# Large-Scale Modeling of Absorbing Aerosols and Their Semi-Direct Effects
## 1 Introduction
In addition to global models focusing mainly on the effects of aerosols on the radiation budget, regional models using reanalyzed wind fields to reproduce meteorological conditions have been used to more describe aerosol patterns and their effects with more detail compared to climate models. Studying the impacts of aerosol absorption on radiation in Central Europe with the regional model system COSMO-MUSCAT [18], [PERSON] et al. [19] find that the aerosol-induced increase in heating rates causes a reduction in cloud cover in this domain. This cloud cover change means a positive semi-direct effect, with a regional average of about 2 Wm\({}^{-2}\) for summer conditions. A different study with this model system for the field experiment in January and February 2008 investigating dust and smoke in the Sahara and Sahel, as well as their transport to the tropical Atlantic for the SAMUM-2 field experiment [20], found a strengthening of the Hadley circulation due to radiative forcing by dust and biomass burning smoke [21]. [PERSON] et al. [22] and [PERSON] and [PERSON] [23] estimated the global semi-direct aerosol effects by decomposing radiative forcing estimates from global climate models into direct, indirect and semi-direct forcings for anthropogenic aerosols. In these studies the semi-direct effects were estimated as residuals from total effective aerosol forcing and instantaneous direct forcing computations. [PERSON] and [PERSON] [23] found a global average negative semi-direct forcing of \(-0.1\) Wm\({}^{-2}\) comparing present-day and preindustrial forcing with the GISS Model E [24]. [PERSON] et al. [22] found the net semi-direct forcing of all anthropogenic aerosol types to be in the order of only 10% of the total aerosol forcing for different setups of the Community Atmosphere Model CAM5.1. The sign of this forcing could be positive or negative depending on the aerosol representation in the model. In their study the number of modes used to describe the aerosol distribution and the efficiency aerosol ageing influenced the results of the semi-direct forcing. In particular the ageing of BC aerosols influences the atmospheric lifetimes of these particles and thus impacts the atmospheric absorption. Those results highlight the large uncertainties and potential model dependencies in describing the semi-direct effect. While those studies show only small global annual average aerosol semi-forcing values compared to estimated direct and indirect aerosol forcings, the overall effect is uncertain and may be model-dependent due to uncertainties in simulated aerosol and cloud distributions and model sensitivities. In addition, the average effect is small due to the cancellation of regionally positive and negative effects of absorbing aerosol on cloud cover and thus top-of-atmosphere radiative flux differences. However, for specific regions the magnitudes of semi-direct aerosol effects may be considerable. Here we investigate the semi-direct effects (or rapid adjustments) of the absorbing aerosol types BC and mineral dust within the global aerosol-climate model ECHAM6-HAM2 (version echam6.3-ham2.3). The general circulation model ECHAM6 distinguishes between clouds generated from moist convection and stratiform clouds that occur as function of relative humidity [25]. The semi-direct effect for an aerosol scenario is computed as residual between the radiative effect (without taking into account changes in indirect aerosol effects) and the instantaneous direct aerosol forcing, as in previous studies. Here we consider the total effects without separating the aerosol distribution into anthropogenic and natural components. In the following we use the term direct radiative effect (DRE) referring to the effect of all aerosol. Thus, the possible maximum effect of absorbing aerosol heating in the atmosphere is evaluated.