Abstract
AbstractChanges 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.
Highlights
Precipitation originating from orographic clouds is crucial for the local hydrology, ecology, and climate in Alpine regions (Roe, 2005)
The lack of hail formation in 1-M as compared to CTRL reduces QICE in 1-M, which in turn contributes to reduced riming processes. These results suggest that the microphysical parametrization, especially the size dependence, strongly impacts the cloud properties and the regional surface precipitation rates
This study provides insights into aerosol–cloud– precipitation interactions in an Alpine environment based on a case-study in the Alps
Summary
Precipitation originating from orographic clouds is crucial for the local hydrology, ecology, and climate in Alpine regions (Roe, 2005). Suppose warm mixed-phase orographic clouds (cloud-top temperatures > −20◦C) are limited in ice particle number concentration In this case, an enhancement in INPs in the liquid-rich environment will favour the riming process over deposition in the growth of ice particles, reducing the importance of the WBF process (Fan et al, 2017). If the droplets are small (due to a high CCN concentration), their freezing temperature is comparatively low, leading to ice formation at higher levels in the cloud, where the released latent heat increases the buoyancy at these high levels This in turn enhances the updraught and invigorates convection, thereby contributing to the thermodynamic effect (Khain et al, 2005; Koren et al, 2005; Wang, 2005). In (a), the grey dashed box denotes the model domain used for the simulations in this study, geographical terms used are given, and the asterisk marks the location of the JFJ high-altitude measuring site [Colour figure can be viewed at wileyonlinelibrary.com]
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