Fabien Solmon

A regional climate modeling study of the effect of desert dust on the West African monsoon

[1]xa0We investigate the effect of the shortwave radiative forcing of Saharan dust on the West African monsoon with a regional climate model interactively coupled to a dust model. Toward this purpose we intercompare sets of 38 summer monsoon season simulations (1969–2006) with and without dust effects over a domain encompassing most of the African continent and adjacent regions. We find that the main effect of the dust radiative shortwave forcing is to reduce precipitation over the Sahel region. This is in response to cooling over the Sahara, which decreases the meridional gradient of moist static energy and results in a weakening of the monsoon energy pump. The dust effects also cause a strengthening of the southern branch of the African Easterly Jet and a weakening of Tropical Easterly Jet. Over the Sahel the dust forcing causes climate response patterns that are similar to those found during dry years over the Sahel, which suggests that Saharan dust feedbacks might have a role in maintaining drought events over the region. Overall, the inclusion of dust also tends to improve the model simulation of the West African monsoon, as well as African and Tropical Easterly jets. This work focuses on climatic feedback associated to shortwave radiation forcing and should be further completed by the study of dust effect on long-wave radiation.

Acidic processing of mineral dust iron by anthropogenic compounds over the north Pacific Ocean

Atmospheric processing of mineral aerosol by anthropogenic pollutants may be an important process by which insoluble iron can be transformed into soluble forms and become available to oceanic biota. Observations of the soluble iron fraction in atmospheric aerosol exhibit large variability, which is poorly represented in models. In this study, we implemented a dust iron dissolution scheme in a global chemistry transport model (GEOS-Chem). The model is applied over the North Pacific Ocean during April 2001, a period when concentrations of dust and pollution within the east Asia outflow were high. Simulated fields of many key chemical constituents compare reasonably well with available observations, although some discrepancies are identified and discussed. In our simulations, the production of soluble iron varies temporally and regionally depending on pollution-to-dust ratio, primarily due to strong buffering by calcite. Overall, we show that the chemical processing mechanism produces significant amounts of dissolved iron reaching and being deposited in remote regions of the Pacific basin, with some seasonal variability. Simulated enhancements in particulate soluble iron fraction range from 0.5% to 6%, which is consistent with the observations. According to our simulations, ∼30% to 70% of particulate soluble iron over the North Pacific Ocean basin can be attributed to atmospheric processing. On the basis of April 2001 monthly simulations, sensitivity tests suggest that doubling SO2 emissions can induce a significant increase (13% on average, up to 40% during specific events) in dissolved iron production and deposition to the remote Pacific. We roughly estimate that half of the primary productivity induced by iron deposition in a north Pacific high-nutrient low-chlorophyll region is due to soluble iron derived from anthropogenic chemical processing of Asian aerosol.

Modeling dust and soluble iron deposition to the South Atlantic Ocean

[1]xa0The global chemical transport model GEOS-Chem, implemented with a dust-iron dissolution scheme, was used to analyze the magnitude and spatial distribution of mineral dust and soluble-iron (sol-Fe) deposition to the South Atlantic Ocean (SAO). The comparison of model results with remotely sensed data shows that GEOS-Chem can capture dust source regions in Patagonia and characterize the temporal variability of dust outflow. For a year-long model simulation, 22 Tg of mineral dust and 4 Gg of sol-Fe were deposited to the surface waters of the entire SAO region, with roughly 30% of this dust and sol-Fe predicted to be deposited to possible high nitrate low chlorophyll oceanic regions. Model-predicted dissolved iron fraction of mineral dust over the SAO was small, on average only accounting for 0.57% of total iron. Simulations suggest that the primary reason for such a small fraction of sol-Fe is the low ambient concentrations of acidic trace gases available for mixing with dust plumes. Overall, the amount of acid added to the deliquesced aerosol solution was not enough to overcome the alkalinity buffer of Patagonian dust and initiate considerable acid dissolution of mineral-iron. Sensitivity studies show that the amount of sol-Fe deposited to the SAO was largely controlled by the initial amount of sol-Fe at the source region, with limited contribution from the spatial variability of Patagonian-desert topsoil mineralogy and natural sources of acidic trace gases. Simulations suggest that Patagonian dust should have a minor effect on biological productivity in the SAO.

Description of the Mesoscale Nonhydrostatic Chemistry model and application to a transboundary pollution episode between northern France and southern England

[1]xa0The mesoscale air quality Mesoscale Nonhydrostatic Chemistry (Meso-NH-C) model is applied to a complex pollution episode over Western Europe during the period 11 to 12 August 1997. As observed in satellite pictures and as simulated, the complexity of this episode is related to the presence of anticyclonic clear-sky areas and regions with deep convective activity in the simulation domain. A brief presentation of the model is made that covers in particular the on-line coupling capability for calculating meteorological and chemical concentration fields at each time step. Then, emphasis is put upon the simulation of transboundary pollution fluxes from London to northern France in a zone of large horizontal wind gradients. Comparison with data from the French Agence De lEnvironnement et de la Maitrise de lEnergie (ADEME) pollution network indicates that ozone concentrations and time of arrival of the pollution plume are correctly predicted at surface stations in northern France. A sensitivity analysis relying upon local ozone production and pollution transport has shown that ∼30% of ozone maxima levels could be attributed to regional transboundary fluxes.

Mixing of boundary layer and upper tropospheric ozone during a deep convective event over Western Europe

Abstract Typically, during summer over Europe, pollution episodes in the boundary layer are interspersed with deep convective events which significantly redistribute all pollutants in the vertical. A 3D mesoscale model with an entraining/detraining plume model coupled on-line with gaseous chemistry (J. Geophys. Res., 2002, in press), is used to study the impact of deep convection upon the redistribution of ozone during a summer pollution episode over northern France combining both stratospheric ozone intrusion and enhanced upward transfers. The model reproduces well the ozone concentrations measured in the upper troposphere during two MOZAIC flights and, through sensitivity analyses, can clearly ascertain to convective transport a 110 ppb ozone peak at 6000 m . This study also emphasizes the impact of convective processes on the ozone spatial distribution near the surface. As a result, convective updrafts and downdrafts affect all chemical concentrations, particularly over a range of ±30 ppb in the ozone surface concentrations. At this stage, our conclusion is that deep convection not only modifies the ozone distribution in the mid and upper troposphere but also has a significant effect at the surface.