Modeling the smoky troposphere of the southeast Atlantic: a comparison to ORACLES airborne observations from September of 2016

Abstract

Abstract. The southeast Atlantic is home to well-defined smoke outflow from Africa coinciding vertically with extensive marine boundary-layer cloud decks, both reaching their climatological maxima in spatial extent around September. A framework is put forth for evaluating the performance of a range of global and regional aerosol models against observations made during the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) airborne mission in September 2016. The sparse airborne observations are first aggregated into 2° grid boxes and into three vertical layers: the cloud-topped marine boundary layer (MBL), the layer from cloud top to 3 km, and the 3–6 km layer. Aerosol extensive properties simulated for the entire study region for all September suggest that the 2016 ORACLES observations are reasonably representative of the regional monthly average, with systematic deviations of 30 % or less. All six models typically place the bottom of the smoke layer at lower altitudes than do the airborne lidar observations by 300–1400 m, whereas model aerosol top heights are within 0–500 m of the observations. All but one of the models that report carbonaceous aerosol masses underestimate the ratio of particulate extinction to the masses, a proxy for mass extinction efficiency, in 3–6 km. Notable findings on individual models include that WRF-CAM5 predicts the mass of black carbon and organic aerosols with minor (~ 10 % or less) biases. GEOS-5 overestimates the carbonaceous particle masses in the MBL by a factor of 3–6. Extinction coefficients in the free troposphere (FT) and above-cloud aerosol optical depth (ACAOD) are 10–30 % lower in WRF-CAM5, 30–50 % lower in GEOS-5, 10–40 % higher in GEOS-Chem, 10–20 % higher in EAM-E3SM except for the practically unbiased 3–6 km extinction, and 20–70 % lower in the Unified Model, than the airborne in situ, lidar and sunphotometer measurements. ALADIN-Climate also underestimates the ACAOD, by 30 %. GEOS-5 and GEOS-Chem predict carbon monoxide in the MBL with small (10 % or less) negative biases, despite their overestimates of carbonaceous aerosol masses. Overall, this study highlights a new approach to utilizing airborne aerosol measurements for model diagnosis.

Improving aerosol activation in the double-moment Unified Model with CLARIFY measurements

“…The biomass burning emissions diameter (specifically, the number geometric mean diameter) is still 120 nm instead of the default for UKCA of 150 nm. This diameter is shown by Shinozuka et al (2019) to give aerosol dry diameters in reasonable agreement (within 40%) with measurements from the parallel NASA ORACLES campaign (ObseRvations of Aerosols above CLouds and their intEractionS), although the diameter is still slightly overestimated compared to ORACLES. For example, in the lower free troposphere most affected by smoke, the simulated dry diameters are biased 37% high.…”

“…In order to simulate the region of interest at 500 m resolution, we start with a global model setup similar to that used by Gordon et al (2018) and identical to that used in the intercomparison study of Shinozuka et al (2019). The horizontal resolution is 0.8 • × 0.55 • (N216) and there are 70 vertical levels from the surface to 85 km altitude.…”

The diurnal cycle of the smoky marine boundary layer observed during August in the remote southeast Atlantic

“…The focus on one month only reduces the convolution of faster cloud responses with larger-scale seasonal meteorological changes. August boundary layer aerosol concentrations are similar to those within the September free troposphere (Shinozuka et al, 2019), but the ability of aerosol particles to absorb sunlight is more pronounced, with single scattering albedos ranging from 0.78 to 0.83 (Zuidema et al, 2018), lower than documented for the September free troposphere (Pistone et al, 2019;Cochrane et al, 2019). The combination of high aerosol concentrations and low single-scatteringalbedo indicate the potential for a clear cloud response to a robust radiative warming of the boundary layer in August.…”

“…1). This contrasts with the more frequent variations in boundary layer smoke loadings occurring during June and July, while in September the boundary layer smoke loadings decrease dramatically, with more of the biomass burning aerosol residing above the boundary layer (e.g., Shinozuka et al, 2019). The focus on one month only reduces the convolution of faster cloud responses with larger-scale seasonal meteorological changes.…”

“…Shortwave-absorbing aerosols above the southeast Atlantic overlay and mix in with one of the Earth's largest stratocumulus decks from July through October. Many studies highlight the presence and radiative impact of absorbing aerosol in the free troposphere (Waquet et al, 2013;Peers et al, 2015;Das et al, 2017;Sayer et al, 2019;Peers et al, 2019;Deaconu et al, 2019), and indeed recent aircraft measurements confirm the biomass-burning aerosol (BBA) is primarily in the free troposphere during the month of September (LeBlanc et al, 2019;Cochrane et al, 2019;Shinozuka et al, 2019). The above-cloud aerosol shortwave absorption can warm the free troposphere, all else being equal, strengthening the capping inversion and reducing entrainment (Johnson et al, 2004;Gordon et al, 2018;Herbert et al, 2019).…”