PNNL

Abstract

Subgrid-scale cloud condensation nuclei (CCN) heterogeneity is not represented in global climate models (GCM) and potentially contributes systematic errors to simulated cloud effects. High-resolution mesoscale model simulations were performed to investigate the impact of assuming a uniform CCN distribution on cloud properties and surface radiation over a region the size of a GCM grid column. Results indicate that a prescribed CCN distribution allowing for vertical and temporal fluctuations does substantially better in simulating cloud properties and radiative effects than does a prescribed uniform and constant CCN distribution. Spatially and temporally averaged net effects on downwelling shortwave radiation are between -3 and -11 W m-2 for the fluctuating and uniform distributions, respectively, versus a control simulation with fully interactive aerosols. Both prescribed CCN distributions produce optically thicker clouds more often than the control, with the mean cloud optical depth increasing by over 25% when using the uniform and constant CCN distribution.