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Scope of SAMUM
The complexity of aerosol processes requires a concerted well focused effort by a team of different research groups. The SAMUM consortium listed in Tab. 1 and their foreign partners offer state-of-the-art numerical and experimental tools, which will be extended, optimized and applied during SAMUM. The experiments are planned to take place in critical regions to advance significantly our understanding of radiative effects of Saharan dust. SAMUM addresses the full range from local microphysical and chemical properties of dust derived from laboratory and field data to regional and global effects as analyzed by airborne and space-borne sensors and as simulated and forecast by advanced weather and climate models.
Local microphysical and chemical properties of Saharan dust
In order to estimate direct and indirect radiative effects of atmospheric aerosol particles, sizeresolved information in the size range from 50 nm in diameter up to approximately 100 µm is required. The size distribution of the total aerosol and the different components (e.g., mineral, biological, anthropogenic components) will be analyzed. Optical properties will be measured of airborne dust and on particulate samples with an emphasis on the characterization and attribution of the light-absorbing fraction (e.g., soot and hematite). A main goal of the analysis is the determination of the complex index of refraction and of the hygroscopic behavior of the different components of the airborne particulate matter originating from desert soils mixed with anthropogenic components. Projects 5 (see Table 1 for project notations), and 6 will contribute most of the related work while projects 1, and 2 will supply complementary data taken on airborne platforms. Local dust source processes will not be studied in SAMUM. Instead, SAMUM will rely on state of the art source models developed by project 3 in cooperation with French partners and validated by the experimental data of SAMUM.
Characterization of the dust-filled atmospheric column
Most radiation studies of dust largely rely on columnar data derived from ground-based and space-borne radiometers. Whereas SAMUM also utilizes this type of data with state-of-the art instrumentation operated in projects 4 and 7 the present experimental approach reaches far beyond columnar information because radiative effects of dust are strongly controlled by the vertical distribution of dust properties. A unique ground-based multi-wavelength lidar will determine profiles of spectral optical dust properties and microphysical characteristics of the dust particles at ambient conditions. For the first time, lidar profiles of the volume extinction coefficient of dust particles at UV and visible wavelengths will be measured within a large dust—related field campaign. Two further ground-based lidars will work in conjunction with the main lidar to yield polarization data that is crucial for the optical characterization of the nonspherical dust particles. Information on the scattering phase function will be derived in an innovative new bi-static approach. The lidar work will be carried out in project 4.
With two aircraft (a small and a larger one) the columnar information is extended with a range of properties that cannot be measured from the ground. Spectral surface albedo is essential for all satellite-derived aerosol characteristics and will be measured with an albedometer in project 2 deployed on the small aircraft. This instrument will also determine profiles of spectral radiative flux densities. Volumetric scattering and absorption properties of dust particles will be determined onboard the same aircraft (project 6). Whereas the smaller aircraft will cover the height range from the surface to about 3 km with detailed radiative flux density measurements, the larger aircraft of project 1 will extend this range to the tropopause region with lidar measurements, onboard particle sampling for projects 5, and 6, in situ measurements of aerosol microphysical and optical properties, and broadband radiation measurements in the solar and terrestrial spectral range.
As a consequence of the detailed measurements at the surface plus the atmospheric column the retrieval of dust properties from concurrent space-borne measurements in project 7 will have all necessary data to optimize the retrieval algorithms for mineral dust and to develop new approaches. The complete data set will allow the first rigorous columnar closure tests of radiative dust properties and effects. At the same time the complete columnar data set will be utilized by a the Third-Order Planetary Boundary Layer-Chemistry-Aerosol Model TOPCAM in project 3 to verify and enhance the sub-grid-scale dust emission parameterization of a regional weather model.
During SAMUM the first dedicated space-borne aerosol Lidar CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) will provide a unique opportunity for the acquisition of global vertical profiles of optical aerosol and cloud characteristics that never have been available for any dust study. SAMUM will greatly profit from CALIPSO data also because investigators of project 4 are involved in the activities of the CALIPSO science team. At the same time CALIPSO is strongly dependent on the ground truth and airborne data derived within SAMUM.
Regional and global radiative effects of Saharan dust
The extrapolation to regional and global radiative effects of Desert dust will be performed within projects 3, and 7 of SAMUM. The modeling project 3 will develop a regional model of dust emissions, transport and deposition for the northern Sahara suitable for simulation of individual dust events and direct comparison with observations. For the use in radiative transfer calculations and for improving dust parameterizations in global climate models mesoscale space-time distribution of dust properties will be computed with this extended state-of-the-art weather model of the German Weather Service (Lokalmodell LM). Finally, the radiative effects of Saharan dust will be quantified from experimentally evaluated mesoscale simulations of Saharan dust storm events. With the ECHAM5 general circulation model (GCM (General Circulation Model)) the climate effects will be estimated with the refined dust parameterization derived in SAMUM. Whereas this estimation with the GCM proceeds along the lines of current climate modeling, the goal for the development of the LM reaches far beyond the state of the art in numerical weather prediction because the strong radiative effects of the dust will be fed back into the atmospheric dynamics part of the model for a more realistic simulation of atmospheric processes.