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As large-eddy simulations (LES), which explicitly simulate aerosol-cloud interactions, are often considered as benchmark simulations in climate science, it is necessary to critically evaluate if these high-resolution models can skillfully represent expected physical phenomena.Here, we focus on the first aerosol indirect aerosol effect in a warm stratocumulus cloud. We investigate if the MIMICA LES (Savre et al., 2014) with a widely used bulk two-moment microphysical scheme (Seifert and Beheng, 2006) can reproduce the susceptibility regimes identified by Reutter et al., (2009). Using a parcel model, Reutter et al. (2009) showed that the cloud droplet number (Nd) responds differently to an increase in aerosol number (Na) depending on ambient updraft strength (w). In the aerosol-limited regime, enough supersaturation can be generated by the updraft motions in the atmosphere so that increasing Na leads to an increase in Nd. Conversely, in the updraft-limited regime, adding aerosol will not increase as activation is limited by the updraft strength and only increasing w will lead to an increase in Na.In the standard setup, the LES cannot simulate the transition from the aerosol- to the updraft-limited regime. Only when implementing a renormalization procedure following Reisin et al., (1996) and, at the same time, increasing the initial droplet radius of newly activated droplets (rdi) to values large than rdi>1µm, a regime transition emerges. However, a clear recommendation for the choice of rdi cannot be made upon physical arguments at this point. Interestingly, the “arbitrarily chosen” droplet mass by Seifert and Beheng (2006) of 1*10-12kg, which corresponds to rdi≈6µm, seems to agree quite well with the expectations from parcel model simulations. The choice is, however, still arbitrary and therefore physically questionable.A potential way to avoid this problem, which mainly occurs at high aerosol concentrations, would be to run the LES with a small enough temporal resolution (Δt≈0.1s) to explicitly resolve all relevant microphysical processes. ReferencesReisin, T., Levin, Z., Tzivion, S., 1996. Rain Production in Convective Clouds As Simulated in an Axisymmetric Model with Detailed Microphysics. Part I: Description of the Model. Journal of Atmospheric Sciences 53, 497–520.Reutter, P., Su, H., Trentmann, J., Simmel, M., Rose, D., Gunthe, S.S., Wernli, H., Andreae, M.O., Pöschl, U., 2009. Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN). Atmospheric Chemistry and Physics 9, 7067–7080.Savre, J., Ekman, A.M.L., Svensson, G., 2014. Technical note: Introduction to MIMICA, a large-eddy simulation solver for cloudy planetary boundary layers. Journal of Advances in Modeling Earth Systems 6, 630–649.Seifert, A., Beheng, K.D., 2006. A two-moment cloud microphysics parameterization for mixed-phase clouds. Part 1: Model description. Meteorol. Atmos. Phys. 92, 45–66.

A coupled climate model (MPI-ESM) with a very idealized set up is used to investigate the efficiency and consequences of regional radiation management.&nbsp

The regional atmospheric model Consortium for Small-scale Modeling (COSMO) coupled to the Multi-Scale Chemistry Aerosol Transport model (Muscat) is extended in this work to represent aerosol-cloud interactions. Previously, only one-way interactions (scavenging of aerosol and in-cloud chemistry) and aerosol-radiation interactions were included in this model. The new version allows for a microphysical aerosol effect on clouds. For this, we use the optional two-moment cloud microphysical scheme in COSMO and the online-computed aerosol information for cloud condensation nuclei concentrations (Cccn), replacing the constant Cccn profile. In the radiation scheme, we have implemented a droplet-size-dependent cloud optical depth, allowing now for aerosol-cloud-radiation interactions. To evaluate the models with satellite data, the Cloud Feedback Model Intercomparison Project Observation Simulator Package (COSP) has been implemented. A case study has been carried out to understand the effects of the modifications, where the modified modeling system is applied over the European domain with a horizontal resolution of 0.25°g × g0.25°. To reduce the complexity in aerosol-cloud interactions, only warm-phase clouds are considered. We found that the online-coupled aerosol introduces significant changes for some cloud microphysical properties. The cloud effective radius shows an increase of 9.5g%, and the cloud droplet number concentration is reduced by 21.5g%.

The regional atmospheric model Consortium for Small Scale Modeling (COSMO) coupled to the MultiScale Chemistry Aerosol Transport model (MUSCAT), is extended in this work to represent aerosol-cloud interactions. Previously, only one-way interactions (scavenging of aerosol and in-cloud chemistry) and aerosol-radiation interactions were included in this model. The new version allows for a microphysical aerosol effect on clouds. For this, we use the optional two-moment cloud microphysical scheme in COSMO and the online-computed aerosol information for cloud condensation nuclei (CCN) concentrations, replacing the constant CCN concentration profile. In the radiation scheme, we implement a droplet-size-dependent cloud optical depth, allowing now for aerosol-cloud-radiation interactions. In order to evaluate the model with satellite data, the Cloud Feedback Model Inter-comparison Project Observational Simulator Package (COSP) has been implemented. A case study has been carried out to understand the effects of the modifications, in which the modified modeling system was applied over the European domain with a horizontal resolution of 0.25° × 0.25°. It is found that the online coupled aerosol introduces significant changes for some cloud microphysical properties. The cloud effective radius shows an increase of 2 to 10 μm, and the cloud droplet number concentration is reduced by 10 to 50 cm−3. For both quantities, the new model version shows a better agreement with the satellite data. The microphysics modifications have a smaller effect on other parameters such as optical depth, cloud water content, and cloud fraction.