Can we trust climate models to realistically represent severe European windstorms?

Tomasz M Trzeciak,Peter Knippertz,Jennifer S R Pirret,Keith D Williams

doi:10.1007/s00382-015-2777-9

Tomasz M Trzeciak, Peter Knippertz + Show 2 more

Open Access

https://doi.org/10.1007/s00382-015-2777-9

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Abstract

Cyclonic windstorms are one of the most important natural hazards for Europe, but robust climate projections of the position and the strength of the North Atlantic storm track are not yet possible, bearing significant risks to European societies and the (re)insurance industry. Previous studies addressing the problem of climate model uncertainty through statistical comparisons of simulations of the current climate with (re-)analysis data show large disagreement between different climate models, different ensemble members of the same model and observed climatologies of intense cyclones. One weakness of such evaluations lies in the difficulty to separate influences of the climate model’s basic state from the influence of fast processes on the development of the most intense storms, which could create compensating effects and therefore suggest higher reliability than there really is. This work aims to shed new light into this problem through a cost-effective “seamless” approach of hindcasting 20 historical severe storms with the two global climate models, ECHAM6 and GA4 configuration of the Met Office Unified Model, run in a numerical weather prediction mode using different lead times, and horizontal and vertical resolutions. These runs are then compared to re-analysis data. The main conclusions from this work are: (a) objectively identified cyclone tracks are represented satisfactorily by most hindcasts; (b) sensitivity to vertical resolution is low; (c) cyclone depth is systematically under-predicted for a coarse resolution of T63 by both climate models; (d) no systematic bias is found for the higher resolution of T127 out to about three days, demonstrating that climate models are in fact able to represent the complex dynamics of explosively deepening cyclones well, if given the correct initial conditions; (e) an analysis using a recently developed diagnostic tool based on the surface pressure tendency equation points to too weak diabatic processes, mainly latent heating, as the main source for the under-prediction in the coarse-resolution runs. Finally, an interesting implication of these results is that the too low number of deep cyclones in many free-running climate simulations may therefore be related to an insufficient number of storm-prone initial conditions. This question will be addressed in future work.

Highlights

Windstorms associated with intense wintertime cyclones from the North Atlantic Ocean are among the most frequent and most devastating natural hazards affecting Europe (Munich 2009; Schwierz et al 2010)
The hindcasts were started from 96 h (4 days) to 18 h before the minimum core pressure in the reanalysis
The reference tracks are determined from data in the original resolution of T255 and from the same data coarse-grained to T63 in order to match the climate model data

Summary

Introduction

Windstorms associated with intense wintertime cyclones from the North Atlantic Ocean are among the most frequent and most devastating natural hazards affecting Europe (Munich 2009; Schwierz et al 2010). Despite all recent progress in climate change research for variables like near-surface temperature, confidence in projections of storm tracks, cyclone intensity and extreme winds in the northern hemisphere remains low according to the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC; Christensen et al 2013) and the UK Climate Projections (UKCP 2009) This low confidence is related to concerns over the skill of many models in realistically representing large-scale features such as the stratosphere (Scaife et al 2012; Manzini et al 2014) and ocean circulation (Woollings et al 2012) as well as the dynamics of individual storm systems, for example due to insufficient horizontal resolution (Willison et al 2013). The consensus between different models on a climate change signal remains rather low (Harvey et al 2012; Zappa et al 2013b)

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