Flood Risk and Atmospheric Internal Variability: A Multiscale Study over Ireland

Based on all the scenarios of future projections in climate change studies, a significant increase in flood risk through most of the main Irish catchments is observed. On the other hand, an increase in temperature with a less clear trend in precipitations are expected to be observed in the atmospheric annual cycle. However, these issues are not enough to explain the expected increase in the flood risk observed by most of climate change modeling studies. Most of the uncertainties in the projected flood risks either in extents or frequency are due to the Global Circulation Model (GCM) more than to the hydrological model selection and parameterization. The studies involving simulations for the past climate show that models appear to agree best with the observed maximum flows when the driving meteorology is precipitation at synoptic scale. Base on that, this work aims to understand the dependency of surface runoff and precipitation in Ireland on the atmospheric internal variability. To decouple the internal variability component, the Community Earth System Model (CESM) is run. This model allows multi-year simulations with prescribed annual variability for the ocean and ice component, as well as the chemistry variables. Therefore, state of the atmosphere of the multi-year run is the evolution of the internal variability. In this work, a ten years long simulation is run with 1-degree resolution. To downscale these results for Ireland, the WRF-ARW model is run with the CESM output as initial and boundary conditions for 7 days at 2 km resolution. This approach aims to gain a better understanding of the parameters linked to flood risk in Ireland, and to assess its dependency to the atmospheric internal variability.