Europe-wide climate change projections at convection permitting scale with the Unified Model

Publications

Steven C. Chan, Elizabeth J. Kendon, Ségolène Berthou, Giorgia Fosser, Elizabeth Lewis, Hayley J Fowler

Climate Dynamics
Work Package 3
Link: https://doi.org/10.1007/s00382-020-05192-8

 

Highlights

Accurately predicting future climate is essential to putting effective adaptation and mitigation plans in place to limit the impacts of climate change. High-resolution, kilometre-scale models provide more realistic representation of rainfall events, especially extreme ones, however supercomputers have only recently become powerful enough to run them over large areas. This study uses such a high-resolution model to project future changes in climate over the whole of Europe, finding greater changes in extreme rainfall than are currently predicted using lower-resolution models. These more accurate predictions allow better preparations to be made to limit the impact of changes to the climate on society.

Background
Convection-Permitting Models (CPMs) are high-resolution models of the Earth’s climate able to improve the simulation of atmospheric convection, a key process in many extreme weather events. They are widely used in weather forecasting, but they require large supercomputing resources, limiting their use in longer-term climate modelling. When applied over a small area CPMs can also suffer from errors introduced by the proximity of their own boundaries. These give insufficient time to properly simulate small scale features which have originated outside the model area before they reach an area of interest. Improvements in our global and regional climate models, as well as increases in supercomputing capacity, have now made it feasible to use a CPM to simulate future climate across the whole of Europe, potentially giving more insight into future weather events than we currently possess. This is what this study aims to do, as well as comparing the results with existing CPM projections for the UK that may suffer from small-area errors.

Results
The CPM used here predicts a greater increase in extreme rainfall across Europe than the lower-resolution global climate model used before. Northern Europe is predicted to see an increase in extreme and overall rainfall in autumn and winter, with drier summers on average, but with a slight increase in extreme rainfall levels. The CPM accurately modelled the western Mediterranean’s autumn peak in extreme hourly rainfall, validating the model against observations, as well as projecting an increase in the future. The model shows that the largest changes in overall and extreme event rainfall will occur in autumn and winter across the continent. This shifts the peak season for extreme hourly rainfall towards autumn. For the UK, the new 2.2 km-scale CPM simulations show a slightly larger summer decrease in mean rainfall than the existing 1.5 km-scale simulations. A larger difference is found for winter, in which mean percentage increases are more than double those predicted in the older 1.5 km simulations. The new simulations also more accurately represent UK rainfall extremes, which were subject to small-area errors and are sensitive to model scale.

Methods
This study ran the Met Office Unified Model at a resolution of 2.2 km over the whole of Europe, a much larger area than normal for CPMs, focussing in detail on Germany, the UK and the western Mediterranean. The model incorporated new global climate model data and new rainfall observational datasets. This model’s future projections were compared against existing simulations of future UK climate at 1.5 km resolution.

Policy relevance
The results of this study help improve high-resolution prediction of European climate, allowing more accurate prediction of future weather extremes and better understanding of future trends. This helps people, businesses and public bodies to better prepare for future climate extremes and limit their impact.

Abstract

For the first time, we analyze 2.2 km UK Met Office Unified Model convection-permitting model (CPM) projections for a pan-European domain. These new simulations represent a major increase in domain size, allowing us to examine the benefits of CPMs across a range of European climates. We find a change to the seasonality of extreme precipitation with warming. In particular, there is a relatively muted response for summer, which contrasts with much larger increases in autumn and winter. This flattens the hourly extreme precipitation seasonal cycle across Northern Europe which has a summer peak in the present climate. Over the Western Mediterranean, where autumn is the main extreme precipitation season, there is a regional increase in hourly extreme precipitation frequency, but local changes for lower precipitation thresholds are often insignificant. For mean precipitation, decreases are projected across Europe in summer, smaller decreases in autumn, and increases in winter; comparable changes are seen in the driving general circulation model (GCM) simulations. The winter mean increase is accompanied by a large decrease of winter mean snowfall. Comparing the driving GCM projections with the CPM ones, the CPMs show a robust enhanced intensification of precipitation extremes at the convection-permitting scale compared to coarser resolution climate model projections across various European regions for summer and autumn.