Extreme windstorms and sting jets in convection-permitting climate simulations over Europe

Colin Manning, Elizabeth J. Kendon, Hayley J. Fowler, Nigel M. Roberts, Ségolène Berthou, Dan Suri, Malcolm J. Roberts

Climate Dynamics
Work Package 3
Link: https://doi.org/10.1007/s00382-021-06011-4

Highlights

Severe windstorms are one of the costliest forms of extreme weather seen in Europe, causing billions of Euros of damage in recent decades. High-resolution climate models have shown promise in more accurately simulating extreme weather events such as these. This study assessed the performance of a high-resolution Convection-Permitting Model (CPM) at simulating extreme windstorms using past data and future projections. The team found that the CPM accurately simulated wind gusts from 15 past storms, while a lower-resolution model underestimated the frequency of extreme wind speeds compared to the CPM. A large increase in the frequency of these events was projected for the future, with the CPM providing more detailed, reliable simulations than its lower-resolution counterpart. Results like this show how useful higher-resolution models can be in projecting future extreme weather, helping inform policies aimed at reducing the impact of these events.

Background
Extra-tropical windstorms are one of the costliest natural hazards that can impact Europe. Many of the most damaging storms are Shapiro-Keyser storms, some of which develop a particularly damaging sting jet. Storms like this have caused billions of Euros worth of damage in recent decades, so understanding more about their future likelihood is potentially of great benefit to climate risk planning. The resolution of climate models has been an obstacle to the accurate simulation of sting jets in the past, however this study utilises new advances to address this shortcoming. Convection-Permitting Models (CPMs) are high-resolution models of the Earth’s climate able to simulate atmospheric convection, a key process in many extreme weather events, rather than using pre-determined convection values. Improved computer power has made the use of these complex models in climate projection more viable. This study assesses the value that a CPM can add to simulations of extreme windstorms, including future projections.

Results
The team found that their CPM was able to accurately simulate the observed maximum wind gusts from 15 extreme windstorms recorded in the past. The simulations were also more accurate than the ERA Interim reanalysis, which blends observations and modelling to increase coverage. A lower-resolution Global Climate Model (GCM) underestimated the frequency of extreme wind speeds compared to the CPM, as the CPM had better simulations of cold conveyor belts and sting jets, two major sources of extreme winds within Shapiro-Keyser cyclones. The CPM’s greater resolution was its key asset here, allowing it to more closely simulate the temperature and pressure gradients that result in higher wind speeds, as well as key processes related to sting jets. The team were able to use these improved simulations to develop an automated method to detect Shapiro-Keyser cyclones, and those that develop sting-jets, in both historical and future simulation data. A large increase in the frequency of extreme windstorms was found in the future simulations, with more reliable and detailed results given by the CPM over the GCM. Results such as this are useful to communities and policymakers in preparing for these dangerous climatic events and limiting their impact.

Methods
This study utilised the UK Met Office’s Unified Model for its CPM, run at 2.2 km resolution. This was used to run a hindcast simulation of the past covering 1999-2018, driven by ERA Interim reanalysis data. A control simulation with current climate forcing was run over 1998-2007, while a future simulation was run over 2091-2100 assuming a high-emissions future scenario (RCP8.5). These simulations were forced by the HadGEM3 GCM run at 25 km resolution. All these simulations were analysed and compared with each other to identify differences between them and determine the potential added value of the higher-resolution CPM.

Policy relevance
These results help inform future policy around preparing for future extreme windstorms, projecting an increase in their frequency. It also proves the utility of using higher-resolution models in projections of extreme weather events. Projections like this are key to helping inform policies aimed at reducing the impact of these damaging events.