Effects of Aerosols on Clear Sky Solar Radiation in the ALADIN-1 HIRLAM NWP System 2 3

14 The direct shortwave radiative effect of aerosols under clear sky conditions in the ALADIN15 HIRLAM numerical weather prediction system was investigated using three shortwave 16 radiation schemes in diagnostic single-column experiments: the IFS scheme, the acraneb2 17 scheme and the hlradia radiation scheme. The aim was to evaluate the strengths and 18 weaknesses of the NWP system in this regard and to prepare it for use of real-time aerosol 19 information. The experiments were run with particular focus on the August 2010 Russian 20 wildfire case. Each of the three radiation schemes accurately (within ± 4% at midday) 21 simulates the direct shortwave aerosol effect when observed aerosol optical properties are 22 used. When the aerosols were excluded from the simulations, errors of more than +15 % in 23 global shortwave irradiance were found at midday, with the error reduced to +10 % when 24 standard climatological aerosols were used. An error of -11 % was seen at midday if only 25 observed aerosol optical depths at 550 nm, and not observation-based spectral dependence 26 of aerosol optical depth, single scattering albedos and asymmetry factors, were included in 1 the simulations. This demonstrates the importance of using the correct aerosol optical 2 properties. The dependency of the direct radiative effect of aerosols on relative humidity 3 was tested and shown to be within ± 6 % in this case. By modifying the assumptions about 4 the shape of the IFS climatological vertical aerosol profile, the inherent uncertainties 5 associated with assuming fixed vertical profiles were investigated. The shortwave heating 6 rates in the boundary layer changed by up to a factor of 2 in response to the aerosol vertical 7 distribution. Finally, we tested the radiative transfer approximations used in the three 8 radiation schemes for typical aerosol optical properties compared to the accurate DISORT 9 model. These approximations are found to be accurate to within ± 13 % even for large 10 aerosol loads. 11