On the differences in the vertical distribution of modeled aerosol optical depth over the southeastern Atlantic
Journal Article
Overview
abstract
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.;
