Occurrence of Vertical Mixing in the Saharan Air Layer Studied with an Integrated Model, Lidar, and Insitu Approach in the Frame of the SALTRACE Campaign
Abstract
Long-range transport of aerosol in the Saharan Air Layer (SAL) from Africa over the Atlantic plays an important role for weather, climate, and ocean fertilization. However, processes occurring within the SAL and their effects on aerosol properties are still unclear. To gain more insight, the Saharan Aerosol Long-range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE) was performed using a wide range of measurement platforms, including ground-based remote sensing and airborne in-situ instruments. In our presentation we study vertical mixing within the SAL based on measured and modeled vertical aerosol profiles in the upper 1 km of the transported SAL. We use ground-based lidar and airborne particle counter data from SALTRACE, measured in the vicinity of Barbados, which is about 4500 km from the western tip of Africa. In addition, CALIOP data collected along the transport path over the Atlantic is considered. In our model we take account of the optical properties and the gravitational settling of irregularly-shaped Saharan dust particles. We test two hypotheses about the occurrence of vertical mixing within the SAL to explain the aerosol profiles observed by the lidars and the particle counter. Our first hypothesis (H1) assumes that no mixing occurs in the SAL leading to an altitude separation of super-micron dust particles as a result of settling. The second hypothesis (H2) assumes that vertical mixing occurs in the SAL during the day allowing large super-micron dust particles to stay airborne longer than with H1. In general, a decrease of the particle linear depolarization ratio towards the SAL top is found in the measured lidar data but the decrease is much weaker than modeled in case of H1. The in-situ data on particle number concentrations show a presence of large particles near the SAL top that is inconsistent with H1. Furthermore, the analysis of the CALIOP data reveals that the average vertical profile of the linear depolarization ratio of the aerosols in the upper 1 km of the SAL does not change along its transport path over the Atlantic. These findings indicate H2 to be much more likely than H1, giving evidence that vertical mixing occurs within the SAL over the Atlantic with significant consequences for the evolution of the size distribution of the super-micron dust particles during transport.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.A14C..01G
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 0365 Troposphere: composition and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSESDE: 1631 Land/atmosphere interactions;
- GLOBAL CHANGE