Intercomparison of Regional Reanalysis Datasets over Italy and Hydrological Applications for the Electro-energy System

The study of climate and of extreme events requires not only the analysis of the future projections, but also the assessment and knowledge of past and present conditions. Reanalysis allows the users to obtain spatially distributed modelled data for a long and consistent period of time with integrated observations, giving an estimate of the atmospheric variables that can be used in multiple applications and among different fields. This study provides an example of regional reanalysis applications, touching the different fields of meteorology, hydrology, and their impacts on the electric and energy sector.

In Italy, the transmission system operators and distribution companies have repeatedly stressed the need to have a reliable and updatable weather dataset with a history of at least 20 years, in order to characterize the extreme events responsible for the impacts on the electric system, to improve the resilience of the system itself, and to quantify the availability of renewable resources (e.g., hydroelectric, wind, solar). From 2019, RSE SpA has developed a reanalysis dataset called MERIDA that is able to represent the atmospheric conditions of Italy at a resolution of 7 km, one hour time step, over a period from 1986 to 2022. MERIDA, being a regional reanalysis, allows the representation of fine-scale processes (such as interaction with orography, land-sea interactions) that global models struggle to reproduce. MERIDA uses the WRF model to dynamically downscale the ERA5 global reanalysis (developed by the European Centre for Medium-Range Weather Forecasts (ECMWF)), integrating modelled values with the observations from the official Italian gauge network, using statistical post-processing techniques. The MERIDA products have been evaluated for the main variables of precipitation and surface air temperature with other Italian and European reanalysis datasets (Copernicus European Regional ReAnalysis (CERRA), developed by the European Copernicus programme, Very High Resolution REAnalysis for ITaly (VHR_REA), provided by the Fondazione Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC)) that are also a downscaling of ERA5, and with gridded observational datasets.

Results show that regional reanalyses can in general reproduce the observed climatology of surface air temperature and precipitation over Italy across different spatial scales. Although the global reanalysis ERA5 totaled the best field-averaged skills in terms of bias and MAE for temperature, higher resolution reanalyses were able to simulate more challenging small-scale processes for both temperature and precipitation, with an enhanced spatial description of the climatological variability over complex terrain (e.g., peaks and valleys alternation) and were able to better reproduce the extremes. Overall, MERIDA reanalysis showed statistical scores that were outstanding compared with other regional reanalyses and the observations.

The final target of this comparison was dual: on one hand it was investigating the added value of using regional downscaled reanalysis vs. the ERA5 global reanalysis to represent the complex climatology and the extreme events in Italy; on the other hand, the purpose is to assess the performance of the MERIDA reanalyses product to use it as a forcing for multiple applications for the electro-energy system. Specifically, in the interest of pushing the modelling skills toward a more integrated hydro-meteorological modelling approach, development is ongoing to use the MERIDA atmospheric reanalysis to force the WRF-Hydro distributed hydrological model to create a 30-year hydro-meteorological retrospective dataset at 250 m resolution over Italy, to reconstruct the impacts of extreme atmospheric events on surface hydrology, river network, and reservoirs. Some test cases of the calibration and validation of the WRF-Hydro model over the Tiber River Basin in the central part of Italy are shown, suggesting promising results in extending the methodology to the rest of the Italian territory.