Long-Term Integrated Water Vapour (IWV) analysis in Southern Spain using remote sensing techniques and reanalysis models

Water vapour (WV) is one of the most significant greenhouse gases and plays a critical role in the majority of the thermodynamic processes that occur within the atmosphere. Thus, it significantly influences the radiative budget and cloud formation mechanisms, being of paramount importance in weather forecasting. Therefore, accurate and detailed characterisation of its spatial and temporal distribution is of undoubtedly great interest. However, measurement techniques often struggle with its variability both in space and time, making it challenging to obtain regular and reliable measurements. 

Although high resolution height-resolved profiles of water vapour mixing ratio are currently being acquired by lidar systems and providing powerful information, such instruments usually suffer from overlap issues in the lower hundred meters, precisely where greater concentrations of water vapour appear. This issue, together with the reduced global representativity due to the scarce number of operative lidar systems, hinders the use of this technique for continuous monitoring of water vapour near the ground. In contrast, other passive and active remote sensing techniques like Microwave Radiometers (MWR), Sun Photometers (SP) or Global Navigation Satellite System (GNSS) are well established and have been globally proven as a feasible and trustworthy alternative for continuous measurements of the total vertical column water vapour concentration (IWV). 

This study addresses the characterisation of IWV over Granada, a city in southern Spain, using remote sensing techniques (MWR, SP and GNSS). These techniques are first validated against in situ data collected from over 70 radiosondes (RS). The study then investigates the IWV evolution over Granada for a 14-year period. The daily, seasonal and annual cycles are described together with the statistical behaviour of the data series in search of tendency changes.

Reanalysis data from Numerical Weather Prediction (NWP) models such as MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, version 2) and ERA5 (fifth generation of European Centre for Medium-Range Weather Forecasts reanalysis) are also validated against remote sensing measurements and then considered to expand the study period to more than 40 years, allowing the climatological study of water vapour in the area. Seasonal decomposition and a Mann-Kendall statistical test discovered an increasing tendency in IWV. Analogous analysis for the temperature in the region also found a positive increase, accentuated since the beginning of the 21st century and reinforcing the results of climate change studies. The relationship between both magnitudes indicates a possible contribution of increased water vapour concentrations to the observed increased temperatures.