School on climate and environmental modelling in the West African region

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radiative forcing of carbonaceous aerosols over two urban environments in northern india

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Assessment of isoprene and near-surface ozone sensitivities to water stress over the Euro-Mediterranean region

Plants emit biogenic volatile organic compounds (BVOCs) in response to changes in environmental conditions (e.g. temperature, radiation, soil moisture). In the large family of BVOCs, isoprene is by far the strongest emitted compound and plays an important role in ozone chemistry, thus affecting both air quality and climate. In turn, climate change may alter isoprene emissions by increasing temperature as well as the occurrence and intensity of severe water stresses that alter plant functioning. The Model of Emissions of Gases and Aerosols from Nature (MEGAN) provides different parameterizations to account for the impact of water stress on isoprene emissions, which essentially reduces emissions in response to the effect of soil moisture deficit on plant productivity. By applying the regional climate–chemistry model RegCM4chem coupled to the Community Land Model CLM4.5 and MEGAN2.1, we thus performed sensitivity simulations to assess the effects of water stress on isoprene emissions and near-surface ozone levels over the Euro-Mediterranean region and across the drier and wetter summers over the 1992–2016 period using two different parameterizations of the impact of water stress implemented in the MEGAN model. Over the Euro-Mediterranean region and across the simulated summers, water stress reduces isoprene emissions on average by nearly 6 %. However, during the warmest and driest selected summers (e.g. 2003, 2010, 2015) and over large isoprene-source areas (e.g. the Balkans), decreases in isoprene emissions range from −20 % to −60 % and co-occur with negative anomalies in precipitation, soil moisture and plant productivity. Sustained decreases in isoprene emissions also occur after prolonged or repeated dry anomalies, as observed for the summers of 2010 and 2012. Although the decrease in isoprene emissions due to water stress may be important, it only reduces near-surface ozone levels by a few percent due to a dominant VOC-limited regime over southern Europe and the Mediterranean Basin. Overall, over the selected analysis region, compared to the old MEGAN parameterization, the new one leads to localized and 25 %–50 % smaller decreases in isoprene emissions and 3 %–8 % smaller reductions in near-surface ozone levels.</p

MODELING THE TRANS-ATLANTIC TRANSPORTATION OF SAHARAN DUST

In the present study we are simulating the trans-Atlantic transport of dust from Sahara to the South-Central America, using the regional climate model RegCM4 and its online dust scheme, for the year 2007. The simulated horizontal and vertical distributions of the mineral dust optical properties were evaluated against the LIVAS CALIPSO satellite dust product. The Trans-Atlantic dust transport is simulated adequately with RegCM4, but there are some spatial discrepancies. Dust optical thickness is overestimated in the eastern Sahara throughout the year by 0.1-0.2, while near the gulf of Guinea is underestimated during winter and spring. Although RegCM4 dust plume is located southern on winter and spring, it doesn't spatially match the dust optical thickness of LIVAS. In summer and autumn the vertical distribution of dust between 3-4km during the Trans-Atlantic transport is simulated by the model adequately up to 30ºW 40ºW longitude. However, during winter-spring RegCM4 misplaces dust loading into higher altitude. Finally, we discuss some possible reasons and mechanisms that might be responsible for the differences between the model and the observations

Radiative and climatic effects of dust over West Africa, as simulated by a regional climate model

International audienceWe 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 forcin

Increase in summer European Ozone amounts due to climate change

Code Ineris : EN-2007-312International audienceThe local and regional distribution of pollutants is significantly influenced by weather patterns and variability along with the spatial patterns of emissions. Therefore, climatic changes which affect local meteorological conditions can alter air quality. We use the regional air quality model CHIMERE driven by meteorological fields from regional climate change simulations to investigate changes in summer ozone mixing ratios over Europe under increased greenhouse gas (GHG) forcing. Using three 30-year simulation periods, we find that daily peak ozone amounts as well as average ozone concentrations substantially increase during summer in future climate conditions. This is mostly due to higher temperatures and reduced cloudiness and precipitation over Europe and it leads to a higher number of ozone events exceeding information and warning thresholds. Our results show a pronounced regional variability, with the largest effects of climate change on ozone concentrations occurring over England, Belgium, Germany and France. The temperature-driven increase in biogenic emissions appears to enhance the ozone production and isoprene was identified as the most important chemical factor in the ozone sensitivity. We also find that summer ozone levels in future climate projections are similar to those found during the exceptionally warm and dry European summer of 2003. Our simulations suggest that in future climate conditions summer ozone might pose a much more serious threat to human health, agriculture and natural ecosystems in Europe, so that the effects of climate trends on pollutant amounts should be considered in future emission control measures

Implementation and evaluation of DROP, a model for the simulation of rainfall distribution below plants described in 3D

Rainfall interception by plants affects environmental processes such as water distribution, erosion and leaching of pollutants. In order to quantify the effect of plant cover for the management of water fluxes we developed the DROP model to simulate the water distribution through plants to the soil surface. It leaned on the discretization of the plant architecture provided by a computerized model or by the digitization of an actual plant. A spatially discrete rainfall table was modified until it reached the soil according to the interception and transmission properties of plant elementary surfaces. Below a maize plant, on sampling areas of 31.2 cm$^2$, the measured transmission ratio (rainfall at the soil surface to incident rainfall) reached 35 for stemflow and varied between 3.5 and 0.05 elsewhere. The DROP model reproduced that distribution fairly well and can be used below plants to map relative water distribution on a grid with a pixel size of 100 cm$^2$.Développement et évaluation de DROP, un modèle simulant la distribution de la pluie sous des plantes décrites en 3D. L'interception de la pluie par les plantes modifie des processus environnementaux comme la distribution de l'eau, l'érosion ou le lessivage de polluants. Pour quantifier l'effet du couvert végétal pour la gestion des flux d'eau, nous avons développé le modèle DROP pour simuler la distribution de l'eau à travers les plantes jusqu'à la surface du sol. Il s'appuie sur la discrétisation de l'architecture de la plante qui peut être fournie par un modèle informatique de plante ou par la digitalisation d'une plante réelle. Une nappe de pluie discrétisée dans l'espace est modifiée jusqu'à ce qu'elle rejoigne le sol, en fonction des propriétés de transmission et d'interception des surfaces élémentaires de la plante. Sous un pied de maïs, pour des surfaces de mesure de 31,2 cm$^2$, le rapport de transmission (pluie à la surface du sol sur pluie incidente) atteint 35 pour le stemflow et varie entre 3,5 et 0,05 ailleurs. Le modèle DROP reproduit correctement cette distribution et peut être utilisé pour cartographier la transmission relative de la pluie sous plantes sur des pixels de 100 cm$^2$

Aerosol modeling for regional climate studies: Application to anthropogenic particles and evaluation over a European/African domain

A simplified anthropogenic aerosol model for use in climate studies is developed and implemented within the regional climate model RegCM. The model includes sulphur dioxide, sulphate, hydrophobic and hydrophilic black carbon (BC) and organic carbon (OC) and is run for the winter and summer seasons of 2000 over a large domain extending from northern Europe to south tropical Africa. An evaluation of the model performance is carried out in terms of surface concentrations and aerosol optical depths (AODs). For sulphur dioxide and sulphate concentration, comparison of simulated fields and experimental data collected over the EMEP European network shows that the model generally reproduces the observed spatial patterns of near-surface sulphate. Sulphate concentrations are within a factor of 2 of observations in 34% (JJA) to 57% (DJF) of cases. For OC and BC, simulated concentrations are compared to different datasets. The simulated and observed values agree within a factor of 2 in 56% (DJF) to 62% (JJA) of cases for BC and 33% (JJA) to 64% (DJF) for OC. Simulated AODs are compared with ground-based (AERONET) and satellite (MODIS, MISR, TOMS) AOD datasets. Simulated AODs are in the range of AERONET and MISR data over northern Europe, and AOD spatial patterns show consistency with MODIS and TOMS retrievals both over Europe and Africa. The main model deficiencies we find are: (i) an underestimation of surface concentrations of sulphate and OC during the summer and especially over the Mediterranean region and (ii) a general underestimation of AOD, most pronounced over the Mediterranean basin. The primary factors we identify as contributing to these biases are the lack of natural aerosols (in particular, desert dust, secondary biogenic aerosols and nitrates), uncertainties in the emission inventories and aerosol cycling by moist convection. Also, in view of the availability of better observing datasets (e.g. as part of the AMMA project), we are currently working on improving these aspects of the model before applying it to climate studies. Despite the deficiencies identified above, we assess that our model shows a performance in line with that other coupled climate/aerosol models and can presently provide a useful tool for sensitivity and process studies

The (uncertain) future of air quality

The assessment of the future evolution of air quality requires accounting for both climate projections and the development of environmental policies. In the context of climate change adaptation, the geophysical changes to be expected in the decades to come will have an impact on chronic and extreme air pollution events (Jacob and Winner, 2009). But air quality is also sensitive to climate mitigation strategies: the social and technological changes required to reduce greenhouse gases emissions will also be accompanied by changes in the emission of air pollutants and precursors thereof. There are potentially large co-benefits between air quality and climate change mitigation that could help in leveraging efforts to engage in win-win strategies. But mitigating climate change can also potentially induce collateral damages to air quality. It is thus very important to precisely identify what are the co-benefits and the possible collateral damages in order to maximize the former while minimizing the later. Here we briefly review recent results on climate change impacts on Mediterranean regional air quality in terms of ozone and particles, and list identified positive and negative feedbacks of climate change on air quality