Reply on RC2

Matthias Gröger

doi:10.5194/esd-2021-49-ac2

Abstract

Atmospheric rivers (AR) are important drivers of heavy precipitation events in western and central Europe and often associated with intense floods. So far, the ARs response to climate change in Europe has been investigated by global climate models within the CMIP5 framework. However, their spatial resolution between 1 and 3° is too coarse for an adequate assessment of local to regional precipitation patterns. Using a regional climate model with 0.22° resolution we downscale an ensemble of 24 global climate simulations following the greenhouse gas scenarios RCP2.6, RCP4.5, RCP8.5. The performance of the model was tested against ER-I reanalysis data. The downscaled simulation notably better represents small-scale spatial characteristics which is most obvious over the terrain of the Iberian Peninsula where the AR induced precipitation pattern clearly reflect eat-west striking topographical elements resulting in zonal bands of high and low AR impact. Over central Europe the model simulates a less far propagation of ARs toward eastern Europe compared to ERA-I but a higher share of AR forced heavy precipitation events especially Norway where 60 % of annual precipitation maxima are related to ARs. We find ARs more frequent and more intense in a future warmer climate especially in the higher emission scenarios whereas the changes are mostly mitigated under the assumption of RCP2.6. They also propagate further inland to eastern Europe in a warmer climate. In the high emission scenario RCP8.5 AR induced precipitation rates increase between 20 and 40 % in western central Europe while mean precipitation rates increase by maximal 12 %. Over the Iberian Peninsula AR induced precipitation rates slightly decrease around −6 % but mean rates decrease around −15 %. The result of these changes is an overall increased contribution of ARs to heavy precipitation with greatest impact over Iberia (15–30 %). Over Norway average AR precipitation rates decline between −5 to −30 %. These reductions most likely the originate from regional dynamical changes. In fact, over Norway we find ARs originating from > 60° N are reduced by up to 20 % while those originating south of 45° N are increased. Also, no clear climate change signal is seen for AR related heavy precipitation and annual maximum precipitation over Norway where the uncertainty of the ensemble is quite large.

Highlights

Atmospheric rivers (ARs) are long and narrow corridors that transport large amounts of moisture from tropical and subtropical origin poleward (e.g. Zhu et al, 1998; Gimeno et al, 2014; Gimeno et al, 2016; Shields et al, 2019)
A high resolution regional climate model with a resolution of 0.22° was applied to investigate the impact of ARs on Europe
In the central and southern part of the Iberian Peninsula the contribution to the regional precipitation budget is strongly modulated by E-W striking topographic signatures

Summary

Introduction

Atmospheric rivers (ARs) are long and narrow corridors that transport large amounts of moisture from tropical and subtropical origin poleward (e.g. Zhu et al, 1998; Gimeno et al, 2014; Gimeno et al, 2016; Shields et al, 2019). On the other hand AR induced heavy precipitation can likewise cause tremendous economical damage due to flooding (e.g. Gimeno et al, 2016; Payne et al, 2020). Due to their intense moisture load they play an important role for the global water cycle. ARs are associated with extraordinary strong low level winds often positioned at the head of a cold front of extra-tropical storm systems (e.g. Dacre et al, 2015; Gimeno et al, 2016). They are modulated by large scale weather regimes as demonstrated by Pasquier et al (2019). ARs can occur during the whole year but due to their strong linkage to extra-tropical storm systems they are more frequent during the cold season in the Northern Hemisphere (Lavers and Villarini 2013; Ramos et al, 2015)

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