Abstract
An aerosol-enabled globally driven regional modeling system has been developed by coupling the National Center for Atmospheric Research's Community Atmosphere Model version 5 (CAM5) with the Weather Research and Forecasting model with chemistry (WRF-Chem). In this modeling system, aerosol-enabled CAM5, a state-of-the-art global climate model is downscaled to provide coherent meteorological and chemical boundary conditions for regional WRF-Chem simulations. Aerosol particle emissions originating outside the WRF-Chem domain can be a potentially important nonlocal aerosol source. As a test case, the potential impacts of nonlocal forest fire aerosols on regional precipitation and radiation were investigated over the northeastern United States during the summer of 2004. During this period, forest fires in Alaska and western Canada lofted aerosol particles into the midtroposphere, which were advected across the United States. WRF-Chem simulations that included nonlocal biomass burning aerosols had domain-mean aerosol optical depths that were nearly three times higher than those without, which reduced peak downwelling domain-mean shortwave radiation at the surface by ~25 W m-2. In this classic twin experiment design, adding nonlocal fire plume led to near-surface cooling and changes in cloud vertical distribution, while variations in domain-mean cloud liquid water path were negligible. The higher aerosol concentrations in the simulation with the fire plume resulted in a ~10% reduction in domain-mean precipitation coincident with an ~8% decrease in domain-mean CAPE. A suite of simulations was also conducted to explore sensitivities of meteorological feedbacks to the ratio of black carbon to total plume aerosols, as well as to overall plume concentrations. Results from this ensemble revealed that plume-induced near-surface cooling and CAPE reduction occur in a wide range of conditions. The response of moist convection was very complex because of strong thermodynamic internal variability. Key Points Nonlocal fire emissions resulted in ~10% precipitation reduction Nonlocal fire emissions reduced peak surface shortwave radiation at by 25 W m-2 An aerosol-enabled globally driven regional modeling system is developed ©2014. American Geophysical Union. All Rights Reserved.
Highlights
Biomass burning significantly affects climate and air quality by providing a major source of greenhouse gases and carbonaceous aerosols, including black carbon and organic carbon [Seiler and Crutzen, 1980; Crutzen and Andreae, 1990]
A globally driven regional modeling system, which coupled aerosol-enabled Community Atmosphere Model version 5 (CAM5) and Weather Research and Forecasting model (WRF)-Chem, was used to explore the meteorological impacts of wildfire emissions over Alaska and western Canada when they are transported over the northeastern United States during the summer of 2004
Forest fire emissions were implemented into CAM5 by vertically distributing Global Fire Emission Database Version 3 (GFEDv3) fire emissions among the CAM5 vertical layers
Summary
Biomass burning significantly affects climate and air quality by providing a major source of greenhouse gases and carbonaceous aerosols, including black carbon and organic carbon [Seiler and Crutzen, 1980; Crutzen and Andreae, 1990]. This type of modeling framework can be integrated for decades into the future to explore potentially interesting sensitivities of regional meteorology and air quality to global-scale changes in emissions and climate We use this modeling framework to explore the potential impacts of wildfire emissions in Alaska and western Canada on meteorological conditions—especially convective systems—over the northeastern United States during a summer 2004 boreal forest fire plume transport event. Nudging forces CAM5 to reproduce observed synoptic plume transport pathways during this period, which carry transient biomass burning aerosols to WRF-Chem’s lateral boundaries This modeling framework provides an opportunity to include coherent meteorological and chemical lateral boundary conditions in the regional model domain where convection is simulated explicitly, despite the fact that some inconsistencies remain between CAM5 and WRF-Chem since they use different representations of the aerosol size distribution and have different meteorological parameterizations and grid structure.
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