Atmos. Chem. Phys. Discuss., 10, 4027-4077, 2010
www.atmos-chem-phys-discuss.net/10/4027/2010/
doi:10.5194/acpd-10-4027-2010
© Author(s) 2010. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
The potential influence of Asian and African mineral dust on ice, mixed-phase and liquid water clouds
A. Wiacek1, T. Peter2, and U. Lohmann2
1Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada
2Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland

Abstract. This modelling study explores the availability of mineral dust particles as ice nuclei for interactions with ice, mixed-phase and liquid water clouds, also tracking the particles' history of cloud-processing. We performed 61 320 one-week forward trajectory calculations originating near the surface of major dust emitting regions in Africa and Asia using high-resolution meteorological analysis fields for the year 2007. Without explicitly modelling dust emission and deposition processes, dust-bearing trajectories were assumed to be those coinciding with known dust emission seasons. We found that dust emissions from Asian deserts lead to a higher potential for interactions with high clouds, despite being the climatologically much smaller dust emission source. This is due to Asian regions experiencing significantly more ascent than African regions, with strongest ascent in the Asian Taklimakan desert at ~25%, ~40% and 10% of trajectories ascending to 300 hPa in spring, summer and fall, respectively. The specific humidity at each trajectory's starting point was transported in a Lagrangian manner and relative humidities with respect to water and ice were calculated in 6-h steps downstream, allowing us to estimate the formation of liquid, mixed-phase and ice clouds. Practically none of the simulated air parcels reached regions where homogeneous ice nucleation can take place (T≲−40 °C) along trajectories that have not experienced water saturation first. By far the largest fraction of cloud forming trajectories entered conditions of mixed-phase clouds, where mineral dust will potentially exert the biggest influence. The majority of trajectories also passed through regions supersaturated with respect to ice but subsaturated with respect to water, where "warm" (T≳−40 °C) ice clouds may form prior to supercooled water or mixed-phase clouds. The importance of "warm" ice clouds and the general influence of dust in the mixed-phase cloud region are highly uncertain due to considerable scatter in recent laboratory data from ice nucleation experiments, which we briefly review in this work. For "classical" cirrus-forming temperatures, our results show that only mineral dust IN that underwent mixed-phase cloud-processing previously are likely to be relevant, and, therefore, we recommend further systematic studies of immersion mode ice nucleation on mineral dust suspended in atmospherically relevant coatings.

Citation: Wiacek, A., Peter, T., and Lohmann, U.: The potential influence of Asian and African mineral dust on ice, mixed-phase and liquid water clouds, Atmos. Chem. Phys. Discuss., 10, 4027-4077, doi:10.5194/acpd-10-4027-2010, 2010.
 
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