Browsing by Autor "V. Hernandez"

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Integrated Water Vapour (IWV) trend analysis from GNSS and NWP reanalyses: a homogenised long-term analysis over Granada
(2026) V. Hernandez; Arlett Díaz Zurita; Onel Rodríguez Navarro; Jorge Andrés Muñiz Rosado; Daniel Pérez Ramírez; David N. Whiteman; Lucas Alados Arboledas; Francisco Navas Guzmán
In a context of climate change and global warming, the characterisation and operational monitoring of greenhouse gases is of uppermost importance for implementing mitigation strategies that could help to reduce the impact of the current climatic emergency in the surrounding ecosystems and society. Among these gases, water vapour can contribute to almost a 60% of the total greenhouse effect. Moreover, its interaction with solar and infrared radiation or its main role in cloud formation, make water vapour a key driver of most atmospheric thermodynamic processes and a crucial component of the Earth's radiative budget. Nevertheless, the large spatial and temporal variability of water vapour hinders the acquisition of reliable operational measurements. Remote sensing techniques such as the Global Navigation Satellite System (GNSS) have been proven to be an accurate and trustworthy alternative for integrated water vapour (IWV) retrievals, providing a valuable platform for continuous operational monitoring and thus enabling long-term characterisation. To further address this challenge, reanalysis data from Numerical Weather Prediction (NWP) models can significantly increase the temporal and spatial coverage of atmospheric variables datasets. In particular, ERA5 (fifth generation of European Centre for Medium-Range Weather Forecasts reanalysis) and MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, version 2) provide validated data for the city of Granada, in southeastern Spain, since 1980.The current study presents a comprehensive analysis of IWV trends retrieved from a 15-year GNSS database and an extended 45-year reanalysis dataset. Special attention is paid to time-series quality control and homogenisation. Small jumps or discontinuities due to GPS receiver updates or changes in the data assimilation strategies of NWP models, can introduce artificial artifacts in the time series and consequently lead to biased or misleading trend esimates. A modified Mann-Kendall test proposed by Coen et al. (2020) that applies a Variance-Corrected Trend-Free Pre-Whitening approach is evaluated against a General Least Square method with a full custom covariance matrix accounting for residual heteroscedasticity and autocorrelation. While both methodologies agree on the sign and uncertainties of the retrieved trends, some discrepancies are found in the magnitudes, reflecting the different nature of both algorithms and highlighting the sensibility of trend detection techniques. Positive increasing IWV trends of a 3% per decade on average were obtained from both datasets and algorithms, being significant to a 95% level when analysing the 45-year time series. Nonetheless, relevant behaviour differences are found between the 1980-2000 and 2000-2024 periods, unveiling the pronounced increasing in IWV experimented during the last 25 years. The results obtained are consistent with previous studies, both regarding the trend magnitude and the uncertainty range, reinforcing the capability of the GNSS technique and NWP models as robust tools for environmental and atmospheric monitoring of complex variables such as water vapour (Parracho et al., 2018; Yuan et al., 2023). However, they also unveil trend discrepancies which are inherent to the chosen retrieval methodologies and that must always be assessed.
Long-term analysis of Raman lidar water vapour profiles over the ACTRIS AGORA Granada station
(2026) Arlett Díaz Zurita; V. Hernandez; David N. Whiteman; Onel Rodríguez Navarro; Jorge Andrés Muñiz Rosado; Daniel Pérez Ramírez; Lucas Alados Arboledas; Francisco Navas Guzmán
Water vapour is a crucial and highly variable greenhouse gas in the Earth's atmosphere that plays a major role in the radiative balance, energy transport and photochemical processes. It can also affect the radiative budget indirectly through cloud formation and by altering the size, shape, and chemical composition of aerosol particles. Moreover, monitoring water vapour remains challenging due to its high temporal and spatial variability. Consequently, systematic and accurate observations of water vapour are essential to improve our understanding of its role at both local and global scales and for enhancing climate projections.Advances in remote sensing techniques have enabled continuous acquisition of precipitable water vapour (PWV) measurements using sun/star photometry, microwave radiometry and the Global Navigation Satellite System (GNSS). Nevertheless, none of these instruments provides information on the vertical distribution of water vapour, a critical information considering that water vapour concentrations typically vary by up to three orders of magnitude between the surface and the upper troposphere. In this context, Raman lidar has demonstrated its ability to capture the spatial and temporal evolution of water vapour in the troposphere. Accurate retrievals of the water vapour mixing ratio from Raman lidar measurements rely on robust and well-characterised calibration procedures as well as on an accurate estimation of the differential atmospheric transmission term, which accounts for extinction differences between the molecular reference (nitrogen and oxygen) and water vapour wavelengths.In this study, the lidar calibration constant was determined using a hybrid calibration method, which combines correlative PWV measurements for lidar calibration with Numerical Weather Prediction (NWP) data to reconstruct the water vapour profile within the incomplete overlap region of the lidar system. The differential transmission was estimated using an automated method to account for the aerosol contribution, based on sun photometer Aerosol Optical Depth (AOD) measurements and an exponential decay function with attitude to model aerosol extinction (Díaz-Zurita et al., 2025). Subsequently, a long-term database of water vapour profiles over the period 2009-2022 was generated, providing high vertical and temporal resolution measurements of water vapour over the city of Granada, in Southern Spain. A comprehensive statistical analysis was conducted to characterise the vertical distribution of water vapour over a 14-year period, representing the first long-term vertical characterisation of water vapour in this region. Mean interannual and seasonal water vapour profiles were derived for the entire study period, and trend analyses were performed to assess long-term variations in water vapour content in the lower troposphere. Additionally, lidar-derived PWV values were compared with those obtained from microwave radiometer and GNSS observations.This research was funded by Grant PID2021-128008OB-I00 funded by MICIU/AEI/ 10.13039/501100011033 by ERDF/EU European Union, and by the Spanish national projects CNS2023-145435, PID2023-151817OA-I00 and Marie Skłodowska-Curie Staff Exchange Actions with the project GRASP-SYNERGY (grant agreement no. 10113163). Diaz-Zurita et al. (2025). Remote Sens. 2025, 17(20), 3444; https://doi.org/10.3390/rs17203444