Abdullah Kahraman, Elizabeth J. Kendon, Steven C. Chan, Hayley J. Fowler
Using very detailed climate model simulations with a 2.2 km grid, this study investigates how climate change may affect intense rainstorms across Europe. Simulations using the high emissions RCP8.5 scenario show a significant future increase in the occurrence of slow-moving, intense rainstorms. These can lead to very high rainfall accumulations as high intensity rain combines with increased duration of exposure locally. This increase in slow-moving storms is likely related to a reduced temperature difference between the poles and the tropics due to global warming, which weakens upper-level winds in the autumn when these intense rainstorms occur most frequently. These findings suggest that these slow-moving storms may be 14 times more frequent across land by the end of the century. Slower storm movement acts to increase the amount of rainfall that accumulates locally, increasing the risk of flash floods across Europe beyond what was expected based on previous studies.
Intense rainstorms are expected to be more frequent due to climate change because warmer air can hold more moisture. Currently, almost stationary intense rainstorms are uncommon in Europe and happen rarely over parts of the Mediterranean Sea. Accurate predictions of future changes in intense rainfall events are key to putting effective adaptation and mitigation plans in place to limit the adverse impacts of climate change. Convection Permitting Models (CPMs) are routinely used for weather forecasting and can better forecast localised extremes, as they are able to capture small-scale processes in the atmosphere, such as the development of storms, not captured by coarser-resolution models. Their use in climate change studies has been limited until now due to their high computational cost. This study uses new CPM climate projections spanning Europe as a whole, carried out at the Met Office.
The team’s results show that storms producing intense rain may move slower with climate change, increasing the duration of exposure to these extremes. Slow-moving storms may be 14 times more frequent across land by the end of the century. This is greater than the projected increase in the frequency of all environments that have the potential for extreme rainfall rates – an increase of around 7 times by 2100 compared to today – suggesting a slowing down of storms. The slower movement of storms that produce intense rain in the future appears to be caused mainly by a reduced temperature difference between the poles and the tropics due to global warming, which weakens upper-level winds in the autumn, when these intense rainstorms occur most frequently. Slower storm movement acts to increase the amount of prolonged heavy rainfall locally, increasing the risk of flash floods across Europe beyond what was expected based on previous studies.
The study applies a new approach from severe weather forecasting to analyse pan-European convection permitting climate simulations using the high emissions RCP8.5 scenario with a 2.2km grid from the UK Met Office Unified Model (v10.1). Environments that have the potential for extreme rainfall rates were calculated based on thresholds of humidity and vertical velocity (how fast air is rising) for each grid cell.
These findings are relevant to climate mitigation and adaptation policy in Europe, with specific implications for future flooding impacts, the design of infrastructure systems, and the management of water resources.
Under climate change, increases in precipitation extremes are expected due to higher atmospheric moisture. However, the total precipitation in an event also depends on the condensation rate, precipitation efficiency, and duration. Here, a new approach following an “ingredients-based methodology” from severe weather forecasting identifies important aspects of the heavy precipitation response to climate change, relevant from an impacts perspective and hitherto largely neglected. Using 2.2 km climate simulations, we show that a future increase in precipitation extremes across Europe occurs, not only because of higher moisture and updraft velocities, but also due to slower storm movement, increasing local duration. Environments with extreme precipitation potential are 7× more frequent than today by 2100, while the figure for quasi-stationary ones is 11× (14× for land). We find that a future reduction in storm speeds, possibly through Arctic Amplification, could enhance event accumulations and flood risk beyond expectations from studies focusing on precipitation rates.