Scale-dependency of Aerosol-Cloud Interactions using Scale-Aware Cloud Microphysics in the Weather Research and Forecasting Model

Of the many challenges in simulating aerosol-cloud interactions (ACI), we address two of them. The First is to determine the factors controlling in-cloud warm region precipitation suppression effects and multi-phase cloud region precipitation invigoration effects. The second issue is to study scale-dependency of ACI: how these interactions can be handled across spatial scales, well into grey scale areas, namely 1-5 km grids by grid-scale cloud and subgrid-scale convective cloud microphysical schemes. In this work, temporally and spatially varying prescribed aerosols generated from the CESM-NCSU are introduced into the Weather Research and Forecasting Model (WRF) to study the impact of aerosol indirect effects on precipitation. We use Morrison's microphysical scheme for grid-scale clouds and the Multi-scale Kain Fritsch cumulus scheme for subgrid-scale using the convective microphysics scheme of Song and Zhang (2011). Additionally, we use the same aerosol activation and ice nucleation parameterizations to ensure consistent aerosol cloud interactions in both the grid-scale and subgrid-scale cloud microphysics. Model simulations are carried out for 96 h starring June 4th 2010 00Z for both a 12km domain consisting of the Eastern U.S. and a 4km domain consisting of the Midwest. Preliminary results indicate that the inclusion of aerosols led to general enhancements in precipitation across the EUS, with approximately 16% (0.10 mm) of the domain average increase attributed to sub-grid scale microphysics and 84% (0.52 mm) of the domain average increase attributed to changes in grid scale microphysics from ACI. Results also indicate suppression in limited regions where usually precipitation is weaker. However, changes in precipitation of the 4 km domain contrast with that in the 12 km results, revealing scale-dependency of ACI. We will present more detailed results illustrating the mechanisms behind: (1) suppression vs invigoration; and (2) the scale dependency of ACI on precipitation and how these mechanisms are impacted by aerosol loadings. Special and routinely available measurements are used to evaluate model performance and uncertainties in the ACI process representations.