Sensitivity of aerosol optical thickness and aerosol direct radiative effect to relative humidity

We present a sensitivity study of the effects of spatial and temporal resolution of atmospheric relative humidity (RH) on calculated aerosol optical thickness (AOT) and the aerosol direct radiative effects (DRE) in a global model. We carry out different modeling experiments using the same aerosol fields simulated in the Global Modeling Initiative (GMI) model at a resolution of 2 degrees latitude by 2.5 degrees longitude, using time-averaged fields archived every three hours by the Goddard Earth Observation System Version 4 (GEOS4), but we change the horizontal and temporal resolution of the relative humidity fields. We find that, on a global average, the AOT calculated using RH at a 1 degrees x 1.25 degrees horizontal resolution is 11% higher than that using RH at a 2 degrees x 2.5 degrees resolution, and the corresponding DRE at the top of the atmosphere is 8-9% and 15% more negative (i.e., more cooling) for total aerosols and anthropogenic aerosol alone, respectively, in the finer spatial resolution case. The difference is largest over surface escarpment regions (e. g. > 200% over the Andes Mountains) where RH varies substantially with surface terrain. The largest zonal mean AOT difference occurs at 50-60 degrees N (16-21%), where AOT is also relatively larger. A similar impact is also found when the time resolution of RH is increased. This increase of AOT and aerosol cooling with the increase of model resolution is due to the highly non-linear relationship between RH and the aerosol mass extinction efficiency (MEE) at high RH (> 80%). Our study is a specific example of the uncertainty in model results highlighted by multi-model comparisons such as AeroCom, and points out one of the many inter-model differences that can contribute to the overall spread among models.