On the differences in the vertical distribution of modeled aerosol optical depth over the southeastern Atlantic
Authors: Chang Ian , Gao Lan , Flynn Connor J. , Shinozuka Yohei , Doherty Sarah J. , Diamond Michael S. , Longo Karla M. , Ferrada Gonzalo A. , Carmichael Gregory R. , Castellanos Patricia , da Silva Arlindo M. , Saide Pablo E. , Howes Calvin , Xue Zhixin , Mallet Marc , Govindaraju Ravi , Wang Qiaoqiao , Cheng Yafang , Feng Yan , Burton Sharon P. , Ferrare Richard A. , LeBlanc Samuel E. , Kacenelenbogen Meloë S. , Pistone Kristina , Segal-Rozenhaimer Michal , Meyer Kerry G. , Ryoo Ju-Mee , Pfister Leonhard , Adebiyi Adeyemi A. , Wood Robert , Zuidema Paquita , Christopher Sundar A. , Redemann Jens 
Published: April 2023

Abstract. The southeastern 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 activities that occur 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 (AODs) 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∘ N–S and 15∘ W–15∘ E (excluding land)
among the ESMs. ALADIN and GEOS-Chem show similar aerosol plume patterns to
a derived above-cloud aerosol product from the Moderate Resolution Imaging
Spectroradiometer (MODIS) during September 2016, but none of the models show
a similar above-cloud plume pattern to MODIS in August 2017. 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. Their similar mean
AODs reflect a cancellation of high and low AOD biases. Compared with
aircraft-based observations, GEOS-FP, MERRA-2, and ALADIN produce 24 %–36 % less AOD and tend to misplace more aerosols in the PBL. 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.