The role of grid resolution, latent heat release, and initialisation time on the simulation of Storm Éowyn and its impacts

Abstract. Extratropical cyclones are primary drivers of damaging winds, heavy precipitation, and coastal flooding in the mid-latitudes. Accurately predicting their track and intensity is important for early warnings, but rapidly-deepening extratropical cyclones can pose a severe forecasting challenge due to their explosive growth, fast propagation, and extreme surface weather conditions. Capturing this rapid development in numerical weather prediction model simulations depends on grid resolution, initialisation time, as well as on the representation of small-scale diabatic processes, in particular latent heating (LH). In this study, we analyse how these factors influence the track, intensity, and surface hazards of the explosively-deepening North Atlantic Storm Éowyn, which occurred in January 2025 and caused extensive wind damage across the British Isles. Using the ICOsahedral Nonhydrostatic (ICON) model in limited-area mode, we perform sensitivity experiments with varying horizontal resolution and forecast lead time. Additionally, we run experiments with scaled LH to isolate its specific role in the development of this high-impact cyclone. The ICON simulations reveal that horizontal resolution plays a critical role for cyclone intensity, with finer grid spacings producing lower central mean sea level pressure values that align more closely with observations. The higher cyclone intensity at higher resolution is linked to a stronger vertical potential vorticity (PV) tower and translates into amplified surface winds and precipitation. With increased resolution, cloud-diabatic processes produce higher low-level PV values, which are associated with more intense surface wind speeds. When LH is fully suppressed, the resulting cyclone is significantly weaker, indicating a diabatic deepening contribution to cyclone intensity of 12 % to 53 %, depending on the forecast initialisation time. The cyclone in the experiments without LH exhibits a distinct northward shift and reduced eastward propagation, such that the cyclone misses the British Isles. The reason for this behaviour in the experiments without LH is an altered steering flow due to a weakened upper-level jet and the absence of downstream ridge building. Finally, initialisation time introduces a non-monotonic sensitivity: initialisation several days prior to cyclogenesis generally yields an underestimation of cyclone intensity and surface winds. Simulations with specific initialisation times substantially underestimate intensity across different model resolutions, highlighting a high sensitivity to early-stage initial conditions and numerical challenges in the convective 'grey zone'. Nevertheless, considering the operational ECMWF ensemble, the spread in cyclone intensity includes the observed evolution already when initialised five days before cyclogenesis. While this study examines a single high-impact cyclone, the large effects of grid resolution, initialisation time, and LH on its representation emphasizes the complex challenges in modelling explosive cyclones and highlights how sensitive these forecasts are to the specific configuration of the simulations.