Analysis of Tropospheric Precipitable Water Vapor Variations Using GNSS Radio Occultation Data and Radiosonde Observations (Case Study: Iran)

Document Type : Research Article

Authors

Department of Geodesy, Faculty of Geodesy and Geomatics Engineering, K. N. Toosi University of Technology, Tehran, Iran.

Abstract

This study investigates the variations of precipitable water vapor (PWV) in Iran using data from the COSMIC satellite mission’s radio occultation (RO) events and radiosonde observations over the period 2007–2019. PWV plays a crucial role in atmospheric energy transfer, the water cycle, and climate variability, making its accurate monitoring essential for meteorological and climatic studies. Currently, PWV is measured through ground-based systems like radiosondes, sun photometers, and microwave radiometers, as well as space-based methods such as GNSS RO, MODIS, and AIRS. While radiosondes provide reliable reference data due to their high accuracy, they suffer from limitations such as sparse spatial coverage and low temporal resolution. In contrast, space-based techniques like COSMIC offer a global coverage and high vertical resolution without being affected by clouds or precipitation, making them particularly valuable in regions with limited ground-based infrastructure. This study utilized 2,398 COSMIC RO events within a 150-kilometer radius of 12 radiosonde stations distributed across Iran, spanning latitudes of 24°N to 41°N and longitudes of 43°E to 64°E. Radiosonde data were preprocessed to remove outliers based on predefined criteria, such as excessive altitude differences between consecutive pressure levels or insufficient vertical layers. PWV values were calculated from both datasets using numerical integration of atmospheric parameters, and statistical metrics like RMSE and MAE were employed to evaluate the agreement between the two sources.
Results indicate that COSMIC-derived PWV generally follows similar trends to radiosonde measurements, but the level of agreement varies across stations. Southern stations like Ahwaz and Bandar Abbas, characterized by humid climates, exhibited higher PWV values and greater discrepancies compared to northern and central stations. The RMSE values ranged from 4.69 mm (Kermanshah) to 7.92 mm (Ahwaz), with an overall mean RMSE of 5.65 mm. Similarly, MAE values varied between 3.72 mm (Kermanshah) and 6.30 mm (Ahwaz), yielding an average MAE of 4.48 mm. Correlation analysis revealed positive relationships between the two datasets, but regression slopes were consistently below 1, indicating that radiosonde measurements tend to underestimate PWV at higher values and overestimate it at lower values compared to COSMIC. The intercepts of the regression equations were positive across all stations, further confirming this trend. Spatially, southern stations demonstrated higher errors, likely due to the complex moisture patterns and high humidity levels in these regions. Despite these discrepancies, the findings suggest that COSMIC-derived PWV can serve as a reliable alternative or supplement to radiosonde measurements, especially in regions lacking sufficient ground-based observational networks. This research highlights the potential of COSMIC data to enhance numerical weather prediction (NWP) and climate studies in underserved areas, while also emphasizing the need for further investigation into the factors influencing the accuracy of COSMIC PWV retrievals under varying atmospheric conditions. Future studies should be focused on improving retrieval algorithms and integrating multi-source data to refine PWV estimation accuracy, particularly in challenging environments like Iran. This work provides a foundation for advancing atmospheric research and operational meteorology in regions with limited access to traditional ground-based data.

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References

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Journal of the Earth and Space Physics
Volume 51, Issue 2
online publication date: 20 September 2025
September 2025
Pages 499-516

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