Simulated temporal scaling dependencies in sub-daily precipitation

Abstract. Rainfall intensity-duration-frequency (IDF) curves are an essential tool in water management, for instance in urban stormwater handling. They are commonly derived by fitting generalised extreme value distributions to observed annual maximum rainfall values, a process that requires long-term, high temporal resolution (sub-daily) observations of precipitation to ensure robust estimates. Alternatively, IDF curves can be approximated with simplified parametric mathematical expressions fitted to empirical data, providing a possibility to avoid the challenging data requirements. In this case, the parametric expression can be based on two key parameters that specify the shape of the curves: the wet-spell mean precipitation μ and the wet-spell frequency f_w. For these two parameters, robust estimates are easier to obtain.

The resulting parametric IDF curves exhibit a fractal dimension and the present study takes a step towards better understanding the conditions influencing this fractal dimension and its spatial and temporal variability. To this end, we explore the dependencies across different timescales, using hourly precipitation data from convection-permitting (3 km) regional climate model simulations carried out with the HCLIM model over northern Europe. The analysis is applied to HCLIM simulations driven by boundary conditions from the ERA-Interim reanalysis, as well as from the EC-Earth and GFDL-CM3 global climate models for current and future climates following the RCP8.5 scenario.

We find that the relationship between wet-spell mean precipitation for different durations, and hence the sub-daily fractal dimension, is influenced by geographical conditions, as is also the wet-spell frequency. The results are consistent across different boundary conditions representing current climate conditions (reanalysis and global climate models), and showed little sensitivity to the driving model, indicating that different meteorological phenomena prevail in different regions and that these are well represented in the models. Future climate projections show changes in the fractal dimension and wet-spell frequency ratios with a general north-south gradient. Overall, the models indicate a shift towards fewer, but more intense wet-hours per wet day.