Introduction - Mineral Dust: A Key Player in the Earth System - page 10

The largest of the four parts of this topic is Part II, which describes the global
dust budget (Chaps. 5 , 6 , 7 , 8 , 9 , 10 ) . This involves the three key elements of the dust
cycle, emission, transport and deposition, as well as information on how the dust
budget is monitored and modelled. Naturally, this part begins with dust emission
processes in Chap. 5 , where Béatrice Marticorena explains the fundamental concept
of an erosion threshold and key physical processes like saltation and self-abrasion,
which ultimately lead to the release of fine particles available for long-range
transport. The theoretical concepts presented here are a key ingredient in numerical
models of dust that are used for weather and climate forecasts and analyses (Chaps.
9 , 10 and 13 ) . One of the most important controlling parameters of dust emission
is near-surface wind speed, and therefore Chap. 6 by Peter Knippertz describes
the meteorological systems and phenomena that create sufficiently high winds
in dust source regions. These systems span scales from thousands of kilometres
(cyclones and anticyclones, fronts) to less than a few tens of metres (dust devils)
and are also involved in the vertical and horizontal transport of freshly emitted
dust, often with strong diurnal cycles. Observing and documenting the resulting
3D dust distribution and its changes in time is a great challenge as detailed in Chap.
7 by Isabelle Chiappello. It requires an ever-increasing range of spaceborne remote-
sensing techniques and platforms in concert with ground-based instruments such
as sun photometers and lidars. The final stage of the dust cycle is deposition, the
physical mechanisms of which are explained by Gilles Bergametti and Gilles Forêt
in Chap. 8 . Deposition is typically divided into dry deposition through gravitational
settling, impaction and diffusion and wet deposition through in-cloud and sub-cloud
scavenging. Particular in-cloud processes are strongly dependent on characteristics
of individual particles (Part I) and how they can interact with cloud microphysics
(Chap. 12 ) . In Chap. 9 , Ina Tegen and Michael Schulz explain how the fundamental
concepts of emission, transport and deposition are implemented into numerical dust
models that can be used for climate and weather simulations following different
approaches for how the dust is treated in the model. Finally in Chap. 10 , Angela
Benedetti et al. document how the dust models described in Chap. 9 are used in
operational forecast systems around the world to produce daily predictions of dust
fields. This includes a discussion of key new approaches like multimodel ensembles
and assimilation of satellite data of the type discussed in Chap. 7 .
Part III is entitled Impacts of dust and comprises the Chaps. 11 , 12 , 13 ,
14 , 15 . The two most important direct impacts of dust on the atmosphere and
climate system are those connected to radiation and clouds, both discussed at the
beginning of this Part. In Chap. 11 , Ellie Highwood and Claire Ryder explain
the fundamental optical properties of dust (dust optical thickness, single-scattering
albedo etc.) and their dependence on composition and size distribution, which
links back to Chap. 2 . In the following Chap. 12 , Athanasios Nenes et al. discuss
how dust particles affect characteristics of clouds (and therefore precipitation)
through acting as cloud condensation nuclei or ice nuclei. These aspects are equally
dependent on composition and size distribution, with ageing processes being of
great importance, as they tend to change the hygroscopicity of dust (see Chap. 4 ) .
The following three chapters discuss more indirect impacts of dust. In Chap. 13 ,