A fully coupled regional climate system model (CNRM-RCSM4) has been used over the Mediterranean region to investigate the direct and semi-direct effects of aerosols, but also their role in the ...radiation–atmosphere–ocean interactions through multi-annual ensemble simulations (2003–2009) with and without aerosols and ocean–atmosphere coupling. Aerosols have been taken into account in CNRM-RCSM4 through realistic interannual monthly AOD climatologies. An evaluation of the model has been achieved, against various observations for meteorological parameters, and has shown the ability of CNRM-RCSM4 to reproduce the main patterns of the Mediterranean climate despite some biases in sea surface temperature (SST), radiation and cloud cover. The results concerning the aerosol radiative effects show a negative surface forcing on average because of the absorption and scattering of the incident radiation. The SW surface direct effect is on average −20.9 Wm
−2
over the Mediterranean Sea, −14.7 Wm
−2
over Europe and −19.7 Wm
−2
over northern Africa. The LW surface direct effect is weaker as only dust aerosols contribute (+4.8 Wm
−2
over northern Africa). This direct effect is partly counterbalanced by a positive semi-direct radiative effect over the Mediterranean Sea (+5.7 Wm
−2
on average) and Europe (+5.0 Wm
−2
) due to changes in cloud cover and atmospheric circulation. The total aerosol effect is consequently negative at the surface and responsible for a decrease in land (on average −0.4 °C over Europe, and −0.5 °C over northern Africa) and sea surface temperature (on average −0.5 °C for the Mediterranean SST). In addition, the latent heat loss is shown to be weaker (−11.0 Wm
−2
) in the presence of aerosols, resulting in a decrease in specific humidity in the lower troposphere, and a reduction in cloud cover and precipitation. Simulations also indicate that dust aerosols warm the troposphere by absorbing solar radiation, and prevent radiation from reaching the surface, thus stabilizing the troposphere. The comparison with the model response in atmosphere-only simulations shows that these feedbacks are attenuated if SST cannot be modified by aerosols, highlighting the importance of using coupled regional models over the Mediterranean. Oceanic convection is also strengthened by aerosols, which tends to reinforce the Mediterranean thermohaline circulation. In parallel, two case studies are presented to illustrate positive feedbacks between dust aerosols and regional climate. First, the eastern Mediterranean was subject to high dust aerosol loads in June 2007 which reduce land and sea surface temperature, as well as air–sea humidity fluxes. Because of northern wind over the eastern Mediterranean, drier and cooler air has been consequently advected from the sea to the African continent, reinforcing the direct dust effect over land. On the contrary, during the western European heat wave in June 2006, dust aerosols have contributed to reinforcing an important ridge responsible for dry and warm air advection over western Europe, and thus to increasing lower troposphere (+0.8 °C) and surface temperature (+0.5 °C), namely about 15 % of this heat wave.
Megacities need adapted tools for the accurate modeling of aerosol impacts. For this purpose a new experimental data processing has been worked out for Beijing aerosols as case study. Size‐resolved ...aerosol particles were extensively sampled during winter and summer 2003 and subsequently fully chemically characterized. The product is an aerosol model presenting a new particle pattern (mode number, size and chemistry) without any prerequisite constrain either on the mode number or on each mode chemical composition. Six modes were found and five of them consistently appear as internally mixed particles organized around a black carbon or a dust core coated by organic and/or inorganic material. Data were checked by robust comparisons with other experimental data (particle number, sunphotometer‐derived derived data). We found the presence of two accumulation modes in different internal mixing and optical calculations show that the Beijing aerosol single scattering albedo (ωo # 0.90) is significantly higher than expected. Such an approach would allow realistic modeling of atmospheric particle impacts under complex situations.
Dust transported from north African source region toward the Mediterranean basin and Europe is a ubiquitous phenomenon in the Mediterranean region. Winds formed by large-scale pressure gradients ...foster dust entrainment into the atmosphere over north African dust source regions and advection of dust downwind. The constellation of centers of high and low pressure determines wind speed and direction, and thus the chance for dust emission over northern Africa and transport toward the Mediterranean. We present characteristics of the atmospheric dust life cycle determining dust transport toward the Mediterranean basin with focus on the ChArMEx (Chemistry-Aerosol Mediterranean Experiment) special observation period in June and July 2013 using the atmosphere–dust model COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdeling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model). Modes of atmospheric circulation are identified from empirical orthogonal function (EOF) analysis of the geopotential height at 850 hPa and compared to EOFs calculated from 1979–2015 ERA-Interim reanalysis. Two different phases are identified from the first EOF, which in total explain 45 % of the variance. They are characterized by the propagation of the subtropical ridge into the Mediterranean basin, the position of the Saharan heat low and the predominant Iberian heat low, and discussed illustrating a dipole pattern for enhanced (reduced) dust emission fluxes, stronger (weaker) meridional dust transport, and consequent increased (decreased) atmospheric dust concentrations and deposition fluxes. In the event of a predominant high-pressure zone over the western and central Mediterranean (positive phase), a hot spot in dust emission flux is evident over the Grand Erg Occidental, and a reduced level of atmospheric dust loading occurs over the western Mediterranean basin. The meridional transport in northward direction is reduced due to prevailing northerly winds. In case of a predominant heat low trough linking the Iberian and the Saharan heat low (negative phase), meridional dust transport toward the western Mediterranean is increased due to prevailing southerly winds resulting in an enhanced atmospheric dust loading over the western Mediterranean. Altogether, results from this study illustrate the relevance of knowing dust source location and characteristics in concert with atmospheric circulation. The study elaborates on the question of the variability of summertime dust transport toward the Mediterranean and Europe with regard to atmospheric circulation conditions controlling dust emission and transport routes of Saharan dust, exemplarily for the 2-month period of June–July 2013. Ultimately, outcomes from this study contribute to the understanding of the variance in dust transport into a populated region.
The present work aims at better understanding regional climate–aerosol interactions by studying the relationships between aerosols and synoptic atmospheric circulation over the Euro-Mediterranean ...region. Two 40-year simulations (1979–2018) have been carried out with version 6.3 of the Centre National de Recherches Météorologiques (National Centre for Meteorological Research) – Aire Limitée Adaptation dynamique Développement InterNational (CNRM-ALADIN) regional climate model, one using interactive aerosols and the other one without any aerosol. The simulation with aerosols has been evaluated in terms of different climate and aerosol parameters. This evaluation shows a good agreement between the model and observations, significant improvements compared to the previous model version and consequently the relevance of using this model for the study of climate–aerosol interactions over this region. A first attempt to explain the climate variability of aerosols is based on the use of the North Atlantic Oscillation (NAO) index. The latter explains a significant part of the interannual variability, notably in winter for the export of dust aerosols over the Atlantic Ocean and the eastern Mediterranean, and in summer for the positive anomalies of anthropogenic aerosols over western Europe. This index is however not sufficient to fully understand the variations of aerosols in this region, notably at daily scale. The use of “weather regimes”, namely persisting meteorological patterns, stable at synoptic scale for a few days, provides a relevant description of atmospheric circulation, which drives the emission, transport and deposition of aerosols. The four weather regimes usually defined in this area in winter and in summer bring significant information to answer this question. The blocking and NAO+ regimes are largely favourable to strong aerosol effects on shortwave surface radiation and near-surface temperature, either because of higher aerosol loads or because of weaker cloud fraction, which reinforces the direct aerosol effect. Inversely, the NAO− and Atlantic Ridge regimes are unfavourable to aerosol radiative effects, because of weaker aerosol concentrations and increased cloud cover. This study thus puts forward the strong dependence of aerosol loads on the synoptic circulation from interannual to daily scales and, as a consequence, the important modulation of the aerosol effects on shortwave surface radiation and near-surface temperature by atmospheric circulation. The role of cloud cover is essential in this modulation as shown by the use of weather regimes.
This study investigates, through regional climate modelling, the surface mass concentration and AOD (aerosol optical depth) evolution of the various (anthropogenic and natural) aerosols over the ...Euro-Mediterranean region between the end of the 20th century and the mid-21st century. The direct aerosol radiative forcing (DRF) as well as the future Euro-Mediterranean climate sensitivity to aerosols have also been analysed. Different regional climate simulations were carried out with the CNRM-ALADIN63 regional climate model, driven by the global CNRM-ESM2-1 Earth system model (used in CMIP6) and coupled to the TACTIC (Tropospheric Aerosols for ClimaTe In CNRM) interactive aerosol scheme. These simulations follow several future scenarios called shared socioeconomic pathways (SSP 1-1.9, SSP 3-7.0 and SSP 5-8.5), which have been chosen to analyse a wide range of possible future scenarios in terms of aerosol or particle precursor emissions. Between the historical and the future period, results show a total AOD decrease between 30 % and 40 % over Europe for the three scenarios, mainly due to the sulfate AOD decrease (between −85 and −93 %), that is partly offset by the nitrate and ammonium particles AOD increase (between +90 and +120 %). According to these three scenarios, nitrate aerosols become the largest contributor to the total AOD during the future period over Europe, with a contribution between 43.5 % and 47.5 %. It is important to note that one of the precursors of nitrate and ammonium aerosols, nitric acid, has been implemented in the model as a constant climatology over time. Concerning natural aerosols, their contribution to the total AOD increases slightly between the two periods. The different evolution of aerosols therefore impacts their DRF, with a significant sulfate DRF decrease between 2.4 and 2.8 W m−2 and a moderate nitrate and ammonium DRF increase between 1.3 and 1.5 W m−2, depending on the three scenarios over Europe. These changes, which are similar under the different scenarios, explain about 65 % of the annual shortwave radiation change but also about 6 % (in annual average) of the warming expected over Europe by the middle of the century. This study shows, with SSP 5-8.5, that the extra warming attributable to the anthropogenic aerosol evolution over Central Europe and the Iberian Peninsula during the summer period is due to “aerosol–radiation” as well as “aerosol–cloud” interaction processes. The extra warming of about 0.2 ∘C over Central Europe is explained by a surface radiation increase of 5.8 W m−2 over this region, due to both a surface aerosol DRF decrease of 4.4 W m−2 associated with a positive effective radiative forcing due to aerosol–radiation interactions (ERFari) of 2.7 W m−2 at the top of the atmosphere (TOA) and a cloud optical depth (COD) decrease of 1.3. In parallel, the simulated extra warming of 0.2∘C observed over the Iberian Peninsula is due to a COD decrease of 1.3, leading to a positive effective radiative forcing due to aerosol–cloud interactions (ERFaci) of 2.6 W m−2 at the TOA but also to an atmospheric dynamics change leading to a cloud cover decrease of about 1.7 % and drier air in the lower layers, which is a signature of the semi-direct forcing. This study thus highlights the necessity of taking into account the evolution of aerosols in future regional climate simulations.
Direct and semi‐direct radiative effects of biomass burning aerosols (BBA) from southern and central African fires are still widely debated, in particular because climate models have been ...unsuccessful in reproducing the low single scattering albedo in BBA over the eastern Atlantic Ocean. Using state‐of‐the‐art airborne in situ measurements and Mie scattering simulations, we demonstrate that low single scattering albedo in well‐aged BBA plumes over southern West Africa results from the presence of strongly absorbing refractory black carbon (rBC), whereas the brown carbon contribution to the BBA absorption is negligible. Coatings enhance light absorption by rBC‐containing particles by up to 210%. Our results show that accounting for the diversity in black carbon mixing state by combining internal and external configurations is needed to accurately estimate the optical properties and henceforth the shortwave direct radiative effect and heating rate of BBA over southern West Africa.
Plain Language Summary
Extensive seasonal fires over southern and central Africa result in the transport of massive amounts of biomass burning aerosols over huge areas of the eastern Atlantic Ocean. Recent field observations highlight that biomass burning aerosols transported from the coast of southern Africa to the far north over southern West Africa were characterized by low single scattering albedo. This finding is of paramount interest, because radiative heating within the absorbing aerosol layer is hypothesized to affect the low cloud deck over this specific region and may ultimately influence the large‐scale circulation. However, debate remains about the causes of the low single scattering albedo by biomass burning aerosols, causing ambiguous parameterizations of their optical properties in climate models. Here we present simultaneous airborne measurements of the composition and optical properties of biomass burning aerosols transported over southern West Africa. We show that black carbon particles dominated the light absorption by biomass burning aerosols at mid‐visible wavelengths. Our findings indicate that the black carbon mixing state plays a significant role in the aerosol optical properties and may be an important modulator to be considered in climate models for simulating direct and semi‐direct radiative effects of biomass burning aerosol over southern West Africa.
Key Points
The low single scattering albedo in biomass burning aerosol (BBA) over southern West Africa results from the presence of refractory black carbon
The brown carbon contribution to the aerosol absorption is negligible in these well‐aged BBA plumes
Accounting for the diversity in black carbon mixing state is needed to accurately estimate BBA optical properties
Aerosols play an important role in Europe and the Mediterranean area where different
sources of natural and anthropogenic particles are present. Among them
ammonium and nitrate (A&N) aerosols may ...have a growing impact on regional
climate. In this study, their representation in coarse and fine modes has
been introduced in the prognostic aerosol scheme of the ALADIN-Climate
regional model. This new aerosol scheme is evaluated over Europe and the
Mediterranean Sea, using two twin simulations over the period 1979–2016 with
and without A&N aerosols. This evaluation is performed at local and
regional scales, using surface stations and satellite measurements. Despite
an overestimate of the surface nitrate concentration, the model is able to
reproduce its spatial pattern including local maxima (Benelux, Po Valley).
Concerning the simulated aerosol optical depth (AOD), the inclusion of A&N
aerosols significantly reduces the model bias compared to both AERONET
stations and satellite data. Our results indicate that A&N aerosols can
contribute up to 40 % of the total AOD550 over Europe, with an
average of 0.07 (550 nm) over the period 2001–2016. Sensitivity studies
suggest that biases still present are related to uncertainties associated
with the annual cycle of A&N aerosol precursors (ammonia and nitric acid).
The decrease in sulfate aerosol production over Europe since 1980 produces
more free ammonia in the atmosphere leading to an increase in A&N
concentrations over the studied period. Analyses of the different aerosol
trends have shown for the first time to our knowledge that, since 2005 over
Europe, A&N AOD550 and A&N shortwave (SW) direct radiative forcing
(DRF) are found to be higher than sulfate and organics, making these the
species with the highest AOD and the highest DRF. On average over the period
1979–2016, the A&N DRF is found to be about −1.7 W m−2 at the
surface and −1.4 W m−2 at the top of the atmosphere (TOA) in all sky
conditions over Europe, with regional maxima located at the surface over the
Po Valley (−5 W m−2). Finally, the dimming effect of A&N aerosols
is responsible for a cooling of about −0.2∘ C over Europe
(summer), with a maximum of −0.4 ∘C over the Po Valley. Concerning
precipitation, no significant impact of A&N aerosols has been found.
Organic aerosols are predominantly emitted from biomass burning and biofuel use. The fraction of these aerosols that strongly absorbs ultraviolet and short visible light is referred to as brown ...carbon (BrC). The life cycle and the optical properties of BrC are still highly uncertain, thus contributing to the uncertainty of the total aerosol radiative effect. This study presents the implementation of BrC aerosols in the Tropospheric Aerosols for ClimaTe In CNRM (TACTIC) aerosol scheme of the atmospheric component of the Centre National de Recherches Météorologiques (CNRM) climate model. This implementation has been achieved using a BrC parameterization based on the optical properties of Saleh et al. (2014). Several simulations have been carried out with the CNRM global climate model, over the period of 2000–2014, to analyze the BrC radiative and climatic effects. Model evaluation has been carried out by comparing numerical results of single-scattering albedo (SSA), aerosol optical depth (AOD), and absorption aerosol optical depth (AAOD) to data provided by Aerosol Robotic Network (AERONET) stations, at the local scale, and by different satellite products, at the global scale. The implementation of BrC and its bleaching parameterization has resulted in an improvement of the estimation of the total SSA and AAOD at 350 and 440 nm. This improvement is observed at both the local scale, for several locations of AERONET stations, and the regional scale, over regions of Africa (AFR) and South America (AME), where large quantities of biomass burning aerosols are emitted. The annual global BrC effective radiative forcing (all-sky conditions) has been calculated in terms of both aerosol–radiation interactions (ERFari, 0.029 ± 0.006 W m−2) and aerosol–cloud interactions (ERFaci, −0.024 ± 0.066 W m−2). This study shows, on an annual average, positive values of ERFari of 0.292 ± 0.034 and 0.085 ± 0.032 W m−2 over the AFR and AME regions, respectively, which is in accordance with the BrC radiative effect calculated in previous studies. This work also reveals that the inclusion of BrC in the TACTIC aerosol scheme causes a statistically significant low-level cloud fraction increase over the southeastern Atlantic Ocean during the burning season partially caused by a vertical velocity decrease at 700 hPa (semi-direct aerosol effect). Lastly, this study also highlights that the low-level cloud fraction changes, associated with more absorbing biomass burning aerosols, contribute to an increase in both solar heating rate and air temperature at 700 hPa over this region.
We used the Regional Circulation Model (RegCM) to investigate the direct effect of dust aerosol on climate over West Africa, with a specific focus on the Sahel region. First, we characterized the ...mechanisms linking dust radiative forcing and convective activity over Sahel and the net impact of dust on precipitation: The mean effect of dust over 11 summer seasons is to reduce precipitation over most of the Sahel region as a result of strong surface cooling and elevated diabatic warming inhibiting convection. However, on the very northern Sahel and in the vicinity of dust sources, a relative increase of precipitation is obtained as a result of enhanced diabatic warming in the lower atmosphere associated with high dust concentrations at low altitude. In the second part of the paper, we investigated the robustness of this signal with regards to different modeling conditions that are thought to be sensitive, namely the extension of the domain, the effect of dust on sea surface temperature, the land surface scheme, the convective scheme and the dust single scattering albedo. The simulated dust induced precipitation anomaly over West Africa is consistent and robust in these tests, but significant variations over the northern Sahel region are nevertheless pointed out. Among different factors, single scattering and surface albedo, as well as the nature of the convective scheme, have the greatest influence on the simulated response of West African climate to dust forcing.
Southern West Africa (SWA) is an African pollution hotspot but a
relatively poorly sampled region of the world. We present an overview of
in situ aerosol optical measurements collected over SWA in ...June and July 2016 as
part as of the DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions in
West Africa) airborne campaign. The aircraft sampled a wide range of air
masses, including anthropogenic pollution plumes emitted from the coastal
cities, long-range transported biomass burning plumes from central and
southern Africa and dust plumes from the Sahara and Sahel region, as well as
mixtures of these plumes. The specific objective of this work is to
characterize the regional variability of the vertical distribution of
aerosol particles and their spectral optical properties (single scattering
albedo: SSA, asymmetry parameter, extinction mass efficiency, scattering
Ångström exponent and absorption Ångström exponent: AAE). The first
findings indicate that aerosol optical properties in the planetary boundary
layer were dominated by a widespread and persistent biomass burning loading
from the Southern Hemisphere. Despite a strong increase in aerosol number
concentration in air masses downwind of urban conglomerations, spectral
SSA were comparable to the background and showed signatures of the absorption
characteristics of biomass burning aerosols. In the free troposphere,
moderately to strongly absorbing aerosol layers, dominated by either dust or
biomass burning particles, occurred occasionally. In aerosol layers
dominated by mineral dust particles, SSA varied from 0.81 to 0.92 at 550 nm
depending on the variable proportion of anthropogenic pollution particles
externally mixed with the dust. For the layers dominated by biomass burning
particles, aerosol particles were significantly more light absorbing than
those previously measured in other areas (e.g. Amazonia, North America), with
SSA ranging from 0.71 to 0.77 at 550 nm. The variability of SSA was mainly
controlled by variations in aerosol composition rather than in aerosol size
distribution. Correspondingly, values of AAE ranged from 0.9 to 1.1, suggesting
that lens-coated black carbon particles were the dominant absorber in the
visible range for these biomass burning aerosols. Comparison with the literature
shows a consistent picture of increasing absorption enhancement of biomass
burning aerosol from emission to remote location and underscores that the
evolution of SSA occurred a long time after emission. The results presented here build a fundamental basis of knowledge about the
aerosol optical properties observed over SWA during the monsoon season and
can be used in climate modelling studies and satellite retrievals. In
particular and regarding the very high absorbing properties of biomass
burning aerosols over SWA, our findings suggest that considering the effect
of internal mixing on absorption properties of black carbon particles in
climate models should help better assess the direct and semi-direct
radiative effects of biomass burning particles.
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