Semi-direct Radiative Effects Research Articles

The radiative impact of biomass burning aerosols on dust emissions over Namibia and the long-range transport of smoke observed during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) campaign

Abstract. The radiative effects of biomass burning aerosols (BBAs) on low-level atmospheric circulation over southern Africa are investigated on 5 September 2017 during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) field campaign. This is conducted using a variety of in situ and remote sensing observations, as well as two 5 d ensemble simulations made with the Meso-NH mesoscale model, one including the direct and semi-direct radiative effects of aerosols and one in which these effects are not included. We show that the radiative impact of BBA building up over a period of 5 d in the Meso-NH simulations can lead to significantly different circulations at low and middle levels, thereby affecting dust emissions over southern Namibia and northwestern South Africa as well as the transport of BBA in a so-called “river of smoke”. While most of the regional-scale dynamics, thermodynamics and composition features are convincingly represented in the simulation with BBA radiative effects, neglecting the radiative impact of BBA leads to unrealistic representations of (i) the low-level jet (LLJ) over the plateau, which is the main low-level dynamic feature fostering dust emission, and (ii) the mid-level dynamics pertaining to the transport of BBA from the fire-prone regions in the tropics to the mid-latitudes. For instance, when the BBA radiative impacts are not included, the LLJ is too weak and not well established over night, and the developing convective planetary boundary layer (PBL) is too deep compared to observations. The deeper convective PBL over Etosha and surrounding areas is related to the enhanced anomalous upward motion caused by the eastern displacement of the river of smoke. This eastern displacement is, in turn, related to the weaker southerly African easterly jet. Both ensemble simulations provide clear evidence that the enhanced near-surface extinction coefficient values detected from observations over Etosha are related to the downward mixing of BBA in the developing convective boundary layer rather than dust being emitted as a result of the LLJ breakdown after sunrise. This study suggests that the radiative effect of BBAs needs to be taken into account to properly forecast dust emissions in Namibia.

Application of the CHIMERE-WRF Model Complex to Study the Radiative Effects of Siberian Smoke Aerosol in the Eastern Arctic

We describe a computational technology for studying the effects of the aerosol-radiation interaction and calculating regional estimates of the direct (DRE) and semidirect radiative effects (SDRE) of biomass burning (BB) aerosol based on simulations with the CHIMERE chemistry transport model coupled with the WRF meteorological model. The technology was applied to numerical studying the radiative effects of Siberian biomass burning aerosol in the eastern Arctic in the period of July 16–31, 2016. The model simulations show that Siberian smokes, on the whole, had a significant cooling effect on the atmosphere in the eastern Arctic in that period due to the DRE, the value of which at top of the atmosphere was, on average, −6.0 W m−2, being minimal over the snow-ice cover of the ocean (−1.2 W m−2). At the same time, the contribution of the Siberian BB aerosol DRE to the radiative balance of the Arctic atmosphere is found to be offset to a certain extent by the SDRE, which is positive (2.0 W m−2), on average. The SDRE is formed as a result of the multihour aerosol effect on meteorological processes and plays the most important role over the snow-ice Arctic covers, where it exceeds the DRE in absolute value. It has been shown that the SDRE of Siberian BBA in our numerical experiments is mainly due to scattering (rather than absorption) of radiation by aerosol particles.

Acceleration of the southern African easterly jet driven by the radiative effect of biomass burning aerosols and its impact on transport during AEROCLO-sA

Abstract. The direct and semi-direct radiative effects of biomass burning aerosols (BBAs) are investigated over southern Africa and the southeastern Atlantic during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) field campaign in September 2017. A reference convection-permitting simulation has been performed using the Meso-NH model with an online dust emission scheme, a strongly absorbing BBA tracer emitted using the daily Global Fire Emissions Database and online-computed backward Lagrangian trajectories. The simulation captures both the aerosol optical depth and the vertical distribution of aerosols as observed from airborne and spaceborne lidars. The occurrence of stratocumulus over the southeastern Atlantic, deep convective clouds over equatorial Africa and the large-scale circulation are all reproduced by the model. If the radiative effects of BBA are omitted in the model, we show that (i) the smoke plume is too low in altitude, (ii) the low-cloud cover is too weak, (iii) the deep convective activity is too frequent but not intense enough, (iv) the Benguela low-level jet is too strong, and (v) the southern African easterly jet is too weak. The Lagrangian analysis indicates that the radiative effect of BBA leads to the transport of BBA to higher altitudes, farther southwest and with a stronger diurnal oscillation. The higher smoke plume altitude can be explained by a combination of three factors: increased upward motion induced by the stronger southern African easterly jet, self-lofting of BBA and reduced subsidence associated with less frequent deep convective activity over western equatorial Africa.

Cloud adjustments dominate the overall negative aerosol radiative effects of biomass burning aerosols in UKESM1 climate model simulations over the south-eastern Atlantic

Abstract. The south-eastern Atlantic Ocean (SEA) is semi-permanently covered by one of the most extensive stratocumulus cloud decks on the planet and experiences about one-third of the global biomass burning emissions from the southern Africa savannah region during the fire season. To get a better understanding of the impact of these biomass burning aerosols on clouds and the radiation balance over the SEA, the latest generation of the UK Earth System Model (UKESM1) is employed. Measurements from the CLARIFY and ORACLES flight campaigns are used to evaluate the model, demonstrating that the model has good skill in reproducing the biomass burning plume. To investigate the underlying mechanisms in detail, the effects of biomass burning aerosols on the clouds are decomposed into radiative effects (via absorption and scattering) and microphysical effects (via perturbation of cloud condensation nuclei – CCN – and cloud microphysical processes). July–August means are used to characterize aerosols, clouds, and the radiation balance during the fire season. Results show that around 65 % of CCN at 0.2 % supersaturation in the SEA can be attributed to biomass burning. The absorption effect of biomass burning aerosols is the most significant on clouds and radiation. Near the continent, it increases the supersaturation diagnosed by the activation scheme, while further from the continent it reduces the altitude of the supersaturation. As a result, the cloud droplet number concentration responds with a similar pattern to the absorption effect of biomass burning aerosols. The microphysical effect, however, decreases the supersaturation and increases the cloud droplet concentration over the ocean, although this change is relatively small. The liquid water path is also significantly increased over the SEA (mainly caused by the absorption effect of biomass burning aerosols) when biomass burning aerosols are above the stratocumulus cloud deck. The microphysical pathways lead to a slight increase in the liquid water path over the ocean. These changes in cloud properties indicate the significant role of biomass burning aerosols for clouds in this region. Among the effects of biomass burning aerosols on the radiation balance, the semi-direct radiative effects (rapid adjustments induced by the radiative effects of biomass burning aerosols) have a dominant cooling impact over the SEA, which offset the warming direct radiative effect (radiative forcing from biomass burning aerosol–radiation interactions) and lead to an overall net cooling radiative effect in the SEA. However, the magnitude and the sign of the semi-direct effects are sensitive to the relative location of biomass burning aerosols and clouds, reflecting the critical task of the accurate modelling of the biomass burning plume and clouds in this region.

Direct and semi-direct radiative forcing of biomass-burning aerosols over the southeast Atlantic (SEA) and its sensitivity to absorbing properties: a regional climate modeling study

Abstract. 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).

Direct and semi-direct radiative effect of North African dust in present and future regional climate simulations

This study explores the direct and semi-direct radiative effect of North African dust in the present and future climate using the regional climate model RegCM4. The simulations cover a historical decade extending from December 1999 to November 2009 and a future decade that spans from December 2089 to November 2099 under the Representative Concentration Pathway 4.5 (RCP4.5), without considering land-cover/land-use changes. For each time-slice a set of two experiments was conducted, namely the “Control”, in which dust is radiatively inactive and the “Feedback”, in which dust interacts with shortwave and longwave radiation. The impact of North African dust on the regional radiative balance is assessed by comparing the “Feedback” and the “Control” experiments during the historical period. The results indicate that the combined effect of dust Direct + Semi-direct Radiative Effect (DSRE) on the shortwave is − 13.8 W m−2 and − 10.7 W m−2 over the Sahel and the Sahara, respectively. The Direct Radiative Effect (DRE) dominates over the Semi-direct Radiative Effect (SRE) in both winter and summer, although during summer over some parts of the desert the SRE in the longwave spectrum accounts for almost 50% of the DSRE. Part of this is due to a noteworthy statistically significant increase of clouds that reaches values up to 3% and stretches across the eastern and western Sahara desert. Dust DSRE intensifies moderately in the future period (− 15.8 W m−2 and − 11.0 W m−2), while its spatial distribution remains the same, suggesting that the effect of climate change in the atmosphere will not alter the radiative effect of dust over North Africa considerably. When taking into account the dust radiative feedback in regional climate simulations the maximum temperature is altered by − 0.2/− 0.2 °C and − 0.3/− 0.6 °C over the Sahel and Sahara regions, respectively, during the summer/winter period, mainly as a result of changes in the shortwave radiative balance. On the contrary, the minimum temperature increases, since it is mostly controlled by the longwave radiation emitted from the Earth’s surface. In the future period the near surface air temperature increases by 1.5–2.5 °C and the fine dust column burden increases by + 4% to + 8% in comparison to the historical period, mainly due to the RCP4.5 forcing. When the dust feedback on climate is active in future simulations it can decrease the summer daily maximum temperature by 0.3 °C over Sahel, and decrease or increase it locally in Sahara by up to 0.2 °C. Prior to the Feedback-Control analysis an extensive evaluation has been conducted for dust optical depth, dust extinction, near surface air temperature and cloud fraction cover using the LIVAS, CRU and CM SAF datasets.

Biomass smoke from southern Africa can significantly enhance the brightness of stratocumulus over the southeastern Atlantic Ocean

Marine stratocumulus clouds cover nearly one-quarter of the ocean surface and thus play an extremely important role in determining the global radiative balance. The semipermanent marine stratocumulus deck over the southeastern Atlantic Ocean is of particular interest, because of its interactions with seasonal biomass burning aerosols that are emitted in southern Africa. Understanding the impacts of biomass burning aerosols on stratocumulus clouds and the implications for regional and global radiative balance is still very limited. Previous studies have focused on assessing the magnitude of the warming caused by solar scattering and absorption by biomass burning aerosols over stratocumulus (the direct radiative effect) or cloud adjustments to the direct radiative effect (the semidirect effect). Here, using a nested modeling approach in conjunction with observations from multiple satellites, we demonstrate that cloud condensation nuclei activated from biomass burning aerosols entrained into the stratocumulus (the microphysical effect) can play a dominant role in determining the total radiative forcing at the top of the atmosphere, compared with their direct and semidirect radiative effects. Biomass burning aerosols over the region and period with heavy loadings can cause a substantial cooling (daily mean -8.05 W m-2), primarily as a result of clouds brightening by reducing the cloud droplet size (the Twomey effect) and secondarily through modulating the diurnal cycle of cloud liquid water path and coverage (the cloud lifetime effect). Our results highlight the importance of realistically representing the interactions of stratocumulus with biomass burning aerosols in global climate models in this region.

Transport of anthropogenic and biomass burning aerosols from Europe to the Arctic during spring 2008

Abstract. During the POLARCAT-France airborne campaign in April 2008, pollution originating from anthropogenic and biomass burning emissions was measured in the European Arctic. We compare these aircraft measurements with simulations using the WRF-Chem model to investigate model representation of aerosols transported from Europe to the Arctic. Modeled PM2.5 is evaluated using European Monitoring and Evaluation Programme (EMEP) measurements in source regions and POLARCAT aircraft measurements in the Scandinavian Arctic. Total PM2.5 agrees well with the measurements, although the model overestimates nitrate and underestimates organic carbon in source regions. Using WRF-Chem in combination with the Lagrangian model FLEXPART-WRF, we find that during the campaign the research aircraft sampled two different types of European plumes: mixed anthropogenic and fire plumes from eastern Europe and Russia transported below 2 km, and anthropogenic plumes from central Europe uplifted by warm conveyor belt circulations to 5–6 km. Both modeled plume types had undergone significant wet scavenging (> 50% PM10) during transport. Modeled aerosol vertical distributions and optical properties below the aircraft are evaluated in the Arctic using airborne lidar measurements. Model results show that the pollution event transported aerosols into the Arctic (> 66.6° N) for a 4-day period. During this 4-day period, biomass burning emissions have the strongest influence on concentrations between 2.5 and 3 km altitudes, while European anthropogenic emissions influence aerosols at both lower (~ 1.5 km) and higher altitudes (~ 4.5 km). As a proportion of PM2.5, modeled black carbon and SO4= concentrations are more enhanced near the surface in anthropogenic plumes. The European plumes sampled during the POLARCAT-France campaign were transported over the region of springtime snow cover in northern Scandinavia, where they had a significant local atmospheric warming effect. We find that, during this transport event, the average modeled top-of-atmosphere (TOA) shortwave direct and semi-direct radiative effect (DSRE) north of 60° N over snow and ice-covered surfaces reaches +0.58 W m−2, peaking at +3.3 W m−2 at noon over Scandinavia and Finland.

Direct and semi-direct aerosol radiative effect on the Mediterranean climate variability using a coupled regional climate system model

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.

Evaluation of black carbon semi‒direct radiative effect in a climate model

AbstractThe semi‒direct effect of black carbon (BC) is studied by using a newly proposed optical property parameterization for cloud droplets with BC inclusions. Based on Atmospheric Model Intercomparison Project‒type climate model simulations, it is found that the cloud amount can be either enhanced or reduced when BC is included in clouds. The decrease of the global annual mean total cloud amount is only about 0.023%. The 3‒D cloud fraction distribution, however, shows larger changes which vary with latitude. A correlation between the changes of the cloud fraction and the vertical velocity is found. The cloud water path is mainly affected by low clouds and so the impact of BC on the cloud water path is particularly strong. It is shown that the BC above clouds tends to stabilize the atmosphere and enhance the cloud amount in the boundary layer. This can be used to explain the relationship between aerosol optical depth and cloud amount according to satellite data. For BC in clouds and above, the global annual mean enhancement of solar absorption is about 0.049 W m−2 and 0.57 W m−2, respectively. The BC semi‒direct radiative forcing is estimated by subtracting the BC direct forcing from the BC total radiative forcing. The global annual mean of BC direct forcing and semi‒direct forcing at the top of the atmosphere are 0.264 W m−2 and 0.213 W m−2, respectively.

Direct and semi-direct radiative effects of anthropogenic aerosols in the Western United States: Seasonal and geographical variations according to regional climate characteristics

The direct and semi-direct radiative effects of anthropogenic aerosols on the radiative transfer and cloud fields in the Western United States (WUS) according to seasonal aerosol optical depth (AOD) and regional climate are examined using a regional climate model (RCM) in conjunction with the aerosol fields from a GEOS-Chem chemical-transport model (CTM) simulation. The two radiative effects cannot be separated within the experimental design in this study, thus the combined direct- and semi-direct effects are called radiative effects hereafter. The CTM shows that the AOD associated with the anthropogenic aerosols is chiefly due to sulfates with minor contributions from black carbon (BC) and that the AOD of the anthropogenic aerosol varies according to local emissions and the seasonal low-level winds. The RCM-simulated anthropogenic aerosol radiative effects vary according to the characteristics of regional climate, in addition to the AOD. The effects on the top of the atmosphere (TOA) outgoing shortwave radiation (OSRT) range from −0.2 Wm−2 to −1 Wm−2. In Northwestern US (NWUS), the maximum and minimum impact of anthropogenic aerosols on OSRT occurs in summer and winter, respectively, following the seasonal AOD. In Arizona-New Mexico (AZNM), the effect of anthropogenic sulfates on OSRT shows a bimodal distribution with winter/summer minima and spring/fall maxima, while the effect of anthropogenic BC shows a single peak in summer. The anthropogenic aerosols affect surface insolation range from −0.6 Wm−2 to −2.4 Wm−2, with similar variations found for the effects on OSRT except that the radiative effects of anthropogenic BC over AZNM show a bimodal distribution with spring/fall maxima and summer/winter minima. The radiative effects of anthropogenic sulfates on TOA outgoing longwave radiation (OLR) and the surface downward longwave radiation (DLRS) are notable only in summer and are characterized by strong geographical contrasts; the summer OLR in NWUS (AZNM) is reduced (enhanced) by 0.52 Wm−2 (1.14 Wm−2). The anthropogenic sulfates enhance (reduce) summer DLRS by 0.2 Wm−2 (0.65 Wm−2) in NWUS (AZNM). The anthropogenic BC affect DLRS noticeably only in AZNM during summer. The anthropogenic aerosols affect the cloud water path (CWP) and the radiative transfer noticeably only in summer when convective clouds are dominant. Primarily shortwave-reflecting anthropogenic sulfates decrease and increase CWP in AZNM and NWUS, respectively, however, the shortwave-absorbing anthropogenic BC reduces CWP in both regions. Due to strong feedback via convective clouds, the radiative effects of anthropogenic aerosols on the summer radiation field are more closely correlated with the changes in CWP than the AOD. The radiative effect of the total anthropogenic aerosols is dominated by the anthropogenic sulfates that contribute more than 80% of the total AOD associated with the anthropogenic aerosols.

Direct and semidirect aerosol effects of southern African biomass burning aerosol

[1] Direct and semidirect radiative effects of biomass burning aerosols from southern African fires during July–October are investigated using 20 year runs of the Community Atmospheric Model (CAM) coupled to a slab ocean model. Aerosol optical depth is constrained using observations in clear skies from Moderate Resolution Imaging Spectroradiometer (MODIS) and for aerosol layers above clouds from Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). Over the ocean, where the aerosol layers are primarily located above cloud, negative top of atmosphere (TOA) semidirect radiative effects associated with increased low cloud cover dominate over a weaker positive all-sky direct radiative effect (DRE). In contrast, over the land where the aerosols are often below or within cloud layers, reductions in cloud liquid water path (LWP) lead to a positive semidirect radiative effect that dominates over a near-zero DRE. Over the ocean, the cloud response can be understood as a response to increased lower tropospheric stability (LTS) which is caused both by radiative heating in overlying layers and surface cooling in response to direct aerosol forcing. The marine cloud changes are robust to changes in the cloud parameterization (removal of the hard-wired dependence of clouds on LTS), suggesting that they are physically realistic. Over land, decreased LWP is consistent with weaker convection driven by increased static stability. Over the entire region the overall TOA radiative effect from the biomass burning aerosols is almost zero due to opposing effects over the land and ocean. However, the surface forcing is strongly negative, which leads to a reduction in precipitation and also a reduction in sensible heat flux. The former is primarily realized through reductions in convective precipitation on both the southern and northern flanks of the convective precipitation region spanning the equatorial rain forest and the Intertropical Convergence Zone (ITCZ) in the southern Sahel. The changes are consistent with the low-level aerosol-forced cooling pattern. The results highlight the importance of semidirect radiative effects and precipitation responses for determining the climatic effects of aerosols in the African region.

Simulation of the direct and semidirect aerosol effects on the southern Africa regional climate during the biomass burning season

The regional climate model RegCM3 was used to simulate the direct and semidirect radiative effects of biomass burning and dust aerosol over southern Africa during the austral winter season. Simulated aerosols were found to induce changes in the regional surface fluxes and atmospheric dynamics. Clear‐sky surface radiative forcing decreased by up to −60 W/m2 in the main biomass burning region, resulting in decreased surface turbulent fluxes and PBL height as well as reduced surface temperatures. The positive temperature bias over the western half of the subcontinent was thus reduced. Radiative absorption by biomass burning aerosols resulted in diabatic warming of the atmosphere, peaking near 700 hPa at a rate of up to 1°C/d. Simulated surface cooling and heating at altitude stabilized the lower troposphere below 700 hPa. Above 700 hPa, stability was reduced in the equatorial region between 5°N and 5°S through an elevated heat pump mechanism, enhancing deep convection and precipitation. The southern branch of the African Easterly Jet was enhanced and shifted southward, likely as a result of the changes in the surface temperature gradient induced by both the reduction in solar radiation reaching the surface and through precipitation‐induced surface cooling in the equatorial region. Daily‐scale aerosol outflow events to the southwest Indian Ocean were also investigated, these events occurring with the passage of a westerly wave. It was found that the aerosol loading enhanced baroclinicity along the leading edge of the frontal system, thus intensifying and narrowing the band of precipitation in this zone.

Modeling of Saharan dust outbreaks over the Mediterranean by RegCM3: case studies

Abstract. The regional climate model RegCM3 coupled with a radiatively active aerosol model with online feedback is used to investigate direct and semi-direct radiative aerosol effects over the Sahara and Europe in a test case of July 2003. The aerosol model includes dust particles in addition to sulfates, hydrophobic and hydrophilic black carbon and organic carbon. The role of the aerosol online feedback on the radiation budget and the direct radiative forcing (short-wave and long-wave) by dust particles are investigated by intercomparing results from three experiments: REF, including all interactive aerosol components, Exp1, not accounting for the aerosol radiative feedback, and Exp2 not accounting for desert dust particles. The comparison of results in the REF experiment with satellite observations, sun/sky radiometer measurements, and lidar profiles at selected Central Mediterranean sites reveals that the spatio-temporal evolution of the aerosol optical depth is reasonably well reproduced by the model during the entire month of July. Results for the dust outbreaks of 17 and 24 July, averaged over the simulation domain, show that the daily-mean SW direct radiative forcing by all particles is −24 Wm−2 and −3.4 Wm−2 on 17 July and −25 Wm−2 and −3.5 Wm−2 on 24 July at the surface and top of the atmosphere, respectively. This is partially offset by the LW direct radiative forcing, which is 7.6 Wm−2 and 1.9 Wm−2 on 17 July and 8.4 Wm−2 and 1.9 Wm−2 on 24 July at the surface and top of the atmosphere, respectively. Hence, the daily-mean SW forcing is offset by the LW forcing of ~30% at the surface and of ~50% at the ToA. It is also shown that atmospheric dynamics and hence dust production and advection processes are dependent on the simulation assumptions and may significantly change within few tens of kilometers. The comparison of REF and Exp1 shows that the aerosol online feedback on the radiation budget decreases the domain-average daily-mean value of the 2 m-temperature, aerosol column burden (CB), and short-wave (SW) atmospheric forcing by −0.52 °C, 14%, and 0.9%, respectively on 17 July and by −0.39 °C, 12% and 12%, respectively on 24 July. The comparison of REF and Exp2 reveals that on 17 July, radiatively-active dust particles decrease the daily-mean 2 m-temperature averaged over the whole simulation domain by 0.4% even if are responsible for 99.8% and 97% of the daily-mean aerosol column burden and SW atmospheric forcing, respectively.