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Content archived on 2024-05-29

Aerosol transport and lifetime studies: Constraining the indirect effect of aerosols on climate

Final Activity Report Summary - ATLIS (Aerosol Transport and Lifetime Studies: Constraining the Indirect Effect of Aerosols on Climate)

Naturally occurring mineral dust is, on a mass basis, the most abundant aerosol in the atmosphere. It is emitted every few days in large quantities from sources in the Sahara in July and August, and a little less frequently from sources in Asia in the spring. It travels great distances westward across the Atlantic in the first case, and eastward across the Pacific in the second. Along the way, it affects biogeochemical cycles and has a radiative impact by strongly reflecting visible radiation and absorbing in the infrared. The radiative processes cause a strong aerosol direct effect, which feeds back on surface temperatures and winds, as well as on climate. One of the biggest uncertainties related to aerosols, however, are their indirect radiative effects via them altering clouds and precipitation patterns. Mineral dust is known to be an efficient nucleus for the formation of ice clouds (cirrus); however, uncertainties in this aerosol's atmospheric lifecycle are very large and currently prevent accurate quantitative estimates of its impact on the ice phase of clouds and precipitation, with their associated climate feedbacks. Uncertainties in cloud cover and precipitation formation are one of the most important problems in computer modelling and future predictions of climate change.

In order to understand dust emission and transport events better, we investigated remote-sensing satellite measurements of mineral dust and cirrus clouds from the CALIPSO LIDAR instrument (launched in 2006) and also from the passive MODIS and OMI instruments (launched in 2002 and 2004). Using meteorological information such as wind, temperature and moisture from the European Centre for Medium-range Weather Forecasts (ECMWF), this work aimed at constraining the role of mineral dust in cold cloud formation. Specifically, in what is known as a Lagrangian trajectory analysis, the ECMWF information was used to trace the transport paths of dust for specific West African and Asian dust emission episodes in 2007. In addition, the potential for dust interactions with cold clouds was assessed in a statistical manner for the duration of 2007 using trajectories originating in the main dust emitting regions of Africa and Asia (West African Desert, Bodélé Depression in Chad, as well as the Taklimakan and Gobi Deserts in China and Mongolia). The main conclusions of this trajectory model-based work are the following:
1) Mineral dust, and especially mineral dust free of condensed water coatings, is very unlikely to exist at altitudes where cirrus clouds can form homogeneously (Temperatures < -35 C);
2) Mineral dust can potentially have a much bigger effect on mixed-phase clouds than on ice-phase-only clouds; and
3) For both cirrus and mixed-phase cloud processes, the Asian dust sources are much more important than the much more abundant African dust sources (in terms of emitted mass).