplifying the NIR radiative transfer equation by assuming linear reflectance change with wavelength in the 0.85–1.25 μm range, facilitating NIR water vapor absorption channel top-of-atmosphere (TOA) radiance simulation without explicit reflectance; (2) partial derivatives of NIR-TIR TOA radiance with respect to the background fields were derived by applying the one-term variational theorem to the radiative transfer equation; (3) optimization approach was employed to adjust the background fields, minimizing the discrepancy between simulated and observed NIR-TIR TOA radiances. The refined water vapor profile was integrated to derive PWV. Three years in situ measurements from the 473 GPS sites and 122 sun photometers in North America were utilized for PWV validation. Additionally, the MODIS MYD05 and MYD07 PWV products were validated using the same in situ measurements. Validation results indicated that the root mean square error (RMSE) of PWV retrieval using the NIR-TIR joint inversion algorithm ranged from 2.40 mm in summer to 1.67 mm in winter, and the mean bias and RMSE were − 0.55 mm and 2.08 mm, respectively, outperforming MODIS PWV products. The bias and RMSE were 3.84 mm and 4.86 mm for MYD05, and 0.41 mm and 4.60 mm for MYD07. Overall, the NIR-TIR joint inversion algorithm provides an effective way to generate comprehensive, long-term, high-resolution PWV data records.

中文翻译：

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一种基于物理的大气可降水蒸气反演算法，通过同步 MODIS 近红外和热红外测量


本研究提出了一种创新的联合反演算法，该算法同步了中分辨率成像光谱仪 （MODIS） 近红外 （NIR） 和热红外 （TIR） 辐射数据，以准确估计晴空可降水蒸气 （PWV）。该算法由三部分组成：（1） 通过假设波长在 0.85–1.25 μm 范围内随波长的线性反射变化来简化 NIR 辐射传输方程，便于在没有明确反射率的情况下进行 NIR 水蒸气吸收通道大气顶部 （TOA） 辐射模拟;（2） 通过将一项变分定理应用于辐射传输方程，推导出 NIR-TIR TOA 辐射度相对于背景场的偏导数;（3） 采用优化方法来调整背景场，最大限度地减少模拟和观测的 NIR-TIR TOA 辐射之间的差异。将精制水蒸气剖面整合得出 PWV。来自北美 473 个 GPS 站点和 122 个太阳光度计的三年原位测量用于 PWV 验证。此外，MODIS MYD05 和 MYD07 PWV 产品使用相同的原位测量进行了验证。验证结果表明，使用 NIR-TIR 联合反演算法检索脉搏波速度的均方根误差 （RMSE） 范围为夏季的 2.40 mm 到冬季的 1.67 mm，均值偏差和 RMSE 分别为 − 0.55 mm 和 2.08 mm，优于 MODIS 脉搏波率产品。MYD05 的偏压和 RMSE 分别为 3.84 mm 和 4.86 mm，MYD07 的偏压和 RMSE 分别为 0.41 mm 和 4.60 mm。总体而言，NIR-TIR 联合反演算法为生成全面、长期、高分辨率的脉压波速度数据记录提供了一种有效的方法。