Temporal Characteristics of Heat Waves and Cold Spells and Their Links to Atmospheric Circulation in EURO-CORDEX RCMs

Eva Plavcová,Jan Kyselý

doi:10.1155/2019/2178321

Abstract

We study summer heat waves and winter cold spells and their links to atmospheric circulation in an ensemble of EURO-CORDEX RCMs in Central Europe. Results of 19 simulations were compared against observations over 1980–2005. Atmospheric circulation was represented by circulation types and supertypes derived from daily gridded mean sea level pressure. We examined observed and simulated characteristics of hot and cold days (defined using percentiles of temperature anomalies from the mean annual cycle) and heat waves and cold spells (periods of at least three hot/cold days in summer/winter). Although the ensemble of RCMs reproduces on average the frequency and the mean length of heat waves and cold spells relatively well, individual simulations suffer from biases. Most model runs have an enhanced tendency to group hot/cold days into sequences, with several simulations leading to extremely long heat waves or cold spells (the maximum length overestimated by up to 2-3 times). All simulations also produce an extreme winter season with (often considerably) higher number of cold days than in any observed winter. The RCMs reproduce in general the observed circulation significantly conducive to heat waves and cold spells. Zonal flow reduces the probability of temperature extremes in both seasons, while advection of warm/cold air from the south-easterly/north-easterly quadrant plays a dominant role in developing heat waves/cold spells. Because of these links, the simulation of temperature extremes in RCMs is strongly affected by biases in atmospheric circulation. For almost all simulations and all circulation supertypes, the persistence of supertypes is significantly overestimated (even if the frequency of a given supertype is underestimated), which may contribute to development of too-long heat waves/cold spells. We did not identify any substantial improvement in the EURO-CORDEX RCMs in comparison to previous ENSEMBLES RCMs, but the patterns of the biases are generally less conclusive as to general RCMs’ drawbacks.

Highlights

Extreme temperature events, namely, heat waves (HWs) in summer and cold spells (CSPs) in winter, are atmospheric phenomena with high impacts on natural ecosystems and human society in midlatitudes
An indication from the ensemble mean that the regional climate models (RCMs) suffer from a common feature is a higher percentage of hot/cold days that occur within HWs/CSPs and the overestimation of the length of the longest HWs/CSPs. e enhanced grouping of days with extreme temperature anomalies is demonstrated by the strong overestimation of the number of cold days in the “coldest” season detected in all RCM simulations. e ensemble-mean results show that by averaging the model biases, one can get the best reproduction of observed patterns [20]
We found an improvement in capturing the frequencies of HWs and CSPs in the ensemble mean of the CORDEX RCMs compared to ENSEMBLES simulations: the overestimation decreased by half

Summary

Introduction

Heat waves (HWs) in summer and cold spells (CSPs) in winter, are atmospheric phenomena with high impacts on natural ecosystems and human society in midlatitudes. Continued changes of climate observed over recent decades are manifested in an increase in global temperature but in changes in all components of the climate system [7]. To understand such changes at regional scales and how they project to influence future climate, including characteristics of HWs and CSPs, regional climate models (RCMs) have been developed and used. It is important to evaluate RCM simulations for the recent or historical climate in order to identify their biases, which may contribute to improvements and further development of the models as well as enhancing interpretation

Results

Discussion

Conclusion