ABSTRACT The global precipitation measurement integrated multi-satellite retrievals (GPM IMERG) provide a new opportunity to estimate precipitation remotely. However, the accuracy of estimations varies in different regions. Therefore, the aim of this study is to improve the IMERG precipitation estimations using two influencing factors including total precipitable water (TPW) and altitude data. The investigations were performed in six rainfall climatic regions of Iran using daily and six-hourly IMERG estimations in years 2015 to 2017. TPW was extracted from a locally developed algorithm for the moderate resolution imaging spectroradiometer (MODIS) measurements. Different models named as M1 to M4 were investigated by considering combinations of the linear and quadratic forms of the IMERG precipitation, TPW, and altitude. The results were evaluated by root mean square error (RMSE), correlation coefficient (r), and Nash-Sutcliffe (NSc). The results showed that the M1 represents the best performance among the proposed models, which uses linear relationship of the IMERG estimations and TPW along with the constant value. The mean values of r, RMSE, and NSc criteria for daily precipitation estimations of IMERG were acquired as 0.46, 5.88, and −2.77 mm, respectively. Those values were improved by M1 to 0.52, 0.18, and 0.27 mm, respectively. Moreover, the evaluation results of IMERG six-hourly precipitation estimations showed r, RMSE, and NSc of 0.65, 11.26, and −33.27 mm. The values of these criteria were improved by M1 to 0.65, 0.07, and 0.32 mm. In general, the results proved the ability of TPW to improve IMERG precipitation estimations in the study area.

中文翻译：

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利用伊朗气候区的大气可降水量和海拔提高全球降水测量综合多卫星反演 (GPM IMERG) 的准确性

摘要 全球降水测量集成多卫星反演（GPM IMERG）为远程估算降水提供了新的机会。然而，估计的准确性在不同地区有所不同。因此，本研究的目的是利用总可降水量 (TPW) 和海拔数据这两个影响因素来改进 IMERG 降水估算。调查是在伊朗的六个降雨气候区进行的，使用的是 2015 年至 2017 年的每日和每 6 小时 IMERG 估计。 TPW 是从当地开发的中等分辨率成像光谱仪 (MODIS) 测量算法中提取的。通过考虑 IMERG 降水、TPW 和海拔的线性和二次形式的组合，研究了命名为 M1 到 M4 的不同模型。结果通过均方根误差 (RMSE)、相关系数 (r) 和 Nash-Sutcliffe (NSc) 进行评估。结果表明，M1 代表了所提出模型中的最佳性能，它使用了 IMERG 估计和 TPW 的线性关系以及常数值。用于 IMERG 每日降水估计的 r、RMSE 和 NSc 标准的平均值分别为 0.46、5.88 和 -2.77 mm。这些值通过 M1 分别提高到 0.52、0.18 和 0.27 毫米。此外，IMERG 六小时降水估算的评估结果显示，r、RMSE 和 NSc 分别为 0.65、11.26 和 -33.27 毫米。这些标准的值通过 M1 提高到 0.65、0.07 和 0.32 毫米。总的来说，结果证明了 TPW 能够改进研究区的 IMERG 降水估计。