The Impact of Temporally and Spatially Varying Prescribed Aerosols on Seasonal Precipitation Patterns using Scale-Aware Microphysics

Aerosols affect Earth’s climate by directly scattering and absorbing incoming solar radiation and by serving as either cloud condensation nuclei (CCN) or ice nuclei (IN). The impacts of CCN or IN on clouds, otherwise known as the aerosol indirect effects, impact both the Earth’s radiation balance by altering cloud albedos, dynamics, and lifetimes and also the hydrologic cycle through changes in surface precipitation. Despite efforts to quantify the indirect effects of aerosols, many of the mechanisms that control the aerosol indirect effects, especially those related to precipitation, are not well understood and modeled. 

In this work temporally and spatially varying prescribed aerosols generated from the Community Earth System Model modified at the North Carolina State University (CESM-NCSU) are introduced into the Weather Research and Forecasting Model (WRF) to study the impact of aerosol indirect effects on precipitation, while minimizing computational expense. The objectives of this study are to (1) determine the impact of both subgrid-scale and grid-scale prescribed aerosol cloud interactions on seasonal precipitation of the Eastern United States, (2) Quantify the differences in grid scale precipitation linked to the use of dynamic prescribed cloud droplet number concentrations, and (3) explore the impact of various background aerosol levels on precipitation patterns of the Eastern United States. To accomplish these goals, the subgrid-scale cloud microphysics scheme of Song and Zhang (2011) has been ported into the Multi-scale Kain Fritsch (MSKF) cumulus scheme within WRF. Additionally, the aerosol activation and ice nucleation parameterizations of Song and Zhang (2011) are also coupled to the Morrison-Double Moment Microphysics scheme to ensure consistent aerosol cloud interactions in both the grid-scale and subgrid-scale cloud microphysics.

Six simulations for the June, July, and August (JJA) 2006 period have been carried out for a domain consisting of the Great Plains and the Eastern United States. The first is a baseline simulation of WRF version 3.8 with default settings; the second is a simulation in which only subgrid-scale microphysics interacts with CESM-derived aerosols; in the third simulation CESM-derived aerosols interact with both the grid-scale and subgrid-scale microphysics; in the fourth simulation grid-scale microphysics interacts with CESM-derived aerosols while the subgrid-scale microphysics interacts with reduced aerosol loadings; in the fifth simulation grid-scale microphysics interacts with reduced aerosol loadings, while the subgrid-scale microphysics interacts with CESM-derived aerosols; and finally in the sixth simulation both grid-scale and subgrid-scale microphysics interact with reduced aerosol loadings. 

Preliminary results indicate that the introduction of sub-grid scale cloud microphysics into MSKF increases domain average precipitation by 1.63 mm, leading to a total increase in precipitated water of 15.2 Pg for the JJA period. This increase in precipitation is attributed to large increases in marine precipitated water of 51.9 Pg. Over the continent precipitated water is reduced by 36.7 Pg compensating the large increases in the marine environment. The majority of the reduction in continental precipitation occurs over the Great Plains states (52.1 Pg), while precipitation is generally increased across both the Northern (4.0 Pg) and Southern (11.3 Pg) states east of the plains. This pattern appears to indicate that the addition of cumulus microphysics increases precipitation in regions of the domain with higher moisture availability (e.g., the Gulf of Mexico and the Atlantic Ocean) and reduces precipitation in drier portions of the domain (e.g., the Great Plains). The pattern in precipitation may also be related to precipitation suppression by the prescribed aerosol, since the northern states have higher prescribed aerosol concentrations compared to the southern states and the continental portion of the domain has higher aerosol levels than the marine environment. Precipitation statistics from both the first and second simulations are similar compared to the Multi-sensor Precipitation Estimate (MPE) product. Overall, the incorporation of both grid-scale and subgrid-scale prescribed aerosol-interactions into the WRF model increases regional heterogeneity in simulated precipitation patterns and is an important step towards a realistic cloud microphysics for the WRF modeling community. More detailed results from simulations three through six will be presented.