# Large-Scale Modeling of Absorbing Aerosols and Their Semi-Direct Effects
## 1 Introduction
Estimates of dust single-scattering albedos at 550 nm summarized by [PERSON] and [PERSON] [10] range from 0.8 to 0.98. Due to their particle size distribution with the majority of the particle mass being contributed by particles larger than 1 \(\upmu\)m, mineral dust particles also absorb and emit longwave radiation in the terrestrial spectral range [11]. In addition to the direct forcing, aerosols impact on the radiation budget by the so-called semi-direct effect or rapid adjustment of the atmosphere to the instantaneous direct radiative effect by aerosols, without considering the effects of surface temperature changes. This additional radiative forcing is mainly caused by the impact of the change in atmospheric heating rates by absorbing aerosols on cloud cover, e.g., due to changes in boundary layer stability or convection. This does not include the indirect aerosol effects on cloud properties by microphysical aerosol-cloud interactions. Semi-direct aerosol effects are complex. In addition to surface albedo, their magnitude and sign depend on both the distribution of absorbing aerosols and clouds, in particular on their relative vertical position. Effects can range from reduction of vertical mixing within the boundary layer as consequence of boundary layer stabilization [12], to shifts of the inter-tropical convergence zone due to the large-scale asymmetric distribution of absorbing aerosol and strengthening the southeast Asian monsoon circulation (e.g., [13]). Due to the partly opposing effects of absorbing aerosols on cloud cover and thus on temperature changes, the semi-direct effect is difficult to quantify on a global scale. Aerosol models embedded in atmospheric global or regional models that consider radiative aerosol effects can be used to estimate magnitudes of such semi-direct aerosol effects. The different effects of the semi-direct effect of soot aerosol have been summarized by [PERSON] and [PERSON] [14]. The semi-direct effect depends on the aerosol distribution, the single scattering albedo (SSA), the presence of clouds, the surface albedo and the vertical distribution of the aerosol. [PERSON] et al. [15] explained the influence of the absorbing aerosol on clouds as reduction and dissolution of clouds as consequence of temperature increase. Using a simplified general circulation model, they estimated that this effect increases the critical SSA below which tropospheric aerosols are warming rather than cooling globally from 0.89 to 0.91. In contrast, for Asia, [PERSON] et al. [16] found in a model study an increase in low cloud cover over oceans and some land regions, i.e., a negative semi-direct effect. For the less absorbing mineral dust aerosol, [PERSON] and [PERSON] [17] also found in a global model study that cloud coverage increases in regions with high dust concentrations. The authors explain this effect by the fact that the warming of the dust layer increases the moisture convergence, which in turn leads to an increase in cloud coverage. For stratocumulus clouds, an overlying absorbent aerosol layer may increase temperature inversion by heating the aerosol containing air layer, causing an increase in cloud cover. The aerosol effect near the ground can increase the convection and thus also increase the cloud coverage. Estimates of the radiative forcing due to the semi-direct effect range from \(-0.4\) to +0.1 Wm\({}^{-2}\), while local values of \(-9.5\) to +11 Wm\({}^{-2}\) can occur [14]. The estimated semi-direct effects of absorbing aerosols is thus lower than the direct effect of anthropogenic aerosols, which is estimated at a range of \(-0.85\) to +0.15 Wm\({}^{-2}\)[2]. The ability of large-scale models to realistically describe the semi-direct effects or rapid adjustments of the atmosphere including changes in cloud properties is limited by their ability to accurately represent aerosol distributions as well as cloud processes.