Using a cloud-resolving model to study the effects of subgrid-scale variations in relative humidity on direct sulphate-aerosol forcing

Cloud-resolving model simulations over a tropical ocean and a mid-latitude continental region have been used to investigate the influence of subgrid-scale variations of relative humidity on the direct radiative forcing of sulphate aerosols. Offline radiation calculations based on output from a cloud-resolving model, with and without sulphate aerosol included, are used to calculate the direct radiative forcing of the aerosol. This forcing is compared with results from single-column radiation calculations typical of those produced by a climate model. The results from this idealized study show that a typical climate model can underestimate the direct radiative forcing of aerosols by up to 80%. The errors in a climate-model calculation are largest when the mean relative humidity is high or there are moist regions within a drier domain; this is usually reflected in a larger standard deviation of the relative humidity. Over the more humid tropical ocean, a climate model may underestimate the direct radiative forcing of sulphate aerosols by 43%, on average (ranging between 30% and 80%). Over the drier continental mid-latitude region the average error is only 10%, but instantaneous values can exceed 50% during times when both the mean relative humidity and its standard deviation are large; this is typically close to convective events.