Cost-benefit analysis of the nesting approach in HARMONIE-AROME for a supercell outbreak case study

Supercells are among the most hazardous convective systems, frequently producing large hail, destructive winds, and severe socio-economic impacts. The enhancement of weather simulations is identified as a primary strategy to optimise short-term forecasting. The present study investigates the performance of two high-resolution configurations of the HARMONIE-AROME model during a severe supercell outbreak over eastern Iberia on 31st July 2015, when six confirmed supercells caused significant damage. The setups tested include a two-step one-way nested approach (2.5 km outer domain and 500 m inner domain), and a single-domain configuration at 500 m resolution. The model outputs, which include reflectivity, precipitation and temperature, are validated against OPERA radar composites and surface observations. At the same time, key convective parameters, derived from the Murcia sounding, are analyzed to assess the pre-convective environment. Although the simulations demonstrate a similar structure to the observed event, the two-domain nested simulation offers a slightly superior depiction of reflectivity and thermodynamic profiles. Nevertheless, precipitation analysis reveals that while nesting improves moderate rainfall representation, it introduces larger errors for the most extreme amounts, limiting its overall benefit. The obtained gain is not sufficient to offset the 30% higher computational cost when the two-domain nested approach is used. The single-domain non-nested configuration demonstrates a superior level of efficiency, exhibiting equivalent accuracy while exhibiting a diminished resource requirement.