Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling

Improving Bulk Microphysics Parameterizations in Simulations of Aerosol Effects

TitleImproving Bulk Microphysics Parameterizations in Simulations of Aerosol Effects
Publication TypeJournal Article
Year of Publication2013
AuthorsWang, Yuan, Fan Jiwen, Zhang Renyi, L. Leung Ruby, and Franklin Charmaine
JournalJournal of Geophysical Research - Atmospheres
Volume118
Number11
Pages5361-5379
Abstract / Summary

To improve the microphysical parameterizations for simulations of the aerosol effects in regional and global climate models, the Morrison double‐moment bulk microphysical scheme presently implemented in the Weather Research and Forecasting model is modified by replacing the prescribed aerosols in the original bulk scheme (Bulk‐OR) with a prognostic double‐moment aerosol representation to predict both aerosol number concentration and mass mixing ratio (Bulk‐2M). Sensitivity modeling experiments are performed for two distinct cloud regimes: maritime warm stratocumulus clouds (Sc) over southeast Pacific Ocean from the VOCALS project and continental deep convective clouds in the southeast of China. The results from Bulk‐OR and Bulk‐2M are compared against atmospheric observations and simulations produced by a spectral bin microphysical scheme (SBM). The prescribed aerosol approach (Bulk‐OR) produces unreliable aerosol and cloud properties throughout the simulation period, when compared to the results from those using Bulk‐2M and SBM, although all of the model simulations are initiated by the same initial aerosol concentration on the basis of the field observations. The impacts of the parameterizations of diffusional growth and autoconversion of cloud droplets and the selection of the embryonic raindrop radius on the performance of the bulk microphysical scheme are also evaluated by comparing the results from the modified Bulk‐2M with those from SBM simulations. Sensitivity experiments using four different types of autoconversion schemes reveal that the autoconversion parameterization is crucial in determining the raindrop number, mass concentration, and drizzle formation for warm stratocumulus clouds. An embryonic raindrop size of 40 µm is determined as a more realistic setting in the autoconversion parameterization. The saturation adjustment employed in calculating condensation/evaporation in the bulk scheme is identified as the main factor responsible for the large discrepancies in predicting cloud water in the Sc case, suggesting that an explicit calculation of diffusion growth with predicted supersaturation is necessary to improve the bulk microphysics scheme. Lastly, a larger rain evaporation rate below clouds is found in the bulk scheme in comparison to the SBM simulation, which may contribute to a lower surface precipitation in the bulk scheme.

URLhttp://onlinelibrary.wiley.com/doi/10.1002/jgrd.50432/abstract
DOI10.1002/jgrd.50432
Journal: Journal of Geophysical Research - Atmospheres
Year of Publication: 2013
Volume: 118
Number: 11
Pages: 5361-5379
Publication Date: 06/2013

To improve the microphysical parameterizations for simulations of the aerosol effects in regional and global climate models, the Morrison double‐moment bulk microphysical scheme presently implemented in the Weather Research and Forecasting model is modified by replacing the prescribed aerosols in the original bulk scheme (Bulk‐OR) with a prognostic double‐moment aerosol representation to predict both aerosol number concentration and mass mixing ratio (Bulk‐2M). Sensitivity modeling experiments are performed for two distinct cloud regimes: maritime warm stratocumulus clouds (Sc) over southeast Pacific Ocean from the VOCALS project and continental deep convective clouds in the southeast of China. The results from Bulk‐OR and Bulk‐2M are compared against atmospheric observations and simulations produced by a spectral bin microphysical scheme (SBM). The prescribed aerosol approach (Bulk‐OR) produces unreliable aerosol and cloud properties throughout the simulation period, when compared to the results from those using Bulk‐2M and SBM, although all of the model simulations are initiated by the same initial aerosol concentration on the basis of the field observations. The impacts of the parameterizations of diffusional growth and autoconversion of cloud droplets and the selection of the embryonic raindrop radius on the performance of the bulk microphysical scheme are also evaluated by comparing the results from the modified Bulk‐2M with those from SBM simulations. Sensitivity experiments using four different types of autoconversion schemes reveal that the autoconversion parameterization is crucial in determining the raindrop number, mass concentration, and drizzle formation for warm stratocumulus clouds. An embryonic raindrop size of 40 µm is determined as a more realistic setting in the autoconversion parameterization. The saturation adjustment employed in calculating condensation/evaporation in the bulk scheme is identified as the main factor responsible for the large discrepancies in predicting cloud water in the Sc case, suggesting that an explicit calculation of diffusion growth with predicted supersaturation is necessary to improve the bulk microphysics scheme. Lastly, a larger rain evaporation rate below clouds is found in the bulk scheme in comparison to the SBM simulation, which may contribute to a lower surface precipitation in the bulk scheme.

DOI: 10.1002/jgrd.50432
Citation:
Wang, Y, J Fan, R Zhang, L Leung, and C Franklin.  2013.  "Improving Bulk Microphysics Parameterizations in Simulations of Aerosol Effects."  Journal of Geophysical Research - Atmospheres 118(11): 5361-5379.  https://doi.org/10.1002/jgrd.50432.