Dry spell representation on regional and global scale using convection-permitting models within the nextGEMS project

As the climate continues to warm, hydrometeorological extremes are extracting a greater toll from society both economically and socially. The need for accurate extreme event projections during acute dry spells was recently highlighted by the January 2025 devastating wildfires in the Los Angeles region. Current CMIP-style global climate models broadly project an increasing frequency and intensity of heavy precipitation and drought. However, the relatively coarse resolution, lack of ocean-atmosphere coupling, and parameterization of convection means they do not capture the spatial heterogeneity and mesoscale processes of complex coasts and topography relevant for simulating extreme events which often introduces model biases.

The ongoing H2020 Next Generation Earth Modelling Systems (nextGEMS) project aims to address these issues with the development of convection-permitting, fully-coupled, Earth-system models. Using the ECMWF Integrated Forecast System (IFS) and Icosahedral Nonhydrostatic Weather and Climate Model (ICON), we examine detailed dry spell characteristics in the Mediterranean region of Europe and then expand our analysis globally. These results are compared against a suite of observations (station and satellite based), reanalysis datasets, and CESM2 simulations.

Using ICON and IFS with about 6 km and 4 km spatial resolution, respectively over a five-year period in the Mediterranean, we find the increased resolution and hybrid/explicit representation of convection improves the representation of dry hour frequency and alleviated the long-standing drizzle bias observed in many GCMs, here illustrated for CESM2. For simulating the maximum length of dry spells over land, switching off the convective parameterization scheme in ICON improves accuracy with similar dry spell lengths as observations and reanalysis. However, the annual maximum length of dry spells over the sea for both ICON and IFS is excessive by 30-50 days. The depiction of dry spells in the Mediterranean region is representative of the nextGEMS’ models performance across the whole mid-latitudes. Ongoing research using recently developed 30-year transient ICON and IFS simulations (2020-2050) looks to investigate how dry extremes evolve globally in a warming world.