What are the Implications of Optical Closure Using Measurements from the Two Column Aerosol Project (TCAP)?
Poster PDF
Authors
Jerome D Fast — Pacific Northwest National Laboratory
Larry Berg — Pacific Northwest National Laboratory
Evgueni Kassianov — Pacific Northwest National Laboratory
Duli Chand — Pacific Northwest National Laboratory
Richard A. Ferrare — NASA - Langley Research Center
Connor J. Flynn — University of Oklahoma School of Meteorology
Mikhail S. Pekour — Pacific Northwest National Laboratory
Arthur J Sedlacek — Brookhaven National Laboratory
John E Shilling — Pacific Northwest National Laboratory
Jason Tomlinson — Pacific Northwest National Laboratory
Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
Description
Relatively large uncertainties remain in climate model predictions of aerosol optical properties and their vertical variations in the atmosphere. The ARM Two-Column Aerosol Project (TCAP) was designed to investigate changes in aerosol microphysical properties, mixing state, optical properties, and radiative forcing in two atmospheric columns: one over Cape Cod, Massachusetts and another located approximately 200 km to the east over the ocean. To have confidence in the measured quantities when assessing climate model performance requires that the diverse set of independent measurements need to be consistent with each other. We therefore perform a column optical closure study in which the measured aerosol concentration, composition, and size are used to drive a radiation model based on Mie theory. Calculated aerosol water is obtained from the MOSAIC aerosol model, driven by observed aerosols, temperature and relative humidity. The calculated scattering, absorption, single scattering albedo (SSA), and extinction profiles from dry aerosols are compared to independent Nephelometer, and PSAP measurements, and similar profiles that include aerosol water are compared with ambient High Spectral Resolution Lidar (HSRL-2) measurements. We show that good agreement between the observed and calculated scattering and SSA is obtained only when the optical counter size distribution measurements are corrected using more realistic refractive indices based on composition measurements (Kassianov et al., 2015). The measured absorption from the PSAP instrument is within the range of calculated absorption from measured SP2 black carbon data. In addition to quantifying the propagation of uncertainties in the measured aerosol composition and size, sensitivity studies are performed to assess assumptions used by Mie theory such as those for mixing rule, refractive indices and their wavelength dependence, and brown carbon. A prognostic regional-scale aerosol model, WRF-Chem, is used to show that errors in simulated optical properties are larger than the uncertainties in measured scattering, absorption, SSA, and extinction. The effect of predicted aerosols on shortwave radiation are compared with similar calculations forced by the observed profile measurements. We show that models can sometimes obtain the aerosol radiative effects that are similar to observed, but for the wrong reasons and that the extensive measurements from TCAP can be used to unravel this conundrum.
