Unraveling Atmospheric Aerosol Layers
Quantification of the processes responsible for vertical variations in aerosol properties over the Mid-Atlantic coast requires higher model resolution.
Previous studies have revealed large errors in the characteristics of simulated aerosol particle layers in global models that subsequently contribute to uncertainties in radiative forcing calculations. Recent research identified the processes that contribute to the vertical distribution of aerosols. Understanding these processes is a necessary first step in better representing the relevant atmospheric processes in climate models.
Much attention has been placed on the role of rapid vertical transport associated with boundary layer turbulence and convective clouds, but slower vertical transport associated with synoptic-scale (large-scale) weather systems is often neglected. A recent study shows that the coarse spatial resolution normally used in global climate models likely underestimates the magnitude of mean vertical motions in the atmosphere, consequently leading to an underprediction of aerosol concentrations in the free troposphere. The higher resolution model used in this study produced stronger vertical motions and better captured the structure of aerosol layers and aerosol concentrations in the free troposphere. The researchers noted that next-generation climate models using “regionally refined” domains will not entirely solve the problem of misrepresenting aerosol layers, since a large fraction of the global domain would still employ relatively coarse resolution.
This research identified atmospheric processes responsible for the structure and composition of the aerosol layer using extensive in situ and remote-sensing measurements collected during the 2012 Two Column Aerosol Project (TCAP), undertaken by the Department of Energy’s (DOE) Atmospheric Radiation Measurement (ARM) Climate Research Facility. The TCAP campaign’s goal was to sample aerosol microphysical properties in two columns: one fixed column near the Cape Cod National Seashore’s Highlands Center on the eastern shore of Cape Cod, Massachusetts, and another movable column several hundred kilometers over the Atlantic Ocean. Aerosol layers were observed on every flight conducted by the research aircraft, although the altitude, thickness, and aerosol concentrations varied daily. A key challenge was to understand the reason for this variability in the aerosol layers, particularly those located in the free troposphere several kilometers above the ocean surface, and identify the source of these aerosols. This research showed that a higher-resolution regional model produced more aerosol mass in the free troposphere than a coarser-resolution global climate model, so that the fraction of aerosol optical thickness in the free troposphere was more consistent with lidar measurements. Simulated aerosol layers in the free troposphere were largely the result of mean vertical motions that transport aerosols from the top of the boundary layer to higher altitudes. The vertical displacement and the time period associated with upward transport in the troposphere depend on the strength of the synoptic system and whether relatively high boundary layer aerosol concentrations are present where convergence occurs. While a parameterization of subgrid-scale convective clouds modulated the concentrations of aerosols aloft, this parameterization did not significantly change the overall altitude and depth of the layers.
Jerome D. Fast
Pacific Northwest National Laboratory
This research is based on work supported by DOE, Office of Science, Office of Biological and Environmental Research (BER), Atmospheric System Research (ASR) program. Logistical support for TCAP came from the ARM Climate Research Facility, a DOE Office of Science user facility sponsored by BER. The regional-scale model simulations were performed on a Cascade supercomputer at DOE’s Environmental Molecular Sciences Laboratory.
Fast, J. D., et al. 2016. “Model Representations of Aerosol Layers Transported from North American over the Atlantic Ocean During the Two-Column Aerosol Project,” Journal of Geophysical Research: Atmospheres 121(16), 9814-48. DOI: 10.1002/2016JD025248.