Response of atmospheric rivers to aerosols treatment in regional climate simulations

Abstract

Atmospheric rivers (ARs) play an essential role in extreme precipitation phenomena. To predictsuch events, a correct simulation of ARs becomes crucial. Since most of the regional climate modelsdo not take aerosols into account in an interactive way, the main objective pursued in this work was toanalyse the role of aerosols in the intensity and behaviour of ARs on the regional scale. The identifi-cation of ARs has always been carried out in global climate simulations applying detection algorithmsthat may not be suitable in regional climate models, due to the presence of boundaries in the spatialdomain.This work presents a new ARs identification algorithm for regional climate simulations (AIRA).The implemented algorithm has proved to be able to properly identify the vapour structures associatedwith ARs. AIRA was applied to a set of hourly data from three regional simulations (BASE, ARI andARCI), covering a period of 20 years. In BASE, aerosols were prescribed, while the model incorporatesaerosols dynamically in both ARI and ARCI. In ARI, aerosols are only incorporated interactively inaerosol-radiation interactions. In ARCI, they are also included in the microphysical processes.AIRA has identified about 250 ARs in the three simulations. Spring and autumn ARs were themost frequent, intense and long-lasting, while they were less frequent, shorter and weaker in summer.The identified ARs explain up to a 30% of the total precipitation in some areas of the Iberian Penin-sula. The differences between the three simulations are significant in the spatial distribution of theprecipitation and in the trajectory and intensity of some ARs. Although the number of detected ARsis similar, the temporal steps with ARs common to the three simulations represent only a 37% of thetotal BASE steps containing ARs. This indicates that the sensitivity to the inclusion of aerosols isrelevant. The common ARs events showed that the BASE and ARI simulations generally present sim-ilar trajectoriesk. However, important differences appear regarding ARCI, speciallywhen ARs are not quite intense.A cluster analysis of the thickness field between 1000 and 850 hPa in ARI identifies three mainpatterns. The comparison between the centroids in ARI and ARCI, reveals that the differences betweenARs in both simulations are mainly related to the aerosols type and concentration. The mainmechanism behind this behaviour is related to the modification of the temperature field dueto aerosol-cloud interactions (indirect effect) while aerosol-radiation effects are less relevant.