Evolution of the physical and chemical properties of the african dust

Cette thèse est consacrée à l’étude de l’évolution des propriétés physico-chimiques d’un panache d’aérosols désertiques nord africains au cours de son transport vers l’Europe du Nord. Cet épisode de mai 2008 est considéré comme l’évènement de transport d’aérosols désertiques le plus important de ces cinq dernières années observé sur le continent européen. La propagation de ce panache a été étudiée en combinant des moyens d’observation (sols, aéroportées, spatiales) au modèle de recherche Méso-NH. L’analyse des processus de dépôt a révélée que les quantités éliminées de l’atmosphère par dépôt sec et humide représentent respectivement 7 et 40 % de la quantité émise. Les valeurs d’épaisseur optique mesurées à 440 nm sur l’Europe variaient de 0.1 à 0.8, avec un maximum proche de 1 au-dessus des Pays-Bas. Au-dessus de cette région, les aérosols désertiques sont principalement situés entre 2.5 et 5.2 km d’altitude, mais aussi proche de la surface. La concentration en aérosols désertiques au sein de ces deux couches a été estimée respectivement à 350 et 450 ��g.m-3. Néanmoins, les mesures réalisées ne présentaient pas les caractéristiques optiques habituellement associées à la présence d’aérosols désertiques. En particulier, la présence d’une importante dépendance spectrale a été observée le 30 mai à proximité de Cabauw. L’exploitation des simulations ont permis de montrer que cela résultait d’une efficacité de lessivage plus importante du mode grossier. La présence d’aérosols désertiques coïncide avec une augmentation de la concentration des noyaux de condensation (CCN). Nos résultats suggèrent que le mélange entre le panache et les aérosols de pollution d’origine anthropique a conduit à une augmentation des capacités hygroscopiques des aérosols désertiques. Ce travail de recherche confirme ainsi que les conditions physico-chimiques de l'atmosphère régissent le cycle de vie des aérosols.This thesis focuses on the evolution of the dust physical and chemical properties through a case of long-range transport of Saharan dust over Northern Europe. This episode of May 2008 is considered as the strongest event of Saharan dust transport to Europe observed since these last five years. This spread of dust is investigated by combining observations (ground-based, airborne, satellite) and the meso-scale model Méso-NH.The analysis of the removal processes reveals that the amounts lost by dry and wet deposition represent 7 and 40 % the total dust emitted respectively. The observed aerosol optical thickness ranged from 0.1 to 0.8 at the wavelength of 440 nm, with a maximum value close to 1 is found over the Netherlands. Over that site, the dust layer was mainly located between 2.5 and 5.2 km, moreover dust was also present at 0.5 km. The concentration of dust inside these two layers is estimated to 350 and 450 ��g.m-3 respectively. Nevertheless, the usual optical characteristics of Saharan dust were not observed. In particular, the scattering coefficient measurements revealed a strong spectral dependence observed during the 30th May, close to Cabauw. The analysis of the numerical experiements revealed that this was due to high precipitation scavenging efficiency for the coarse mode. The presence of Saharan dust coincides with an increase of the cloud condensation nuclei (CCN) concentration. Our results suggest that the mixing processes between the Saharan dust and anthropogenic particles have led to an increase of the Saharan dust hygroscopic properties. Thus, this thesis confirms that physical and chemical conditions of the atmosphere govern the life cycle of dust

Évolution des propriétés physico-chimiques des aérosols désertiques issus de l’outflow africain

This thesis focuses on the evolution of the dust physical and chemical properties through a case of long-range transport of Saharan dust over Northern Europe. This episode of May 2008 is considered as the strongest event of Saharan dust transport to Europe observed since these last five years. This spread of dust is investigated by combining observations (ground-based, airborne, satellite) and the meso-scale model Méso-NH.The analysis of the removal processes reveals that the amounts lost by dry and wet deposition represent 7 and 40 % the total dust emitted respectively. The observed aerosol optical thickness ranged from 0.1 to 0.8 at the wavelength of 440 nm, with a maximum value close to 1 is found over the Netherlands. Over that site, the dust layer was mainly located between 2.5 and 5.2 km, moreover dust was also present at 0.5 km. The concentration of dust inside these two layers is estimated to 350 and 450 ��g.m-3 respectively. Nevertheless, the usual optical characteristics of Saharan dust were not observed. In particular, the scattering coefficient measurements revealed a strong spectral dependence observed during the 30th May, close to Cabauw. The analysis of the numerical experiements revealed that this was due to high precipitation scavenging efficiency for the coarse mode. The presence of Saharan dust coincides with an increase of the cloud condensation nuclei (CCN) concentration. Our results suggest that the mixing processes between the Saharan dust and anthropogenic particles have led to an increase of the Saharan dust hygroscopic properties. Thus, this thesis confirms that physical and chemical conditions of the atmosphere govern the life cycle of dust.Cette thèse est consacrée à l’étude de l’évolution des propriétés physico-chimiques d’un panache d’aérosols désertiques nord africains au cours de son transport vers l’Europe du Nord. Cet épisode de mai 2008 est considéré comme l’évènement de transport d’aérosols désertiques le plus important de ces cinq dernières années observé sur le continent européen. La propagation de ce panache a été étudiée en combinant des moyens d’observation (sols, aéroportées, spatiales) au modèle de recherche Méso-NH. L’analyse des processus de dépôt a révélée que les quantités éliminées de l’atmosphère par dépôt sec et humide représentent respectivement 7 et 40 % de la quantité émise. Les valeurs d’épaisseur optique mesurées à 440 nm sur l’Europe variaient de 0.1 à 0.8, avec un maximum proche de 1 au-dessus des Pays-Bas. Au-dessus de cette région, les aérosols désertiques sont principalement situés entre 2.5 et 5.2 km d’altitude, mais aussi proche de la surface. La concentration en aérosols désertiques au sein de ces deux couches a été estimée respectivement à 350 et 450 ��g.m-3. Néanmoins, les mesures réalisées ne présentaient pas les caractéristiques optiques habituellement associées à la présence d’aérosols désertiques. En particulier, la présence d’une importante dépendance spectrale a été observée le 30 mai à proximité de Cabauw. L’exploitation des simulations ont permis de montrer que cela résultait d’une efficacité de lessivage plus importante du mode grossier. La présence d’aérosols désertiques coïncide avec une augmentation de la concentration des noyaux de condensation (CCN). Nos résultats suggèrent que le mélange entre le panache et les aérosols de pollution d’origine anthropique a conduit à une augmentation des capacités hygroscopiques des aérosols désertiques. Ce travail de recherche confirme ainsi que les conditions physico-chimiques de l'atmosphère régissent le cycle de vie des aérosols

Lightning activity within tropical cyclones in the South West Indian Ocean

International audienceLightning activity within 70 tropical cyclones in the South West Indian Ocean is studied using a large data set (2005–2013) provided by the World Wide Lightning Location Network (WWLLN). The radial and azimuthal distributions of lightning flashes are analyzed in three different regions of the basin: the open ocean, the Mozambique Channel, and the oceanic region up to 400 km off the eastern coast of Madagascar (ECM). Over the open ocean, lightning activity detected by the WWLLN is mainly found in the eyewall and decreases outward, regardless of storm intensity. Lightning activity in the eyewall of tropical cyclones is higher than in the eyewall of tropical storms. The front and the right quadrants (225 • to 45 •) relative to the storm motion are the regions where lightning flashes are preferentially detected. Near the ECM, lightning density in the eyewall, the inner rainbands, and the outer rainbands is quite similar, presumably owing to the proximity of land. When the system reaches tropical cyclone strength, lightning activity is mainly found in the left and rear quadrants relative to storm motion. In the Mozambique Channel, the radial and azimuthal distributions of lightning flashes are complex due to the geographical configuration of this subdomain. The relationships between lightning activity and intensity change have also been investigated for systems over the open ocean. The proportion of periods with lightning activity is higher during rapid intensity changes of tropical cyclones. During tropical storm stage, lightning activity in the outer rainbands starts increasing 18 h before a rapid intensification period

Tropopause Characteristics and Variability from 11 yr of SHADOZ Observations in the Southern Tropics and Subtropics

International audienceIn this paper, tropopause characteristics observed from tropical to subtropical Southern Hemisphere stations using Southern Hemisphere Additional Ozonesonde (SHADOZ) data are presented for the 11-yr period of 1998-2008. Three different definitions of tropopause--cold-point tropopause (CPT), lapse-rate tropopause (LRT), and ozone tropopause (OT)--are determined, and their variability for nine different SHADOZ sites is studied for the purpose of evaluating their usefulness as indicators of possible tropopause trends. For each station, the OT is uniquely defined by the ozone gradient and is found to be more variable than either LRT or CPT. The OT roughly coincides with the upper boundary of the region of most active convective mixing over the western Pacific Ocean and with the lower boundary of the transition region from the troposphere to the lower stratosphere that is generally referred to as the tropical tropopause layer. The monthly and year-to-year variations in the tropopause are examined, and the annual cycle in OT, the dominant signal, is described. The distance of separation of the OT from the CPT or LRT is smaller for the tropics (stations at 0°-15°S) than for the subtropics (15°-25°S). The decadal trend in tropopause heights is measured using a statistical model that accounts for natural variations expressed in El Niño-Southern Oscillation, the quasi-biennial oscillation, and the Indian Ocean dipole. The decadal trend estimation shows no statistically significant trend for the CPT and LRT in the tropics, in contrast to other studies. A decrease in altitude for the OT is significant. In the subtropics, the CPT and LRT decline significantly, by −240 and −190 m (10 yr)−1, respectively, but the OT increases

Tropopause characteristics and variability from 11-year SHADOZ observations in the southern tropics and sub-tropics

International audienceCommunication about Tropopause characteristics and variability from 11-year SHADOZ observations in the southern tropics and sub-tropic

An exploratory analysis of low-ozone events during spring and summer months over Cape Point, South Africa

International audienceDecreased stratospheric ozone levels contribute towards increased ultraviolet (UV) radiation at the Earth's surface with potential negative impacts on public health. This study sought to determine whether or not the break-up of the polar vortex has an effect on stratospheric ozone levels and the resulting UV-B radiation at Cape Point. Using a dynamical model, it was shown that the polar vortex has a limited effect on ozone levels at Cape Point in September and that tropical air-masses had a larger impact on ozone levels during summer months. Decreased levels of stratospheric ozone resulted in increased UV radiation at the surface

Use of the TREND RUN model to deduce trends in South African Weather Service (SAWS) atmospheric data: Case study over Addo (33.568°S, 25.692°E) Eastern Cape, South Africa

International audienceThis paper reports on the use of a multi-regression model adapted at the University of Reunion Island, that was formulated for surface temperature trend estimates in South Africa. Depending on the location of the observing site, the selected geophysical signal is expressed in the form of a sum of forcings, which account for most of its variability. The trend values are then derived from the residual terms as a linear function. The atmospheric forcings included in the model are: Annual, Semi-annual, Quasi-Biennial Oscillation, El-Nino Southern Oscillation (ENSO), the 11-year solar cycle (SSN) and Indian Ocean Dipole (IOD). Long-term databases of more than 20 years are typically considered for the measurement of decadal trends and to determine the contribution of different atmospheric forcings. In this study, the surface minimum and maximum temperature datasets collected from 1980 to 2004 (25 years) at the South African Weather Service (SAWS) Addo station (33.568°S; 25.692°E), located in the Eastern Cape, South Africa are employed. The first results obtained for this station are presented and illustrate the validity of the model to account for observed trends

Long-range transport of Saharan dust over Northern Europe: EUCAARI 2008 campaign

International audienceThe evolution of dust optical properties is illustrated in this paper through a case of long-range transport of Saharan dust over northwestern Europe during the European Integrated Project on Aerosol-Cloud-Climate and Air Quality Interactions (EUCAARI) experimental campaign in 2008. This spread of dust over northwestern Europe is investigated by combining satellite, airborne, ground-based observations and the non- hydrostatic meso-scale model Meso-NH. The total dust amount emitted during the study period is estimated to 185 Tg. The analysis of the removal processes reveals that only 12.5 Tg is lost by dry deposition, and that wet deposition is the main process of dust removal (73 Tg). The observed aerosol optical thickness ranged from 0.1 to 0.5 at the wavelength of 440 nm, with a maximum value close to 1 is found over the Netherlands (51.97 N, 4.93 E). Over that site, the main dust layer is located between 2.5 and 5.2 km above sea level (asl), moreover dust was also present at 0.9 km asl. The nephelometer measurements on board the ATR-42 aircraft revealed a strong wavelength dependence of the scattering coefficient over the Netherlands. The Angström exponent is greater than 0.5, whereas usually it approaches zero in presence of Saharan dust. This is due to high precipitation scavenging efficiency for the coarse mode, particularly below 4 km. Our results confirm that atmospheric conditions govern the life cycle of dust microphysical phenomena, providing conditions for transformation processes during transport, and removal of particles from the atmosphere