TT7: Characterisation of Aerosols during the dry season and analysis of their radiative impact

TT-leaders: Jim Haywood, Jacques Pelon, Karine Desboeufs, Paola Formenti, Ellie Highwood, Béatrice Marticorena

Detailed information: TT7 document: Characterisation of Aerosols during the dry season and analysis of their radiative impact (latest update: 23.01.2006, pdf, 1.8 MB)

Scientific justification and objectives

The African continent is the largest global source of both mineral dust aerosols and biomass burning aerosols. These aerosols are known to significantly affect the solar and terrestrial radiation of the African region thereby modifying the planetary albedo and the outgoing longwave radiation and reducing the radiation flux available to the ocean. In addition to attenuation by scattering, these aerosols absorb a significant proportion of both solar and terrestrial radiation in the atmospheric column and therefore modify the heating rates of the atmosphere. They also significantly influence the surface radiation balance with subsequent effects on the sensible and latent heat fluxes over land surface which may affect the atmospheric dynamics and hydrology of the region. Additionally, the aerosol produced during the dry season may play a role in the interannual variability through the modulation of the radiation energy stored in the ocean in the Benin Gulf. On the reverse, rainfall and the development of the vegetation, linked to the penetration of West African Monsoon, indirectly controls dust emission and biomass burning emissions during the dry season. Deposition of west African dust to the Atlantic ocean provides ocean nutrients, including iron and subsequent changes in ocean productivity affects the global carbon cycle.

The importance of the radiative forcing of aerosols and potential impact on dynamics has been shown at different scales. The lack of inclusion of mineral dust aerosol has recently been highlighted as a major model deficiency in numerical weather prediction models. On the other hand, aerosols are now included in climate models to predict how they affect long-term climate changes acting directly on radiative budget or indirectly through the modification of cloud life cycle. To address these issues, the Met Office Unified model has been developed to include mineral dust generation, transport and deposition in both the global and CAM (crisis area model) versions. However interaction processes still need to be better understood and parameterised.

Until now, dust and biomass aerosol properties and radiative impact have been characterized in regions where they have been studied independently. Dust uptake from the surface to the atmosphere has extensively been analysed and parameterisations have been developed for modelling. Source regions for dust and biomass burning particles as well as horizontal transport processes near sources can be identified by radiometry from satellite, but one of the main difficulties remains to precisely identify the vertical extension of the transported particles, which is critical for further dispersion and to the radiative impact due to their significant absorption. Far from the sources, dust or biomass burning aerosols can be frequently observed in elevated layers. In 1999, lidar observations from LITE were first used to analyse the dispersion of dust particles over the Atlantic ocean. Combination of in situ, passive and active remote sensing during the SHADE campaign have more recently allowed a better characterization of their radiative impact. Biomass burning aerosols have been extensively characterized during the SAFARI campaign. Although their radiative forcing is negative over low reflectance surfaces, the observations made during the SAFARI campaign have first evidenced a positive radiative forcing due to elevated biomass particle layers over water clouds.

Dust and biomass burning aerosol particles have different microphysical and radiative properties (size, shape, absorption) and their mixing is leading to complex modifications. Mixing and, further on, aging, sedimentation and cloud processing occurring over land and ocean will then result in a variable forcing which needs to be better understood. This is the purpose of the AMMA SOP0 field experiment.

The SOLAS observational programme Dust Outflow and Deposition in the Ocean (DODO) will be conducted in collaboration with AMMA and will extend observations of dust outbreaks further west towards Cape Verde and the Atlantic Ocean. It will be complemented by dust modelling, laboratory studies and remote sensing products, to produce a quantitative climatology of dust deposition to the Atlantic Ocean, including extensive analysis of iron content. In situ measurements of aerosol chemical and microphysical properties will be made using the FAAM aircraft and associated instrument suite during dust outflow events.

To summarize, the primary objectives of the AMMA SOP0 are:

• To perform high quality in situ and remote sensing measurements of the optical and physical properties of mineral dust aerosols, anthropogenic biomass burning aerosols, and combinations of the two from sub-Sahelian West Africa.
• To provide high quality spectral measurements of the solar and terrestrial radiative effects of both mineral dust and biomass burning aerosols.
• To determine the consistency between in situ measurements/satellite and surface based remote sensing methods of the effects on the radiation budget of the Earth of the composite biomass and mineral dust aerosols.
• To measure and model the effect of the presence of aerosols on the radiation budget at the top of the atmosphere, throughout the atmospheric column, and at the surface of the Earth over land and ocean.
• To contribute to new satellite validation and allow data to be further used in the analysis at the regional and global scale.
• To improve and validate numerical models (global and regional) to better understand the impact of aerosols on the radiation budget, hence the climate.
• To better determine the winter-time source regions, source strengths and emission factors of mineral dust and biomass burning aerosols.
• To better determine the winter-time outflow and deposition of mineral dust aerosols.
  

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