02 Sep 2022
02 Sep 2022
Status: this preprint is currently under review for the journal ACP.

On the differences in the vertical distribution of modeled aerosol optical depth over the southeast Atlantic

Ian Chang1, Lan Gao1, Connor J. Flynn1, Yohei Shinozuka2,3, Sarah J. Doherty4,5, Michael S. Diamond5,a,b, Karla M. Longo6, Gonzalo A. Ferrada7, Gregory R. Carmichael7, Patricia Castellanos8, Arlindo M. da Silva8, Pablo E. Saide9,10, Calvin Howes10, Zhixin Xue11, Marc Mallet12, Ravi Govindaraju13, Qiaoqiao Wang14, Yafang Cheng15, Yan Feng16, Sharon P. Burton17, Richard A. Ferrare17, Samuel E. LeBlanc2,3, Meloë S. Kacenelenbogen2, Kristina Pistone3,2, Michal Segal-Rozenhaimer2,3,18, Kerry G. Meyer8, Ju-Mee Ryoo2,19, Leonhard Pfister2, Adeyemi A. Adebiyi20, Robert Wood5, Paquita Zuidema21, Sundar A. Christopher11,22, and Jens Redemann1 Ian Chang et al.
  • 1School of Meteorology, University of Oklahoma, Norman, Oklahoma, USA
  • 2NASA Ames Research Center, Moffett Field, California, USA
  • 3Bay Area Environmental Research Institute, Moffett Field, California, USA
  • 4Cooperative Institute for Climate, Ocean and Ecosystem Studies, University of Washington, Seattle, Washington, USA
  • 5Department of Atmospheric Science, University of Washington, Seattle, WA, USA
  • 6National Institute for Space Research, São José dos Campos, Brazil
  • 7Center for Global and Regional Environmental Research, University of Iowa, Iowa City, Iowa, USA
  • 8NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
  • 9Institute of the Environment and Sustainability, University of California, Los Angeles, Los Angeles, California, USA
  • 10Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, California, USA
  • 11Department of Atmospheric and Earth Science, University of Alabama in Huntsville, Huntsville, Alabama, USA
  • 12Centre National de Recherches Météorologiques, UMR3589, Météo-France-CNRS, Toulouse, France
  • 13Science Systems and Applications, Inc., Greenbelt, Maryland, USA
  • 14Institute for Environmental and Climate Research, Jinan University, 510632 Guangzhou, China
  • 15Minerva Research Group, Max Planck Institute for Chemistry, 55128 Mainz, Germany
  • 16Environmental Science Division, Argonne National Laboratory, Argonne, Illinois, USA
  • 17NASA Langley Research Center, Hampton, Virginia, USA
  • 18Department of Geophysics, Porter School of the Environment and Earth Sciences, Tel-Aviv University, Israel
  • 19Science and Technology Corporation (STC), Moffett Field, CA, USA
  • 20School of Natural Sciences, University of California, Merced, Merced, California, USA
  • 21Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida, USA
  • 22Earth System Science Center, University of Alabama in Huntsville, Huntsville, Alabama, USA
  • anow at: NOAA Chemical Sciences Laboratory (CSL), Boulder, Colorado, USA
  • bnow at: Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, USA

Abstract. The southeast Atlantic is home to an expansive smoke aerosol plume overlying a large cloud deck for approximately a third of the year. The aerosol plume is mainly attributed to the extensive biomass burning activity that occurs in southern Africa. Current Earth system models (ESMs) reveal significant differences in their estimates of regional aerosol radiative effects over this region. Such large differences partially stem from uncertainties in the vertical distribution of aerosols in the troposphere. These uncertainties translate into different aerosol optical depths (AOD) in the planetary boundary layer (PBL) and the free troposphere (FT). This study examines differences of AOD fraction in the FT and AOD differences among ESMs (WRF-CAM5, WRF-FINN, GEOS-Chem, EAM-E3SM, ALADIN, GEOS-FP, and MERRA-2) and aircraft-based measurements from the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign. Models frequently define the PBL as the well-mixed surface-based layer, but this definition misses the upper parts of decoupled PBLs, in which most low-level clouds occur. To account for the presence of decoupled boundary layers in the models, the height of maximum vertical gradient of specific humidity profiles from each model is used to define PBL heights. Results indicate that the monthly mean contribution of AOD in the FT to the total-column AOD ranges from 44 % to 74 % in September 2016 and from 54 % to 71 % in August 2017 within the region bounded by 25°S–0° and 15°W–15°E (excluding land) among the ESMs. Using the second-generation High Spectral Resolution Lidar (HSRL-2) to derive an aircraft-based constraint on the AOD and the fractional AOD, we found that WRF-CAM5 produces 40 % less AOD than those from the HSRL-2 measurements, but it performs well at separating AOD fraction between the FT and the PBL. AOD fractions in the FT for GEOS-Chem and EAM-E3SM are, respectively, 10 % and 15 % lower than the AOD fractions from the HSRL-2 and their similarities in the mean AODs are the result of cancellation of high and low AOD biases. GEOS-FP, MERRA-2, and ALADIN produce 24 %–36 % less AOD and tend to misplace more aerosols in the PBL compared to aircraft-based observations. The models generally underestimate AODs for measured AODs that are above 0.8, indicating their limitations at reproducing high AODs. The differences in the absolute AOD, FT AOD, and the vertical apportioning of AOD in different models highlight the need to continue improving the accuracy of modeled AOD distributions. These differences affect the sign and magnitude of the net aerosol radiative forcing, especially when aerosols are in contact with clouds.

Ian Chang et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-496', Anonymous Referee #1, 29 Nov 2022
  • RC2: 'Comment on acp-2022-496', Anonymous Referee #2, 13 Jan 2023
  • EC1: 'Comment on acp-2022-496', Yves Balkanski, 17 Jan 2023

Ian Chang et al.


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Short summary
Abundant aerosols are present above low-level liquid clouds over the southeast Atlantic during austral winter and spring. The model simulation differences in the proportion of aerosol residing in the planetary boundary layer and in the free troposphere can greatly affect the regional aerosol radiative effects. This study examines aerosol optical depths and planetary boundary layer depths among various models and evaluates them against measurements from the NASA ORACLES field campaign.