Less frequent but more intense summertime precipitation in Finland: results from a convection-permitting climate model

Changes in short-term precipitation events have significant local impacts of climate change, yet are poorly captured by coarse-resolution global climate models. We analyse the projected changes in warm season precipitation events in Finland from a convection-permitting regional climate model HARMONIE-Climate, operating at 3-kilometer resolution. Realistic modeled precipitation characteristics are verified against multiple observational datasets for 1986–2018, and projected changes in precipitation events are analyzed until 2041–2060 and 2081–2100.

Our results show that all simulations agree on an increase in mean wet hour precipitation intensity and a decrease in the number of wet hours. The proportion of wet hours with respect to the all hours (calculated from the whole area of Finland) decrease from 11–13 % to 9–11 % by mid-century, and further reducing to around 9 % across all simulations and scenarios by late century. We also find that as climate change proceeds, the frequency of precipitation events over 2 mm h⁻¹ increases and the changes become greater for the categories of higher intensities, while lower intensity events become less frequent.

Of particular interest are the projected changes in intensity categories used in alert classification by the Finnish Meteorological Institute, wherein heavy rain is identified at a threshold of 7 mm h⁻¹, and the national alert level for potentially dangerous rainfall is set at 20 mm h⁻¹. According to the simulations, the frequency of such events in Finland will increase greatly as the climate change proceeds and their contribution to overall wet hours increase. In a strong climate change scenario (RCP8.5), extremely heavy precipitation exceeding 20 mm h⁻¹ will become twice to three times as common (three to six times) in 2041–2060 (2081–2100) compared to 1986–2005, while simultaneously the total number of wet hours is projected to decrease by 12–16 % (18–25 %).