The Role of Water Vapour in Shaping Mediterranean Summer Climate: Findings from MESSA-DIN 2021 measurement campaing in southern Italy

Water vapour fluxes, originating mainly from the Atlantic, North Africa, and the Mediterranean region, play a critical role in shaping the climate dynamics of the Mediterranean Basin, especially during the summer months. These fluxes significantly influence relative humidity levels in the troposphere, affecting both local and regional weather patterns, such as intense rainfall events and prolonged droughts, while also contributing to the amplification of heatwaves through enhanced surface radiation trapping. This study uses observational data collected during the Mediterranean Experiment for Sea Salt and Dust Ice Nuclei (MESSA-DIN) from July to September 2021 in Soverato, southern Italy, to characterise the synoptic conditions of the severe summer of 2021.

A combination of ground-based remote sensing instruments revealed intense and persistent water vapour transport in the mid-troposphere. ERA5 data were used to identify the moisture dynamics over the Mediterranean Basin. The comparison between ERA5 reanalysis data and ground-based measurements further highlighted discrepancies in the representation of water vapour, particularly a dry bias in relative humidity in the range between 500 hPa and 300 hPa. While ERA5 provided a coherent and detailed representation of synoptic patterns and showed general agreement in the time evolution of the atmospheric vertical structure with observations, it exhibited a dry bias in relative humidity (RH) values compared to a ground-based microwave profiler (MWP). However, the magnitude of the bias also depends on the bias affecting the MWP retrieval, typically within 10-15% RH in the mid-troposphere. ERA5 also overestimates the presence of both cold and warm clouds, while ground instruments detected much less frequent cloud cover. This emphasizes the need for improving reanalysis performance in complex coastal and orographic settings. The bias in ERA5 was further assessed using GRUAN data from the Potenza station and regular upper-air data from Mediterranean stations.

The study underscores the importance of ground-based measurements, such as those from microwave radiometers, in improving weather forecasts for extreme events. Despite their lower vertical resolution, these instruments—both on their own and when combined with higher-resolution measurement techniques such as Raman lidars and upper-air soundings—provide continuous, real-time measurements of atmospheric water vapour. These measurements are essential for enhancing our understanding of water vapour fluxes and their impact on cloud formation, as well as for improving the accuracy of high-resolution forecasting models, especially in the representation of extreme weather events in the Mediterranean and Central Europe.