Open cells can decrease the mixing of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer

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 5 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 10 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 sub-cloud 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 ∼few cm −3 ) in the upper region of the 15 boundary layer, that 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 airmass 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 20 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 1 https://doi.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 MODIS Terra imagery. This shows that the maxima in open cell frequency (> 0.25) occurs far offshore and in a...

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 SSA assumption not only has a large impact on the scattering AOT, it also influences, to a lesser extent, the AAOT. Peers et al (2019) also showed that the AAOT is sensitive to the assumed asymmetry factor. The asymmetry factor assumption for the MOD06ACAERO retrieval is AOT dependent (with the asymmetry factor taking values of 0.60 and 0.62 at a 30 wavelength of 0.55 µm for AOT value of 0.5 and 1.5, respectively) and is smaller than the asymmetry factor assumed for the SEVIRI algorithm (i.e.…”

“…The South East Atlantic Ocean (SEAO) is an ideal region to analyse the effects of 5 partially absorbing aerosols on the radiative budget, the atmospheric stability, clouds and precipitation. The biomass burning aerosols emitted from July to October in southern Africa are mostly transported westward in the residual continental boundary layer in the free troposphere (Abel et al, 2019). These absorbing biomass burning particles are frequently observed above the extensive stratocumulus deck covering the SEAO.…”

“…In the companion paper (Part 1; Peers et al, 2019), an algorithm to retrieve aerosols above clouds from the geostationary SEVIRI instrument was presented. The first step of the method consists of correcting the SEVIRI-5 measured reflectances for the large impact of the transmittance of atmospheric gases.…”

“…For SEVIRI, the retrieval is performed for COT SEVIRI larger than 3 and poorly fitted measurements due to cloud edges, inhomogeneous clouds and observations in the glory backscattering region are removed. Readers are referred to Meyer et al (2015) and 15 Peers et al (2019) for a complete description of the MODIS and the SEVIRI filters. Here, the aerosol and cloud properties from both methods are aggregated onto a 0.1° ´ 0.1° grid.…”

“…This can be explained mainly by the 15 differences in the aerosol model assumed for the retrieval. In the companion paper (Peers et al, 2019), it is shown that the assumed aerosol Single Scattering Albedo (SSA) can have a large impact on the retrieved above-cloud AOT. For the MOD06ACAERO algorithm, the assumed microphysical properties of aerosols are a function of the AOT (Levy et al, 2009).…”