Evaluation of Hourly PWV Products Derived From ERA5 and MERRA-2 Over the Tibetan Plateau Using Ground-Based GNSS Observations by Two Enhanced Models

Precipitable water vapor (PWV), despite being the key factor for assessing climate change over the Tibetan Plateau (TP), is undersampled in this region due to the complex terrain and climate conditions therein. The fifth-generation European Center for Medium-Range Weather Forecasts Reanalysis (ERA5) and Second Modern-Era Retrospective Analysis for Research and Applications (MERRA-2) data sets are the two newly released global reanalyzes that provide hourly PWV products with a high spatiotemporal resolution for water vapor research. The accuracies of these two PWV products on the TP require a comprehensive and cautious assessment. In this study, the hourly PWV values derived from ERA5 and MERRA-2 are systematically assessed by a comparison with 33 Global Navigation Satellite System (GNSS) stations from 2017 to 2018 over the TP. To improve the reliability of this evaluation, an enhanced T-m model and a PWV vertical correction model, namely, TP-T-m and TP-PWVC, respectively, are developed, and both models show better performance when compared against other widely used models. The accuracies of the ERA5- and MERRA-2-derived PWV at the 33 GNSS stations are very high; ERA5 has the smallest mean bias and root mean square (RMS) error of -0.08 and 1.77 mm, respectively, compared to those of 0.53 and 2.12 mm for MERRA-2. In addition, the biases and RMS errors show significant regional and seasonal differences. The performance of ERA5 is slightly better than MERRA-2, mainly due to its higher spatial resolution. The spatial distributions of the PWV derived from the two reanalyzes are consistent with the GNSS-derived PWV distribution. The diurnal PWV variations derived from ERA5 are in good agreement with the diurnal variation of the GNSS-derived PWV, whereas significant differences are observed for MERRA-2 at several stations. These results indicate that ERA5 can provide more reliable hourly PWV estimates over the TP, which can help users further understand the strengths and weaknesses of these two PWV products and promote climate change research.

Automated summary

This study systematically assessed the hourly PWV values derived from ERA5 and MERRA-2 by comparing them with 33 GNSS stations over the Tibetan Plateau from 2017 to 2018. It was found that ERA5 had slightly higher accuracy compared to MERRA-2, possibly due to its higher spatial resolution.