# Large-Scale Modeling of Absorbing Aerosols and Their Semi-Direct Effects
## 2 Methods
### Model Setup  
The radiative effects of absorbing aerosols were evaluated with eight model experiments, summarized in Table 1. All model simulations were carried out at a spectral resolution of T63 (corresponding to a horizontal grid size of approximately 1.9 degree, or 210 km at the Equator) and 47 vertical layers. For all cases except GFAS biomass burning emissions from the ACCMIP dataset are used. As this results in comparably low BC concentrations [49], a sensitivity study using GFAS fire emissions multiplied with the scaling factor 3.4 is included (GFAS) resulting in higher BC loads. The initial model aerosol concentration is zero. Each model simulation was carried out for eight model years, whereby the first three modeled months were disregarded for the further analysis of the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Experiment** & **Aerosol Types** & **Aerosol-Radiation Interaction** & **Aerosol-Cloud Interaction** \\ \hline BASE & All & yes & no \\ GFAS & All & yes & no \\ noBC & no BC & yes & no \\ noABS & no BC, no dust & yes & no \\ noDust & No dust & yes & no \\ BASEind & all & yes & yes \\ noBCind & no BC & yes & yes \\ NoAer & all & no & no \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of model experiments, summarizing the included aerosol species and information whether aerosol-radiation and aerosol-cloud activation is computed prognostically using the simulated aerosol distributions. In the experiment GFAS the aerosol emissions from fires are computed using the GFAS dataset [34], all other cases are simulated with ACCMIP emissions. results. Sea surface temperatures were considered fixed at climatological values of the average for years 1979-2008. From all model years climatological averages of monthly means and variabilities were constructed for the considered model variables. These include the net solar and thermal radiation (here only analyzed for the top of the atmosphere (TOA)), the accumulated solar and thermal (all-sky and clear-sky) radiation flux anomalies at TOA, atmospheric hearing rates, and the vertically integrated cloud cover. We calculate the direct radiative and semi-direct radiative effects and radiative forcing by aerosol as in [PERSON] et al. (2012). The radiative fluxes at the top of the atmosphere (\(F_{net}\)) are diagnosed as net solar \(F_{net,s}\) and thermal fluxes (\(F_{net,t}\)), including effects of scattering and absorption by clouds and aerosol particles. As is [PERSON] et al. (2012) the radiative forcing for the thermal part of the spectrum was only considered for cloudy conditions to exclude effects that originate from changes in land surface temperatures. Thus, in this study the net radiative fluxes were computed as:  
\[F_{net}=F_{net,s}+F_{net,t}-F_{net,t,clear}. \tag{1}\]  
\(F_{net,t,clear}\) includes only cloud-free conditions, but includes aerosol forcing (which is small for thermal wavelengths). The direct radiative effect \(DRE\) of an aerosol compound is then computed as difference between the radiative fluxes for the model simulation including the regarded aerosol species and the simulation not including this aerosol type:  
\[DRE_{aerosol}=F_{net, aerosol}-F_{net,no\_aerosol} \tag{2}\]  
The instantaneous direct radiative forcing \(IDR\) by the different aerosol species is computed by double calls to the radiation routine to diagnose the radiative forcing without influencing the atmospheric conditions driven by this forcing, but include the solar and thermal net fluxes at the top of the atmosphere. Again, the TOA aerosol direct forcing \(IDR_{aerosol}\) is computed as difference between the radiation fluxes \(F_{0}\) at the top of the atmosphere of the model simulation including and simulation results excluding this aerosol type. The double radiation call was performed in the runs that simulate the aerosol effects.