Effets direct et semi-direct des aérosols en Afrique de l'ouest pendant la saison sèche

Ces travaux de thèse présentent l'étude du forçage radiatif direct et semi-direct ainsi que les impacts climatiques associés, qu'exercent les particules d'aérosols désertiques et de feux de biomasse sur le climat régional ouest Africain pendant la saison sèche. Dans ce cadre, le modèle de climat à été utilisé en lien avec les observations in-situ des campagnes DABEX/AMMA-SOP0, les mesures photométriques (AERONET/PHOTONS) et satellitaire (PARASOL,MODIS,OMIetMISR). Le modèle RegCM3 configuré spécifiquement pour représenter les aérosols d'Afrique de l'ouest a été évalué au cours d'une simulation de la saison sèche 2006. Dans cette configuration, le modèle s'est montré capable d'estimer raisonnablement les quantités d'aérosols pour des applications climatiques et les variations d'albédo de simple diffusion. Pendant les mois de décembre et janvier, l'albédo de simple diffusion simulé au dessus du Sahel se situe entre 0.81 et 0.83 (à 440 nm) quand les aérosols de feux de biomasse dominent le mélange atmosphérique. Pendant les mois de mars et avril, pour lesquels les aérosols désertiques dominent, l'albédo de simple diffusion simulé se situe entre 0.90 et 0.92 (à 440 nm). Le forçage radiatif direct au sommet de l'atmosphère (visible + infrarouge) est majoritairement négatif sur l'ensemble du domaine et compris entre -5.0 W /m2 et -4.0 W /m2. Sur le Sahara, le forçage radiatif direct TOA est proche de zéro (-0.15 W /m2). La grande divergence entre le forçage radiatif direct au sommet de l'atmosphère et en surface indique que l'absorption est importante au sein de l'atmosphère (forçage radiatif direct atmosphèrique de +11.47 et +24.40 W /m2 au dessus du Sahara et du Sahel, respectivement). Du fait de leur albédo de simple diffusion relativement bas, les aérosols de feux de biomasse, contribuent principalement à ce rechauffement atmosphérique. Ceci se traduit à l'échelle régionale par un taux d'échauffement radiatif atmosphérique (dans le visible) compris entre +0.2 et +0.6 K /jour en moyenne journalière dans la couche d'aérosol de feux de biomasse localisée entre 2 et 5 km. Deux simulations à plus longue échéance sur la période 2001-2006 ont été menées pour étudier les conséquences de ce forçage radiatif sur le climat régional pendant la saison sèche. Une simulation DUST (aérosols désertiques) et BBDUST (aérosols désertiques + aérosols de feux) sont réalisées en prenant en compte les rétroactions liées au forçage radiatif direct. L'important forçage radiatif en surface réduit l'énergie radiative disponible au sol. Ceci conduit à des perturbations significatives du bilan energétique en surface. Au dessus du Sahara, les réductions de flux de chaleur sensible sont proches dans les expériences DUST et BBDUST (respectivement -5.52 W /m2 et -6.65 W /m2). Au niveau du Sahel en revanche, l'inclusion des aérosols de feux de biomasse diminue plus fortement le flux de chaleur sensible (-16.59 W /m2 dans l'expérience BBDUST et -5.37 W /m2 dans l'expérience DUST). La réponse du flux de chaleur latente est plus complexe et dépend à la fois de la localisation des sources d'aérosols et des espèces considérées. Ainsi, la réponse des champs de précipitations simulés due aux effets radiatifs direct et semi-direct des aérosols diffère fortement entre les deux expériences. Dans l'expérience DUST, les précipitations sont réduites sur la majorité du domaine avec une diminution maximum au centre du continent. Dans l'expérience BBDUST, les aérosols de feux de biomasse augmentent les précipitations pour cette sous-région. L'augmentation des précipitations semble reliée à une augmentation locale de l'activité convective au dessus de 500 hPa sous l'effet d'un mécanisme de pompe thermique.This work investigates direct and semi-direct aerosol radiative forcing and the associated climatic impacts over the West African region during the dry-season. The regional climate model version 3 (RegCM3) is used in combination with in-situ observations from the AMMA­SOP0/DABEX field campaigns and remote sensing observations from sunphotometry (AE­RONET/PHOTON) and satellite platforms (PARASOL, MODIS, OMI and MISR). RegCM3 is specifically configured to represent West African aerosols and is evaluated for the 2006 dry season. In this setup, RegCM3 is found to represent aerosol loadings accurately enough for climatic applications, with the model simulating consistent aerosol single scattering albedo variations. In december and January, when smoke aerosols dominate the background aerosol loading, the aerosol single scattering albedo over the Sahel ranges from 0.81 0.83 (at 440 nm). During the months of march and april, when dust aerosol are mainly observed, the simu­lated aerosol single scattering albedo ranges between 0.90 and 0.92 (at 440 nm). The direct aerosol radiative forcing (visible + infrared) estimated at top of the atmosphere is essentially negative over the whole domain, with values ranging from -5 W /m2 to -4.0 W /m2. Over the Sahara, the direct aerosol radiative forcing at top of the atmosphere (TOA) is close to zero (-0.15 W /m2). The large difference between the TOA and surface direct radiative forcing in­dicates strong radiative absorption in the atmosphere (+11.47 and +24.40 W /m2 over the Sahara and Sahel, respectively). Due to their relatively low single scattering albedo, smoke aerosols are the dominant contributors to atmospheric heating. At the regional scale, this results in a daily average atmospheric heating rates ranging between +0.2 and +0.6 K /day within the main smoke layers (approximately 2 and 5 km above the ground surface). Two lon­ger simulations covering the 2001-2006 period are also conducted in order to investigate the effects of this radiative forcing on the regional climate during the dry season. A simulation including dust aerosols (DUSTexp) and a simulation including both dust and smoke aerosols (BBDUSTexp) are performed in order to take into account the dynamical feedbacks associa­ted with direct and semi-direct aerosol radiative forcing. The strong aerosol radiative forcing at surface decreases available radiation, which leads to significant perturbations of the sur­face energy balance. Over the Sahara, sensible heat flux anomalies are similar in the two experiments (-5.52 W /m2 and -6.65 W /m2, in the DUSTexp and BBDUSTexp, respectively). Over the Sahel, the decrease is more significant in BBDUSTexp simulation (-16.59 W /m2 compared to -5.37 W /m2 in DUSTexp). Changes in latent heat fluxes are more complex and depend simultaneously on aerosols emission locations and the aerosol species present. As a result, the precipitation changes due to aerosol radiative effects are very different within the two experiments. In the DUSTexp, precipitation is decreased over most of the domain with a maximum decrease over the central part of the continent. For the BBDUSTexp, smoke aero­sols tend to enhance precipitation over this sub-region. This increase seems to be related to a local increase of convective activity above 500 hPa, resulting from an elevated heat pump mechanism

Substantial cooling effect from aerosol-induced increase in tropical marine cloud cover

With global warming currently standing at approximately +1.2 °C since pre-industrial times, climate change is a pressing global issue. Marine cloud brightening is one proposed method to tackle warming through injecting aerosols into marine clouds to enhance their reflectivity and thereby planetary albedo. However, because it is unclear how aerosols influence clouds, especially cloud cover, both climate projections and the effectiveness of marine cloud brightening remain uncertain. Here we use satellite observations of volcanic eruptions in Hawaii to quantify the aerosol fingerprint on tropical marine clouds. We observe a large enhancement in reflected sunlight, mainly due to an aerosol-induced increase in cloud cover. This observed strong negative aerosol forcing suggests that the current level of global warming is driven by a weaker net radiative forcing than previously thought, arising from the competing effects of greenhouse gases and aerosols. This implies a greater sensitivity of Earth’s climate to radiative forcing and therefore a larger warming response to both rising greenhouse gas concentrations and reductions in atmospheric aerosols due to air quality measures. However, our findings also indicate that mitigation of global warming via marine cloud brightening is plausible and is most effective in humid and stable conditions in the tropics where solar radiation is strong

Opportunistic experiments to constrain aerosol effective radiative forcing

Aerosol–cloud interactions (ACIs) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The nonlinearity of cloud-state changes to aerosol perturbations make it challenging to attribute causality in observed relationships of aerosol radiative forcing. Using correlations to infer causality can be challenging when meteorological variability also drives both aerosol and cloud changes independently. Natural and anthropogenic aerosol perturbations from well-defined sources provide “opportunistic experiments” (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatiotemporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct a synthesis of the available satellite datasets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Opportunistic experiments have significantly improved process-level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change

Strong constraints on aerosol-cloud interactions from volcanic eruptions.

Aerosols have a potentially large effect on climate, particularly through their interactions with clouds, but the magnitude of this effect is highly uncertain. Large volcanic eruptions produce sulfur dioxide, which in turn produces aerosols; these eruptions thus represent a natural experiment through which to quantify aerosol-cloud interactions. Here we show that the massive 2014-2015 fissure eruption in Holuhraun, Iceland, reduced the size of liquid cloud droplets-consistent with expectations-but had no discernible effect on other cloud properties. The reduction in droplet size led to cloud brightening and global-mean radiative forcing of around -0.2 watts per square metre for September to October 2014. Changes in cloud amount or cloud liquid water path, however, were undetectable, indicating that these indirect effects, and cloud systems in general, are well buffered against aerosol changes. This result will reduce uncertainties in future climate projections, because we are now able to reject results from climate models with an excessive liquid-water-path response

The CLoud–Aerosol–Radiation interaction and forcing: year 2017 (CLARIFY-2017) measurement campaign

The representations of clouds, aerosols, and cloud–aerosol–radiation impacts remain some of the largest uncertainties in climate change, limiting our ability to accurately reconstruct past climate and predict future climate. The south-east Atlantic is a region where high atmospheric aerosol loadings and semi-permanent stratocumulus clouds are co-located, providing an optimum region for studying the full range of aerosol–radiation and aerosol–cloud interactions and their perturbations of the Earth’s radiation budget. While satellite measurements have provided some useful insights into aerosol–radiation and aerosol–cloud interactions over the region, these observations do not have the spatial and temporal resolution, nor the required level of precision to allow for a process-level assessment. Detailed measurements from high spatial and temporal resolution airborne atmospheric measurements in the region are very sparse, limiting their use in assessing the performance of aerosol modelling in numerical weather prediction and climate models. CLARIFY-2017 was a major consortium programme consisting of five principal UK universities with project partners from the UK Met Office and European- and USA-based universities and research centres involved in the complementary ORACLES, LASIC, and AEROCLO-sA projects. The aims of CLARIFY-2017 were fourfold: (1) to improve the representation and reduce uncertainty in model estimates of the direct, semi-direct, and indirect radiative effect of absorbing biomass burning aerosols; (2) to improve our knowledge and representation of the processes determining stratocumulus cloud microphysical and radiative properties and their transition to cumulus regimes; (3) to challenge, validate, and improve satellite retrievals of cloud and aerosol properties and their radiative impacts; (4) to improve the impacts of aerosols in weather and climate numerical models. This paper describes the modelling and measurement strategies central to the CLARIFY-2017 deployment of the FAAM BAe146 instrumented aircraft campaign, summarizes the flight objectives and flight patterns, and highlights some key results from our initial analyses

Opportunistic experiments to constrain aerosol effective radiative forcing

Aerosol–cloud interactions (ACIs) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The nonlinearity of cloud-state changes to aerosol perturbations make it challenging to attribute causality in observed relationships of aerosol radiative forcing. Using correlations to infer causality can be challenging when meteorological variability also drives both aerosol and cloud changes independently. Natural and anthropogenic aerosol perturbations from well-defined sources provide “opportunistic experiments” (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatiotemporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct a synthesis of the available satellite datasets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Opportunistic experiments have significantly improved process-level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change

Simulation of aerosol radiative effects over West Africa during DABEX and AMMA SOP-0

International audienceThe regional climate model RegCM3 has been used to assess optical properties and clear-sky direct radiative forcing (DRF) of mineral dust and carbonaceous aerosols over West Africa for the period October 2005 to April 2006. Our results display a significant seasonal variation of the aerosol single scattering albedo (SSA) due to varying contributions from biomass burning (BB) and dust. During December-January, simulated SSA values dropped to around 0.81-0.83 at 440 nm and to 0.80-0.85 at 675 nm when absorbing aerosols from biomass burning dominate the mixture. During March and April, when mineral dust dominates, simulated SSA values increased reaching around 0.90-0.92 at 440 nm and 0.94-0.96 at 675 nm. The simulated aerosol optical thickness (AOT) was maximum over central Africa where it far exceeded estimates of AOT from satellite which showed the greatest AOT in the gulf of Guinea. This discrepancy was linked to an overestimation of BB emissions in central Africa and a possible underestimation of AOT over central Africa due a high occurrence of cloud and associated difficulties in cloud screening. The DRF calculations were extremely sensitive to aerosol optical properties and underlying surface albedo. Over dark surfaces, the sum of shortwave (SW) and longwave (LW) top of the atmosphere (TOA) direct radiative forcing averaged from December to February was negative (−5.25 to −4.0 W/m2) while over bright surfaces it was close to zero (−0.15 W/m2). Large differences between SW surface and SW TOA direct radiative forcing indicated that SW absorption had an important influence on the radiative budget. The SW radiative heating rate associated with the aerosol reached 1.2 K/d at local noon (diurnal mean of 0.40 K/d) over Niamey (∼13.5°N, 2°E) and peaked at altitudes of 2-4 km, corresponding to the BB aerosol layer

Cloud physics from space

A review of the progression of cloud physics from a subdiscipline of meteorology into the global science it is today is described. The discussion briefly touches on the important post‐war contributions of three key individuals who were instrumental in developing cloud physics into a global science. These contributions came on the heels of the post‐war weather modification efforts that influenced much of the early development of cloud physics. The review is centered on the properties of warm clouds primarily to limit the scope of the paper and the connection between the early contributions to cloud physics and the current vexing problem of aerosol effects on cloud albedo is underlined. Progress toward estimating cloud properties from space and insights on warm cloud processes are described. Measurements of selected cloud properties, such as cloud liquid water path are now mature enough that multi‐decadal time series of these properties exist and this climatology is used to compare to analogous low cloud properties taken from global climate models. The too wet (and thus too bright) and the too dreary biases of models are called out underscoring the challenges we still face in representing warm clouds in Earth system models. We also provide strategies for using observations to constrain the indirect radiative forcing of the climate system