Open cells exhibit weaker entrainment of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer

Abstract

Abstract. This work presents synergistic satellite, airborne and surface-based observations of a pocket of open cells (POC) in the remote south-east Atlantic. The observations were obtained over and upwind of Ascension Island during the CLouds and Aerosol Radiative Impacts and Forcing (CLARIFY) and the Layered Smoke Interacting with Clouds (LASIC) field experiments. A novel aspect of this case study is that an extensive free-tropospheric biomass burning aerosol plume that had been transported from the African continent was observed to be in contact with the boundary layer inversion over the POC and the surrounding closed cellular cloud regime. The in situ measurements show marked contrasts in the boundary layer thermodynamic structure, cloud properties, precipitation and aerosol conditions between the open cells and surrounding overcast cloud field. The data demonstrate that the overlying biomass burning aerosol was mixing down into the boundary layer in the stratocumulus cloud downwind of the POC, with elevated carbon monoxide, black carbon mass loadings and accumulation-mode aerosol concentrations measured beneath the trade-wind inversion. The stratocumulus cloud in this region was moderately polluted and exhibited very little precipitation falling below cloud base. A rapid transition to actively precipitating cumulus clouds and detrained stratiform remnants in the form of thin quiescent veil clouds was observed across the boundary into and deep within the POC. The subcloud layer in the POC was much cleaner than that in the stratocumulus region. The clouds in the POC formed within an ultra-clean layer (accumulation-mode aerosol concentrations of approximately a few cm−3) in the upper region of the boundary layer, which was likely to have been formed via efficient collision–coalescence and sedimentation processes. Enhanced Aitken-mode aerosol concentrations were also observed intermittently in this ultra-clean layer, suggesting that new particle formation was taking place. Across the boundary layer inversion and immediately above the ultra-clean layer, accumulation-mode aerosol concentrations were ∼ 1000 cm−3. Importantly, the air mass in the POC showed no evidence of elevated carbon monoxide over and above typical background conditions at this location and time of year. As carbon monoxide is a good tracer for biomass burning aerosol that is not readily removed by cloud processing and precipitation, it demonstrates that the open cellular convection in the POC is not able to entrain large quantities of the free-tropospheric aerosol that was sitting directly on top of the boundary layer inversion. This suggests that the structure of the mesoscale cellular convection may play an important role in regulating the transport of aerosol from the free troposphere down into the marine boundary layer. We then develop a climatology of open cellular cloud conditions in the south-east Atlantic from 19 years of September Moderate Resolution Imaging Spectroradiometer (MODIS) Terra imagery. This shows that the maxima in open cell frequency (> 0.25) occurs far offshore and in a region where subsiding biomass burning aerosol plumes may often come into contact with the underlying boundary layer cloud. If the results from the observational case study applied more broadly, then the apparent low susceptibility of open cells to free-tropospheric intrusions of additional cloud condensation nuclei could have some important consequences for aerosol–cloud interactions in the region.

Observation of absorbing aerosols above clouds over the south-east Atlantic Ocean from the geostationary satellite SEVIRI – Part 2: Comparison with MODIS and aircraft measurements from the CLARIFY-2017 field campaign

“…The CER retrieved from the 1.6 µm channel is lower than the one retrieved from the 2.1 µm by 0.5 µm, which is consistent with the analysis from Platnick (2000). Differences in the cloud parameterization, such as the refractive index and the effective variance, also affect the CER retrieval (Arduini et al, 2005;Painemal and Zuidema, 2011;Platnick et al, 2019), although the impact is expected to depend on the observed scattering angle. Biases could also arise from an offset in the absolute calibration of the SEVIRI 1.64 µm band compared to MODIS (Meirink et al, 2013).…”

Aerosols enhance cloud lifetime and brightness along the stratus-to-cumulus transition

“…Higher effective radius and rain rates were observed inside this POC during the CLouds-Aerosol-Radiation Interactions and Forcing flight campaign (20 to 40 µm) compared with the surrounding cloud deck (10 to 20 µm) as described in Abel et al (20). Similar behavior was observed in open cellular broken cloud conditions during the Variability of the American Monsoon Systems Ocean-Cloud-Atmosphere-Land Study campaign (21) and from a climatology of POCs in satellite observations (20). We conclude that there is confidence in the ability of the satellite data to capture microphysical responses along the trajectories used in this study.…”

“…Satellite retrievals are generally more uncertain in broken cloudy conditions due to the general inability to account for complex three-dimensional (3D) radiative transfer and absorption (19). Higher effective radius and rain rates were observed inside this POC during the CLouds-Aerosol-Radiation Interactions and Forcing flight campaign (20 to 40 µm) compared with the surrounding cloud deck (10 to 20 µm) as described in Abel et al (20). Similar behavior was observed in open cellular broken cloud conditions during the Variability of the American Monsoon Systems Ocean-Cloud-Atmosphere-Land Study campaign (21) and from a climatology of POCs in satellite observations (20).…”

Impact of the variability in vertical separation between biomass burning aerosols and marine stratocumulus on cloud microphysical properties over the Southeast Atlantic

“…BBAs over the Southeast Atlantic have 500 nm singlescattering albedo ranging between 0.83 and 0.89 (Pistone et al, 2019), which indicates a significant absorbing component to the BBA layer. The warming associated with shortwave absorption by BBAs over the Southeast Atlantic can be amplified by the evaporation of cloud droplets, the semidirect effect (Hansen et al, 1997;Ackerman et al, 2000). Aerosols above a reflective cloud layer absorb more solar radiation than aerosols below or within cloud, which affects cloud formation (Haywood and Shine, 1997) and the region's aerosol direct radiative effect (Keil and Haywood, 2003;Cochrane et al, 2019).…”