A limited-area-model case study of the effects of sub-grid scale variations in relative humidity and cloud upon the direct radiative forcing of sulfate aerosol

A limited-area non-hydrostatic model with a horizontal spatial resolution of 2km by 2km is used to assess the importance of sub-grid scale variations in relative humidity and cloud upon the direct radiative forcing (DRF) by tropospheric sulfate aerosols. The DRF from the limited-area model for both clear and cloudy regions is analyzed and the results compared against those obtained using general circulation model (GCM) parameterizations that perform the computations over coarse horizontal grids. In this idealized model study, the GCM calculations underestimate the clear sky DRF by approximately 73% and the cloudy sky DRF by approximately 60%. These results indicate that, for areas where the relative humidity is high and where there is substantial spatial variability in relative humidity and cloud, GCM calculations may considerably underestimate the DRF.