Aerosol–cloud–precipitation interactions during a Saharan dust event – A summertime case‐study from the Alps

Abstract

Changes in the ambient aerosol concentration are known to affect the microphysical properties of clouds. Especially regarding precipitation formation, increasing aerosol concentrations are assumed to delay the precipitation onset, but may increase precipitation rates via convective invigoration and orographic spillover further downstream. In this study, we analyse the effect of increased aerosol concentrations on a heavy precipitation event observed in summer 2017 over northeastern Switzerland, an event which was considerably underestimated by the operational weather forecast model. Preceding the precipitation event, Saharan dust was advected towards the Alps, which could have contributed to increased precipitation rates north of the Alpine ridge. To investigate the potential impact of the increased ambient aerosol concentrations on surface precipitation, we perform a series of sensitivity simulations using the Consortium for Small‐scale Modeling (COSMO) model with different microphysical parametrizations and prognostic aerosol perturbations. The results show that the choice of the microphysical parametrization scheme in terms of a one‐ or two‐moment scheme has the relatively largest impact on surface precipitation rates. In the one‐moment scheme, surface precipitation is strongly reduced over the Alpine ridge and increased further downstream. Simulated changes in surface precipitation in response to aerosol perturbations remain smaller in contrast to the impact of the microphysics scheme. Elevated cloud condensation nuclei (CCN) concentrations lead to increased cloud water and decreased cloud ice mass, especially in regions of high convective activity south of the Alps. These altered cloud properties indeed increase surface precipitation further downstream, but the simulated change is too small to explain the observed heavy precipitation event. Additional ice‐nucleating particles (INPs) increase cloud ice mass, but only trigger local changes in downstream surface precipitation. Thus, increased aerosol number concentrations during the Saharan dust outbreak are unlikely to have caused the heavy precipitation event in summer 2017.

Water isotopic characterisation of the cloud-circulation coupling in the North Atlantic trades. Part 1: A process-oriented evaluation of COSMOiso simulations with EUREC4A observations

“…A high N c leads to a reduction of the cloud droplets' size since the available liquid water is distributed over more droplets. This, in turn, reduces the efficiency of the collisional growth process and can delay or even prevent the formation of precipitation (see, e.g., Eirund et al, 2022). In other words, N c directly affects the autoconversion rate, the rate at which COSMO iso turns cloud water into rain water.…”

Heavy snowfall event over the Swiss Alps: Did wind shear impact secondary ice production?

“…The Consortium for Small Scale Modelling (COSMO; Baldauf et al, 2011) non-hydrostatic model, version 5.4.1b, was used for this case study. COSMO has been used to study wintertime (Lohmann et al, 2016;Henneberg et al, 2017;Dedekind et al, 2021) and summertime (Dedekind, 2021;Eirund et al, 2021) orographic MPCs in the Swiss Alps. The model domain roughly covers a region of 500 km×600 km (44.5 to 49.5 • N and 4 to 13 • E) at a horizontal grid spacing of 1.1 km×1.1 km (Fig.…”

“…We use a detailed two-moment bulk cloud microphysics scheme within COSMO with six hydrometeor categories, including cloud droplets, rain, ice, snow, graupel and hail . The 2M scheme has been used extensively to study the evolution, lifetime, persistence and aerosol-cloud interactions of MPCs Baldauf et al, 2011;Lohmann et al, 2016;Possner et al, 2017;Henneberg, 2017;Glassmeier and Lohmann, 2018;Sullivan et al, 2018;Eirund et al, 2019Eirund et al, , 2021. We refer to ice particles as any combination of the hail, graupel, snow or ice categories.…”