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Simulations are performed for the period 2000–2015 by two different regional climate models, ALADIN and RegCM, to quantify the direct and semi-direct radiative effects of biomass-burning aerosols (BBAs) in the southeast Atlantic (SEA) region. Different simulations have been performed using strongly absorbing BBAs in accordance with recent in situ observations over the SEA. For the July–August–September (JAS) season, the single scattering albedo (SSA) and total aerosol optical depth (AOD) simulated by the ALADIN and RegCM models are consistent with the MACv2 climatology and MERRA-2 and CAMS-RA reanalyses near the biomass-burning emission sources. However, the above-cloud AOD is slightly underestimated compared to satellite (MODIS and POLDER) data during the transport over the SEA. The direct radiative effect exerted at the continental and oceanic surfaces by BBAs is significant in both models and the radiative effects at the top of the atmosphere indicate a remarkable regional contrast over SEA (in all-sky conditions), with a cooling (warming) north (south) of 10 ∘S, which is in agreement with the recent MACv2 climatology. In addition, the two models indicate that BBAs are responsible for an important shortwave radiative heating of ∼0.5–1 K per day over SEA during JAS with maxima between 2 and 4 km a.m.s.l. (above mean sea level). At these altitudes, BBAs increase air temperature by ∼0.2–0.5 K, with the highest values being co-located with low stratocumulus clouds. Vertical changes in air temperature limit the subsidence of air mass over SEA, creating a cyclonic anomaly. The opposite effect is simulated over the continent due to the increase in lower troposphere stability. The BBA semi-direct effect on the lower troposphere circulation is found to be consistent between the two models. Changes in the cloud fraction are moderate in response to the presence of smoke, and the models differ over the Gulf of Guinea. Finally, the results indicate an important sensitivity of the direct and semi-direct effects to the absorbing properties of BBAs. Over the stratocumulus (Sc) region, DRE varies from +0.94 W m−2 (scattering BBAs) to +3.93 W m−2 (most absorbing BBAs).

Aerosol absorption is a key property to assess the radiative impacts of aerosols on climate at both global and regional scales. The aerosol physico-chemical and optical properties remain not sufficiently constrained in climate models, with difficulties to properly represent both the aerosol load and their absorption properties in clear and cloudy scenes, especially for absorbing biomass burning aerosols (BBA). In this study we focus on biomass burning (BB) particle plumes transported above clouds over the southeast Atlantic (SEA) region off the southwest coast of Africa, in order to improve the representation of their physico-chemical and absorption properties. The methodology is based on aerosol regional numerical simulations from the WRF-Chem coupled meteorology–chemistry model combined with a detailed inventory of BB emissions and various sets of innovative aerosol remote sensing observations, both in clear and cloudy skies from the POLDER-3/PARASOL space sensor. Current literature indicates that some organic aerosol compounds (OC), called brown carbon (BrOC), primarily emitted by biomass combustion absorb the ultraviolet-blue radiation more efficiently than pure black carbon (BC). We exploit this specificity by comparing the spectral dependence of the aerosol single scattering albedo (SSA) derived from the POLDER-3 satellite observations in the 443–1020 nm wavelength range with the SSA simulated for different proportions of BC, OC and BrOC at the source level, considering the homogeneous internal mixing state of particles. These numerical simulation experiments are based on two main constraints: maintaining a realistic aerosol optical depth both in clear and above cloudy scenes and a realistic BC/OC mass ratio. Modelling experiments are presented and discussed to link the chemical composition with the absorption properties of BBA and to provide estimates of the relative proportions of black, organic and brown carbon in the African BBA plumes transported over the SEA region for July 2008. The absorbing fraction of organic aerosols in the BBA plumes, i.e. BrOC, is estimated at 2 % to 3 %. The simulated mean SSA are 0.81 (565 nm) and 0.84 (550 nm) in clear and above cloudy scenes respectively, in good agreement with those retrieved by POLDER-3 (0.85±0.05 at 565 nm in clear sky and at 550 nm above clouds) for the studied period.

Aerosols have a direct effect on the Earth's radiative budget and can also affect cloud development and lifetime, and the aerosols above clouds (AAC) are particularly associated with high uncertainties in global climate models. Therefore, it is a prerequisite to improve the description and understanding of these situations. During the austral winter, large loadings of biomass burning aerosols originating from fires in the southern African subcontinent are lifted and transported westwards, across the southeastern Atlantic Ocean. The negligible wet scavenging of these absorbing aerosols leads to a near-persistent smoke layer above one of the largest stratocumulus cloud decks on the planet. Therefore, the southeastern Atlantic region is a very important area for studying the impact of above-cloud absorbing aerosols, their radiative forcing and their possible effects on clouds. In this study we aim to analyse and quantify the effect of smoke loadings on cloud properties using a synergy of different remote sensing techniques from A-Train retrievals (methods based on the passive instruments POLDER and MODIS and the operational method of the spaceborne lidar CALIOP), collocated with ERA-Interim re-analysis meteorological profiles. To analyse the possible mechanisms of AAC effects on cloud properties, we developed a high and low aerosol loading approach, which consists in evaluating the change in radiative quantities (i.e. cloud-top cooling, heating rate vertical profiles) and cloud properties with the smoke loading. During this analysis, we account for the variation in the meteorological conditions over our sample area by selecting the months associated with one meteorological regime (June–August). The results show that the region we focus on is primarily under the energetic influence of absorbing aerosols, leading to a significant positive shortwave direct effect at the top of the atmosphere. For larger loads of AACs, clouds are optically thicker, with an increase in liquid water path of 20 g m−2 and lower cloud-top altitudes by 100 m. These results do not contradict the semi-direct effect of above-cloud aerosols, explored in previous studies. Furthermore, we observe a strong covariance between the aerosol and the water vapour loadings, which has to be accounted for. A detailed analysis of the heating rate profiles shows that within the smoke layer, the absorbing aerosols are 90 % responsible for warming the ambient air by approximately 5.7 K d−1. The accompanying water vapour, however, has a longwave effect at distance on the cloud top, reducing its cooling by approximately 4.7 K d−1 (equivalent to 7 %). We infer that this decreased cloud-top cooling in particular, in addition with the higher humidity above the clouds, might modify the cloud-top entrainment rate and its effect, leading to thicker clouds. Therefore, smoke (the combination of aerosol and water vapour) events would have the potential to modify and probably reinforce the underlaying cloud cover.

The Aerosol, Radiation and Clouds in southern Africa (AEROCLO-sA) project investigates the role of aerosols on the regional climate of southern Africa. This is a unique environment where natural and anthropogenic aerosols and a semipermanent and widespread stratocumulus (Sc) cloud deck are found. The project aims to understand the dynamical, chemical, and radiative processes involved in aerosol–cloud–radiation interactions over land and ocean and under various meteorological conditions. The AEROCLO-sA field campaign was conducted in August and September of 2017 over Namibia. An aircraft equipped with active and passive remote sensors and aerosol in situ probes performed a total of 30 research flight hours. In parallel, a ground-based mobile station with state-of-the-art in situ aerosol probes and remote sensing instrumentation was implemented over coastal Namibia, and complemented by ground-based and balloonborne observations of the dynamical, thermodynamical, and physical properties of the lower troposphere. The focus laid on mineral dust emitted from salty pans and ephemeral riverbeds in northern Namibia, the advection of biomass-burning aerosol plumes from Angola subsequently transported over the Atlantic Ocean, and aerosols in the marine boundary layer at the ocean–atmosphere interface. This article presents an overview of the AEROCLO-sA field campaign with results from the airborne and surface measurements. These observations provide new knowledge of the interactions of aerosols and radiation in cloudy and clear skies in connection with the atmospheric dynamics over southern Africa. They will foster new advanced climate simulations and enhance the capability of spaceborne sensors, ultimately allowing a better prediction of future climate and weather in southern Africa

The direct radiative effect (DRE) of aerosols above clouds has been found to be significant over the south-east Atlantic Ocean during the African biomass burning season due to elevated smoke layers absorbing radiation above the cloud deck. So far, global climate models have been unsuccessful in reproducing the high DRE values measured by various satellite instruments. Meanwhile, the radiative effects by aerosols have been identified as the largest source of uncertainty in global climate models. In this paper, three independent satellite datasets of DRE during the biomass burning season in 2006 are compared to constrain the south-east Atlantic radiation budget. The DRE of aerosols above clouds is derived from the spectrometer SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), the polarimeter Polarization and Directionality of the Earth's Reflectances (POLDER), and collocated measurements by the spectrometer Ozone Monitoring Instrument (OMI) and the imager Moderate Resolution Imaging Spectroradiometer (MODIS). All three datasets confirm the high DRE values during the biomass season, underlining the relevance of local aerosol effects. Differences between the instruments can be attributed mainly to sampling issues. When these are accounted for, the remaining differences can be explained by a higher cloud optical thickness (COT) derived from POLDER compared to the other instruments and a neglect of aerosol optical thickness (AOT) at shortwave infrared (SWIR) wavelengths in the method used for SCIAMACHY and OMI–MODIS. The higher COT from POLDER by itself can explain the difference found in DRE between POLDER and the other instruments. The AOT underestimation is mainly evident at high values of the aerosol DRE and accounts for about a third of the difference between POLDER and OMI–MODIS DRE. The datasets from POLDER and OMI–MODIS effectively provide lower and upper bounds for the aerosol DRE over clouds over the south-east Atlantic, which can be used to challenge global circulation models (GCMs). Comparisons of model and satellite datasets should also account for sampling issues. The complementary DRE retrievals from OMI–MODIS and POLDER may benefit from upcoming satellite missions that combine spectrometer and polarimeter measurements.

Aerosols have important effects on both local and global climate, as well as on clouds and precipitations. We present some original results of the airborne AErosol RadiatiOn and CLOud in Southern Africa (AEROCLO-sA) field campaign led in Namibia in August and September 2017. In order to quantify the aerosols radiative impact on the Namibian regional radiative budget, we use an innovative approach that combines the OSIRIS polarimeter and lidar data to derive heating rate of the aerosols. To calculate this parameter, we use a radiative transfer code and meteorological parameters provided by dropsondes. This approach is evaluated during massive transports of biomass burning particles above clouds. We present vertical profiles of heating rates computed in the solar and thermal parts of the spectrum. Our results indicated strong positive heating rate values retrieved above clouds due to aerosols, between +2 and +5 Kelvin per day (vertically averaged). Within the smoke layer, water vapor's cooling effect through infrared radiation generally balances its warming effect from solar radiation. At the top of the layer, a stronger cooling effect of −1.5 K/day often dominates due to water vapor. In order to validate this methodology, we use irradiance measurements acquired during sounding performed with the aircraft during dedicated parts of the flights, which provides direct measurements of irradiances distribution and heating rates in function of the altitude. Finally, we discuss the possibility to apply this method to available and future spaceborne passive and active sensors.

Aerosols have important effects on both local and global climate, as well as on clouds and precipitations. We present original results derived from the airborne observations acquired from the AErosol RadiatiOn and CLOud in Southern Africa (AEROCLO-sA) field campaign led in Namibia in August and September 2017. In order to quantify the aerosols' radiative impact on the Namibian regional radiative budget, we use an innovative approach that combines the OSIRIS polarimeter, an airborne prototype of the future 3MI polarimeter of ESA, and lidar data to derive the heating rate of the aerosols. To calculate this parameter we use a radiative transfer code, meteorological parameters provided by dropsondes, and OSIRIS-retrieved aerosol optical thickness, size, and absorption above clouds. This approach is evaluated during massive transports of biomass-burning particles above clouds. We present vertical profiles of heating rates computed in the solar and thermal parts of the spectrum. Our results indicate strong positive heating rate values retrieved above clouds due to aerosols, between +2 and +5 K per day (vertically averaged). Within the smoke layer, water vapor's cooling effect through infrared radiation generally balances its warming effect from solar radiation. At the top of the layer, a stronger cooling effect of −1.5 K d−1 often dominates due to water vapor. In order to validate this methodology, we use irradiance measurements acquired by aircraft during spiral descent during dedicated parts of their flights, which provides direct measurements of irradiance distribution and heating rates as functions of the altitude. Despite the challenges posed by cloud horizontal variability observed during the spiral descent, simulated and measured results generally agree in most cases. Finally, we discuss the possibility of applying this method to available and future spaceborne passive and active sensors.

We analyse the airborne measurements of above-cloud aerosols from the AErosol, RadiatiOn, and CLOuds in southern Africa (AEROCLO-sA) field campaign performed in Namibia during August and September 2017. The study aims to retrieve the aerosol above-cloud direct radiative effect (DRE) with well-defined uncertainties. To improve the retrieval of the aerosol and cloud properties, the airborne demonstrator of the Multi-Viewing, Multi-Channel, Multi-Polarization (3MI) satellite instrument, called the Observing System Including PolaRisation in the Solar Infrared Spectrum (OSIRIS), was deployed on-board the SAFIRE (Service des Avions Français Instrumentés pour la Rechercheen Environnement) Falcon 20 aircraft during 10 flights performed over land, over the ocean, and along the Namibian coast. The airborne instrument OSIRIS provides observations at high temporal and spatial resolutions for aerosol above clouds (AACs) and cloud properties. OSIRIS was supplemented with the Photomètre Léger Aéroporté pour la surveillance des Masses d'Air version 2 (PLASMA2). The combined airborne measurements allow, for the first time, the validation of AAC algorithms previously developed for satellite measurements. The variations in the aerosol properties are consistent with the different atmospheric circulation regimes observed during the deployment. Airborne observations typically show strong aerosol optical depth (AOD; up to 1.2 at 550 nm) of fine-mode particles from biomass burning (extinction Ångström exponent varying between 1.6 and 2.2), transported above bright stratocumulus decks (mean cloud top around 1 km above mean sea level), with cloud optical thickness (COT) up to 35 at 550 nm. The above-cloud visible AOD retrieved with OSIRIS agrees within 10 % of the PLASMA2 sun photometer measurements in the same environment. The single scattering albedo (SSA) is one of the most influential parameters on the AAC DRE calculation that remains largely uncertain in models. During the AEROCLO-sA campaign, the average SSA obtained by OSIRIS at 550 nm is 0.87, which is in agreement within 3 %, on average, with previous polarimetric-based satellite and airborne retrievals. The strong absorption of the biomass burning plumes in the visible range is generally consistent with the observations from the Aerosol Robotic Network (AERONET) ground-based sun photometers. This, however, shows a significant increase in the particles' absorption at 440 nm in northern Namibia and Angola, which indicates more absorbing organic species within the observed smoke plumes. Biomass burning aerosols are also vertically collocated, with significant amounts of water content up to the top of the plume at around 6 km height in our measurements. The detailed characterization of aerosol and cloud properties, water vapour, and their uncertainties obtained from OSIRIS and PLASMA2 measurements allows us to study their impacts on the AAC DRE. The high-absorbing load of AAC, combined with high cloud albedo, leads to unprecedented DRE estimates, which are higher than previous satellite-based estimates. The average AAC DRE calculated from the airborne measurements in the visible range is +85 W m−2 (standard deviation of 26 W m−2), with instantaneous values up to +190 W m−2 during intense events. These high DRE values, associated with their uncertainties, have to be considered as new upper cases in order to evaluate the ability of models to reproduce the radiative impact of the aerosols over the southeastern Atlantic region.

We analyse the airborne measurements of above-cloud aerosols from the AErosol, RadiatiOn, and CLOuds in southern Africa (AEROCLO-sA) field campaign performed in Namibia during August and September 2017. The study aims to retrieve the aerosol above-cloud direct radiative effect (DRE) with well-defined uncertainties. To improve the retrieval of the aerosol and cloud properties, the airborne demonstrator of the Multi-Viewing, Multi-Channel, Multi-Polarization (3MI) satellite instrument, called the Observing System Including PolaRisation in the Solar Infrared Spectrum (OSIRIS), was deployed on-board the SAFIRE (Service des Avions Français Instrumentés pour la Rechercheen Environnement) Falcon 20 aircraft during 10 flights performed over land, over the ocean, and along the Namibian coast. The airborne instrument OSIRIS provides observations at high temporal and spatial resolutions for aerosol above clouds (AACs) and cloud properties. OSIRIS was supplemented with the Photomètre Léger Aéroporté pour la surveillance des Masses d'Air version 2 (PLASMA2). The combined airborne measurements allow, for the first time, the validation of AAC algorithms previously developed for satellite measurements. The variations in the aerosol properties are consistent with the different atmospheric circulation regimes observed during the deployment. Airborne observations typically show strong aerosol optical depth (AOD; up to 1.2 at 550 nm) of fine-mode particles from biomass burning (extinction Ãngström exponent varying between 1.6 and 2.2), transported above bright stratocumulus decks (mean cloud top around 1 km above mean sea level), with cloud optical thickness (COT) up to 35 at 550 nm. The above-cloud visible AOD retrieved with OSIRIS agrees within 10 % of the PLASMA2 sun photometer measurements in the same environment.

Simulations of total and polarized cloud reflectance angular signatures such as the ones measured by the multi-angular and polarized radiometer POLDER3/PARASOL are used to evaluate cloud heterogeneity effects on cloud parameter retrievals. Effects on optical thickness, albedo, effective radius and variance of the cloud droplet size distribution and aerosol parameters above cloud are analyzed. Three different clouds that have the same mean optical thicknesses were generated: the first with a flat top, the second with a bumpy top and the last with a fractional cloud cover. At small scale (50 m), for oblique solar incidence, the illumination effects lead to higher total but also polarized reflectances. The polarized reflectances even reach values that cannot be predicted by the 1-D homogeneous cloud assumption. At the POLDER scale (7 km × 7 km), the angular signature is modified by a combination of the plane–parallel bias and the shadowing and illumination effects. In order to quantify effects of cloud heterogeneity on operational products, we ran the POLDER operational algorithms on the simulated reflectances to retrieve the cloud optical thickness and albedo. Results show that the cloud optical thickness is greatly affected: biases can reach up to −70, −50 or +40 % for backward, nadir and forward viewing directions, respectively. Concerning the albedo of the cloudy scenes, the errors are smaller, between −4.7 % for solar incidence angle of 20∘ and up to about +8 % for solar incidence angle of 60∘. We also tested the heterogeneity effects on new algorithms that allow retrieving cloud droplet size distribution and cloud top pressures and also aerosol above clouds. Contrary to the bi-spectral method, the retrieved cloud droplet size parameters are not significantly affected by the cloud heterogeneity, which proves to be a great advantage of using polarized measurements. However, the cloud top pressure obtained from molecular scattering in the forward direction can be biased up to about 60 hPa (around 550 m). Concerning the aerosol optical thickness (AOT) above cloud, the results are different depending on the available angular information. Above the fractional cloud, when only side scattering angles between 100 and 130∘ are available, the AOT is underestimated because of the plane–parallel bias. However, for solar zenith angle of 60∘ it is overestimated because the polarized reflectances are increased in forward directions.