Radiative and climate effects of aerosol scattering in long-wave radiation based on global climate modeling

Abstract. The few studies that considered aerosol scattering in the long-wave (LW) typically relied on artificially increasing it. In order to analyze the radiative and climatic effects of physically accounting for this process, simulations have been performed with the ARPEGE-Climat atmospheric global climate model over the 1985–2014 period, using the ecRad radiation scheme, and updated optical properties of coarse aerosols, particularly dust. The evaluation of the model coarse aerosol optical depth (AOD) against AERONET data over North Africa and Arabian Peninsula shows the ability of ARPEGE-Climat to capture spatio-temporal variations of coarse AOD, despite regional biases. The comparison of simulations with and without aerosol scattering in the LW shows that this process leads to a significant increase in downwelling surface LW radiation in dust-emitting regions (+5 W m^-2 on average) between March and September, in line with the maximum coarse AOD. This increase results in a rise in minimum near-surface temperatures of up to +1 °C. It is also associated with an outgoing LW radiation decrease at the top of the atmosphere (TOA). However, during certain months and regions, near-surface temperatures can be significantly reduced due to short-wave surface radiation decreases related to increases in low-level clouds. A precipitation increase over Sahel during September linked to wetter atmospheric layers is also simulated. Neglecting LW aerosol scattering in climate simulations has therefore significant impacts on climate, notably in dust-emitting regions. Globally, the LW aerosol scattering contribution to radiation is of 0.4 W m^-2 at both surface and TOA.