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Discussion papers
https://doi.org/10.5194/acp-2018-900
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-900
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 27 Sep 2018

Research article | 27 Sep 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Influence of cloud microphysical processes on black carbon wet removal, global distributions, and radiative forcing

Jiayu Xu1, Jiachen Zhang2, Junfeng Liu1, Kan Yi1, Songlin Xiang1, Xiurong Hu1, Yuqing Wang1, Shu Tao1, and George Ban-Weiss2 Jiayu Xu et al.
  • 1Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing, China
  • 2Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, CA, USA

Abstract. Parameterizations that impact wet removal of black carbon remain uncertain in global climate models. In this study, we enhance the default wet deposition scheme for BC in the Community Earth System Model (CESM) to (a) add relevant physical processes that were not resolved in the default model, and (b) facilitate understanding of the relative importance of various cloud processes on BC distributions. We find that the enhanced scheme greatly improves model performance against HIPPO observations relative to the default scheme. We find that convection scavenging, aerosol activation, ice nucleation, evaporation of rain/snow, and below cloud scavenging dominate wet deposition of BC. BC conversion rates for processes related to in-cloud water/ice conversion (i.e., riming, the Bergeron processes, and evaporation of cloud water sedimentation) are relatively smaller, but have large seasonal variations. We also conduct sensitivity simulations that turn off each cloud process one at a time to quantify the influence of cloud processes on BC distributions and radiative forcing. Convective scavenging is found to most significantly influence BC concentrations at mid-altitudes over the tropics and even globally. In addition, BC is sensitive to all cloud processes over the Northern Hemisphere at high latitudes. As for BC vertical distributions, convective scavenging has a dominant influence. Aerosol activation mainly increases the fraction of column BC below 5km whereas ice nucleation decreases that above 10km. During wintertime, the Bergeron process also significantly increases BC concentrations at lower altitudes over the Arctic. Our simulation yields a global BC burden of 85Gg; corresponding direct radiative forcing (DRF) of BC estimated using the Parallel Offline Radiative Transfer (PORT) is 0.13Wm−2, much lower than previous studies. The range of DRF derived from sensitivity simulations is large, 0.09–0.33Wm−2, corresponding to BC burdens varying from 73Gg to 151Gg. Due to differences in BC vertical distributions among each sensitivity simulation, fractional changes in DRF (relative to the baseline simulation) are always higher than fractional changes in BC burdens; this occurs because relocating BC in the vertical influences the radiative forcing per BC mass. Our results highlight the influences of cloud microphysical processes on BC concentrations and radiative forcing.

Jiayu Xu et al.
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Short summary
In this study, we fully describe black carbon wet removal coupled with all cloud processes from cloud microphysics scheme in a climate model, and conduct sensitivity simulations that turn off each cloud process one at a time. We find that convective scavenging, aerosol activation, ice nucleation, evaporation of rain/snow, and below cloud scavenging dominate wet deposition of BC. In addition, the range of direct radiative forcing derived from sensitivity simulations is large, 0.08-0.46 W m-2.
In this study, we fully describe black carbon wet removal coupled with all cloud processes from...
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