Pierre Tulet

Quantifying uncertainty in estimates of mineral dust flux: An intercomparison of model performance over the Bodélé Depression, northern Chad

Mineral dust aerosols play an important role in the climate system. Coupled climate-aerosol models are an important tool with which to quantify dust fluxes and the associated climate impact. Over the last decade or more, numerous models have been developed, both global and regional, but to date, there have been few attempts to compare the performance of these models. In this paper a comparison of five regional atmospheric models with dust modules is made, in terms of their simulation of meteorology, dust emission and transport. The intercomparison focuses on a 3-day dust event over the Bodele depression in northern Chad, the worlds single most important dust source. Simulations are compared to satellite data and in situ observations from the Bodele Dust Experiment (BoDEx 2005). Overall, the models reproduce many of the key features of the meteorology and the large dust plumes that occur over the study domain. However, there is at least an order of magnitude range in model estimates of key quantities including dust concentration, dust burden, dust flux, and aerosol optical thickness. As such, there remains considerable uncertainty in model estimates of the dust cycle and its interaction with climate. This paper discusses the issues associated with partitioning various sources of model uncertainty.

Diurnal cycle of dust and cirrus over West Africa as seen from Meteosat Second Generation satellite and a regional forecast model.

A brightness temperature difference (BTD) technique is used to evaluate the dust and cirrus forecasts of a regional meteorological model. The technique based on a contrasted absorption property of dust and cirrus at two wavelengths within the atmospheric infrared window is applied to 3-hourly Meteosat Second Generation (MSG) observations in the 10.8- and 12-μm bands over West Africa. The satellite observation of dust coverage over the Sahara shows a well marked diurnal cycle associated with the boundary layer activity peaking at 15 UTC. A similar signature is obtained from the regional model when the dust scheme is activated. The cirrus cover over West Africa is maximum at 12 UTC as seen both from MSG and the model. The use of prognostic dust aerosol, instead of climatology, furthermore better captures the observed convective activity.

The 7–13 March 2006 dust storm over West Africa: Generation, transport, and vertical stratification

Several studies have shown the importance of desert dust aerosols in weather forecast models. Nevertheless, desert dust has been poorly represented in such models and is the source of some prediction errors, in particular for tropical and subtropical regions. The purpose of this paper is to illustrate the formation and the three-dimensional transport of a severe dust storm which occurred in March 2006 over West Africa. An intense dust haze was transported southward over the Gulf of Guinea thereby generating an aerosol optical thickness (AOT) greater than 4 over Nigeria. The MesoNH mesoscale atmospheric model coupled with new dust parameterization schemes has been used to illustrate the three-dimensional transport of the dust plume and the vertical layering of this desert air mass above the lower atmosphere monsoon flux layer. It is modeled that more than 50 g m−2 of dust was emitted during this event from the surface by a strong Harmattan wind over the Sahel region. It is also shown that when the dust layer is located over the boundary layer, it can modify the atmospheric stability by as much as 9.5 K in terms of potential temperature in the lowest 2000 m of the atmosphere.

Development of a reduced chemical scheme for use in mesoscale meteorological models

Abstract A new Regional Lumped Atmospheric Chemical Scheme (ReLACS) is introduced. This mechanism is intended to be valid for clean to polluted conditions. ReLACS is derived from a new reactivity weighting approach which may be applied to any atmospheric chemical scheme. In this work, ReLACS is based upon the reduction of the regional atmospheric chemistry mechanism (RACM) (Stockwell et al., 1997. Journal of Geophysical Research D22, 25847–25849) on a given moderately polluted emission scenario. However, sensitivity tests around this scenario show that ReLACS compares favorably with RACM, not only for ozone but also for other important atmospheric oxidants. The gain obtained from this lumping in the number of species allows ReLACS to be suitable to perform, otherwise costly, three dimensional pollution studies when coupled with mesoscale meteorological models.

Estimate of Sahelian dust emissions in the intertropical discontinuity region of the West African Monsoon

A three-dimensional mesoscale numerical simulation has been performed to investigate the dust emissions over the Sahel associated with strong near-surface winds in the region of the West African Inter Tropical Discontinuity (ITD) during the summer, when the ITD is located over Niger and Mali around 18°N. The study focuses on the period from 2 to 12 July 2006, in the framework of the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1. The comparison with observations suggests that the model can be used reliably to analyze and quantify the dust emissions associated with the strong near-surface winds blowing over the Sahelian dust sources during the period of interest. The daily mean values of dust load related to the strong winds on both side of the ITD, as estimated from the simulation within the model domain (2°W–16°E, 12–28°N), are in excess of 2 Tg on some of the days of the 2–12 July 2006 period. In the present case, the dust load associated with the strong winds south of the ITD accounts for between one third and two thirds of the total load mobilized in the ITD region over the entire domain on a given day. It is simulated to range between 0.5 and 0.8 Tg on average. This study suggests that emissions driven by strong surface winds occurring on both sides of the ITD while lying across the Sahel may contribute significantly to the total dust load over West and North Africa observed annually.

Dry cyclogenesis and dust mobilization in the intertropical discontinuity of the West African Monsoon: A case study

Three-dimensional mesoscale numerical simulations were performed over Niger in order to investigate dry cyclogenesis in the West African intertropical discontinuity (ITD) during the summer, when it is located over the Sahel. The implications of dry cyclogenesis on dust emission and transport over West Africa are also addressed using the model results, together with spaceborne observations from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). The study focuses on the case of 7-8 July 2006, during the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1. Model results show the formation of three dry cyclones in the ITD during a 24-h period. Simulations are used to investigate the formation and the development of one of these cyclones over Niger in the lee of the Hoggar-Air Mountains. They show the development of the vortex to be associated with (1) strong horizontal shear and low-level convergence existing along the monsoon shearline and (2) enhanced northeasterly winds associated with orographic blocking of air masses from the Mediterranean Sea. The dry cyclone was apparent between 0700 and 1300 UTC in the simulation, and it was approximately 400 km wide and 1500 m deep. Potential vorticity in the center of vortex reached nearly 6 PVU at the end of the cyclogenesis period (1000 UTC). The role of the orography on cyclogenesis along the ITD was evaluated through model simulations without orography. The comparison of the characteristics of the vortex in the simulations with and without orography suggests that the orography plays a secondary but still important role in the formation of the cyclone. Orography and related flow splitting tend to create low-level jets in the lee of the Hoggar and Air mountains which, in turn, create conditions favorable for the onset of a better defined and more intense vortex in the ITD region. Moreover, orography blocking appears to favor the occurrence of a longer-lived cyclone. Furthermore, model results suggested that strong surface winds (11 m s-1) enhanced by the intensification of the vortex led to the emission of dust mass fluxes as large as 3 µg m-2 s-1. The mobilized dust was mixed upward to a height of 4-5 km to be made available for long-range transport. This study suggests that the occurrence of dry vortices in the ITD region may contribute significantly to the total dust activity over West Africa during summer. The distribution of dust over the Sahara-Sahel may be affected over areas and at time scales much larger than those associated with the cyclone itself.

Development of an online radiative module for the computation of aerosol optical properties in 3-D atmospheric models: validation during the EUCAARI campaign

Obtaining a good description of aerosol optical properties for a physically and chemically complex evolving aerosol is computationally very expensive at present. The goal of this work is to propose a new numerical module computing the optical properties for complex aerosol particles at low numerical cost so that it can be implemented in atmospheric models. This method aims to compute the optical properties online as a function of a given complex refractive index deduced from the aerosol chemical composition and the size parameters corresponding to the particles. The construction of look-up tables from the imaginary and the real part of the complex refractive index and size parameters will also be explained. This approach is validated for observations acquired during the EUCAARI (European integrated project on aerosol cloud climate air quality interactions) campaign on the Cabauw tower during May 2008 and its computing cost is also estimated. These comparisons show that the module manages to reproduce the scattering and absorbing behaviour of the aerosol during most of the fifteen-day period of observation with a very cheap computationally cost.

Long‐range transport of Saharan dust over northwestern Europe during EUCAARI 2008 campaign: Evolution of dust optical properties by scavenging

The 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 nonhydrostatic 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 Angstrom 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.

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.

Air pollution modelling at a regional scale

Abstract A mesoscale simulation system Meso-NH-C allowing for on line coupling between dynamics and chemistry is presented. Further advantages of this system lie in high vertical resolution, nesting capabilities and a full set of parametrization schemes. This system is applied from 9 to 11 August 1997 for a pollution episode over Western Europe. In this episode, both a European and a regional scale analysis have been performed (Greater Paris area). The possibilities of such a modelling system have been emphasized, particularly regarding the evaluation of transboundary fluxes.