Simulating marine boundary layer clouds over the eastern Pacific in a regional climate model with double-moment cloud microphysics

A double-moment cloud microphysics scheme with a prognostic treatment of aerosols inside clouds has been implemented into the International Pacific Research Center Regional Atmospheric Model (iRAM) to simulate marine boundary layer clouds over the eastern Pacific and to study aerosol-cloud interactions, including the aerosol indirect effect. This paper describes the new model system and presents a comparison of model results with observations. The results show that iRAM with the double-moment cloud microphysics scheme is able to reproduce the major features, including the geographical patterns and vertical distribution of the basic cloud parameters such as cloud droplet number, liquid water content, or droplet effective radii over the eastern Pacific reasonably well. However, the model tends to underestimate cloud droplet number concentrations near the coastal regions strongly influenced by advection of continental aerosols and precursor gases. In addition, the average location of the stratocumulus deck off South America is shifted to the northwest compared with the satellite observations. We apply the new model system to assess the indirect aerosol effect over the eastern Pacific by comparing a simulation with preindustrial aerosol to an otherwise identical simulation with present-day aerosol. Resulting changes in the cloud droplet number concentration are particularly pronounced in Gulf of Mexico and along the Pacific coastlines with local changes up to 70 cm(-3) (50% of the present-day value). The modeled domain-averaged 3-month (August-October) mean change in top-of-atmosphere net cloud forcing over the ocean owing to changes in the aerosol burden by anthropogenic activities is -1.6W m(-2)