Impact of volcanic aerosols on clouds in cloud-system-resolving simulations and satellite observations

Increased anthropogenic aerosols result in an enhancement in cloud droplet number concentration (Nd), which consequently modifies cloud and precipitation processes. It is unclear how exactly cloud liquid water path (LWP) and cloud fraction respond to aerosol perturbations. A volcanic eruption may help to better understand and quantify the cloud response to external perturbations, with a focus on short-term cloud adjustments. The goal of the present study is to understand and quantify the response of clouds to a se- lected volcanic eruption and to thereby advance the fundamental understanding of the cloud response to external forcing. In this study we used the ICON (ICOsahedral Non-hydrostatic) model in its numerical weather prediction setup at a cloud-system-resolving resolution of 2.5 km horizontally, to simulate the region around the Holuhraun volcano for one week (1 – 7 September 2014). The ICON-NWP version employed in this study does not include an interactive aerosol model. Therefore a new method for cloud condensation nuclei (CCN) activation in a mi- crophysics scheme that was developed specifically for this study is introduced. The CCN values were determined by interpolating from look-up tables and considering the corresponding pressure (p) and vertical velocity (w) values within each grid-box of the atmospheric model. Moreover, in order to improve the comparison between cloud microphysical variables and satellite retrievals, the MODIS simulator from the COSP (CFMIP Observation Simulator Package) framework was implemented into the source code of ICON-NWP. A pair of simulations, with and without the volcanic aerosol plume, allowed us to as- sess the simulated effective radiative forcing and its mechanisms, as well as its impact on adjustments of LWP and cloud fraction to the perturbations of Nd. In comparison to MODIS (Moderate Resolution Imaging Spectroradiometer) satellite retrievals, a clear enhancement of Nd due to the volcanic aerosol is detected and attr