Citation:

Glotfelty, T., Kiran Alapaty, J. He, P. Hawbecker, X. Song, AND G. Zhang. Studying Scale Dependency of Aerosol Cloud Interactions using Scale-Aware Cloud Formulations. Monthly Weather Review. American Meteorological Society, Boston, MA, , 1-57, (2020). https://doi.org/10.1175/JAS-D-19-0203.1

Impact/Purpose:

Particulate matter, PM, (aerosol) pollution impacts clouds and weather at various spatial scales. In particular, PM impacts on warm period clouds such as thunderstorm clouds is currently missing in almost all models. New science has been developed to incorporate this missing science into a weather model used for air quality and hydrology research at EPA to study how PM impacts various clouds at different spatial scale in the eastern and western U. S. using 36, 12, 4, and 1 km grid spacing for short-term periods during the summer of 2006. This is accomplished by comparing simulations with current climatological aerosol levels to simulations where aerosol levels have been reduced by 90%. The cloud lifetime effect weakens with decreasing grid spacing due to a decrease in the relative importance of two different cloud processes, autoconversion compared to accretion. PM impacts on spatially small clouds are dominant at 36 km while PM impacts on spatially large clouds are dominant at 4 and 1 km, however, PM impacts are comparable at 12 km for both types of clouds.

Description:

The WRF-ACI model (documented in Part-I) is used to investigate the scale dependency of aerosol-cloud interactions (ACI) across the “grey zone” scales for grid scale and subgrid scale clouds. The impacts of ACI are examined across regions in the eastern and western U. S. at 36, 12, 4, and 1 km grid spacing for short-term periods during the summer of 2006. ACI impacts are determined by comparing simulations with current climatological aerosol levels to simulations where aerosol levels have been reduced by 90%. The aerosol cloud lifetime effect is found to be the dominant ACI process leading to suppressed precipitation in regions of the eastern U.S., while regions in the western U. S. experience offsetting impacts on precipitation from the cloud lifetime and thermodynamic invigoration effects of aerosols. Generally, the cloud lifetime effect weakens with decreasing grid spacing due to a decrease in the relative importance of autoconversion compared to accretion. Subgrid scale ACI are dominant at 36 km while grid scale ACI are dominant at 4 and 1 km, however, grid scale and subgrid scale ACI are comparable at 12 km. The equivalent impacts of grid scale and subgrid scale processes at 12 km lead to undesirable behavior of ACI, indicating that 12 km may not be the optimal configuration for the study of ACI with the current WRF model configuration. The work implies including subgrid scale cloud microphysics and ice/mixed phase cloud ACI processes may be necessary in weather and climate models to study ACI.

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