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A Comprehensive Validation of Global Precipitation Measurement Satellite Products Over a Western Mediterranean Region

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Universitat de Barcelona. Facultat de Física

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[eng] Precipitation estimation is essential for understanding atmospheric processes, water resource management, and climate modeling. Advances in remote sensing, particularly the Global Precipitation Measurement (GPM) mission, have improved global precipitation coverage, especially in regions where traditional methods are insufficient. Since its launch in 2014, GPM has become one of the most comprehensive efforts to quantify precipitation globally, with continuous updates to its products. The complex Mediterranean climate, characterized by high variability and uncertainty in precipitation projections, highlights the importance of validating these products. This thesis focuses on validating GPM precipitation estimates over a Western Mediterranean region.
The thesis is structured into three parts, based on three scientific publications and a preprint. It begins with the validation of Integrated Multi-satellite Retrievals for GPM (IMERG) products across multiple temporal scales, particularly their performance in detecting intense rainfall in the Mediterranean. The impact of cloud top phase on satellite retrievals is also examined through 18 case studies, along with a comparison to Support to Operational Hydrology and Water Management (H SAF) products. Finally, rain estimates and drop size distributions from the GPM Dual-frequency Precipitation Radar (DPR) are validated.
Many GPM validation studies lack evaluations at sub-daily scales or in mountainous regions. This thesis addresses these gaps by assessing IMERG Early, Late, and Final runs at different temporal scales in Catalonia, using ground stations from 2015 to 2020. While IMERG Final reduces errors at all scales, it underestimates precipitation in areas like the Pyrenees, and both Early and Late runs tend to overestimate rainfall. IMERG also shows high bias and low correlation at sub-daily scales, indicating challenges in estimating precipitation at high temporal resolution. Very heavy rainfall is significantly underestimated, by more than 80%.
The second part of the study focuses on extreme precipitation events, using IMERG Early and Late products to assess retrievals’ performance. Stratified results based on the microphysical properties of clouds show a general underestimation of precipitation, which worsens with increased rainfall intensity and temporal resolution. Passive microwave (PMW) sensors showed less bias than infrared (IR) sensors, although including IR increased errors. IMERG performed better in ice-phase clouds compared to warm and mixed-phase clouds.
This analysis was extended to compare IMERG with H SAF products in 18 extreme rainfall cases, using a pixel-to-point approach to reduce discrepancies between satellite and ground data. H64 performed best at daily scales, and H68 at hourly detection, although accuracy decreased with increasing rainfall intensity. Despite biases, the IMERG Late product was the most effective at detecting extreme precipitation events.
The final part of the thesis evaluates the GPM Core satellite’s Dual-frequency Precipitation Radar (DPR), focusing on seven disdrometers in various topographic regions from 2014 to 2023. Radar reflectivity, drop size distributions, and precipitation intensity were compared. GPM DPR captured variability in observed drop size distributions but overestimated the mass-weighted mean diameter and underestimated the
intercept parameter. Errors were highest for rainfall rate and the intercept parameter, but lowest for radar reflectivity and the mass-weighted mean diameter. The classification of stratiform and convective rainfall by GPM DPR also showed an overestimation of stratiform cases.
This research is one of the first in the Iberian Peninsula to validate IMERG products with a detailed focus on orographic, climatic, and precipitation intensity factors at high temporal resolution. By comparing GPM and H SAF products and evaluating updates to DPR version 7, this study provides valuable insights into satellite precipitation estimation and lays the foundation for future research.

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[spa] La estimación cuantitativa de la precipitación es esencial para comprender los procesos atmosféricos, gestionar los recursos hídricos, mejorar las predicciones meteorológicas y los modelos climáticos. El uso de satélites, como la misión Global Precipitation Measurement (GPM), han mejorado significativamente la cobertura global de precipitaciones. Desde su lanzamiento en 2014, los productos del GPM han sido actualizados continuamente para mejorar sus algoritmos. En regiones como el Mediterráneo, con alta variabilidad climática, la validación rigurosa de estos productos es crucial. Esta tesis tiene como objetivo validar las estimaciones de precipitación de GPM en una región del Mediterráneo Occidental.
Estructurada en tres partes, la tesis valida los productos de IMERG, enfocados en eventos de lluvia intensa. Se evalúan los errores en la detección de la precipitación, incluyendo el impacto de las fases de la nube en los datos satelitales. Además, se compara IMERG con productos del Operational Hydrology and Water Management (H SAF, por sus siglas en inglés) y se valida la estimación de lluvia y la distribución de tamaños de gotas obtenidas del Dual Frequency Precipitation Radar (DPR) de GPM.
Los resultados muestran que, aunque IMERG reduce errores, subestima la precipitación en áreas montañosas como los Pirineos, y sufre altos sesgos en escalas subdiarias. La lluvia muy intensa es subestimada en más del 80%, teniendo en cuenta observaciones de pliviómetros. En cuanto a eventos extremos, IMERG muestra un sesgo menor con sensores de microondas, pero los errores aumentan al incluir datos de infrarrojos. En la comparación con productos de H SAF, el producto H64 fue el mejor a escala diaria, la versión IMERG Late destacó en la detección de eventos intensos. Finalmente, la validación del radar DPR muestra que, aunque captura bien la variabilidad en la distribución de tamaños de gotas, subestima el parámetro de intersección (Nw) y sobreestima el diámetro medio ponderado (Dm).
Este estudio, uno de los primeros en la Península Ibérica en validar productos de IMERG y de la última versión del DPR, proporciona información clave para mejorar el uso de datos satelitales en la estimación de precipitaciones.

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A Comprehensive Validation of Global Precipitation Measurement Satellite Products Over a Western Mediterranean Region