Meteorological Aspects of Dust Storms - Mineral Dust: A Key Player in the Earth System - page 121

6.1 Introduction

Dust emission occurs from multiple sources around the globe (Chap. 3 ) , when
the near-surface wind exceeds the emission threshold determined by the local
soil conditions (Chap. 5 ) . This threshold is strongly influenced by factors such
as rainfall, soil moisture, flooding (e.g. of ephemeral lakes), vegetation and snow
cover. Soil and wind related factors in combination create strong regional, seasonal,
interannual and longer-term variations in dust emissions. This chapter concentrates
on meteorological mechanisms for the generation of strong winds over dust sources
during the parts of the year, when the soil is typically dry and exposed enough to
be deflated, leading to local peaks in the annual dust emission (for an overview, see
Littmann 1991 ).
These meteorological drivers will be separated by their spatial scales into
continental-scale circulation systems like monsoons (Sect. 6.2 ), synoptic-scale
weather systems in the southern midlatitudes, subtropics and tropics (Sect. 6.3 ),
mesoscale moist convective systems (Sect. 6.4 ) and microscale processes in the
dry-convective planetary boundary layer (PBL; Sect. 6.5 ). Subsequently, diurnal
variations (Sect. 6.6 ) and topographic effects such as lee cyclogenesis and oro-
graphic channelling (Sect. 6.7 ) will be described. Section 6.8 discusses how well
the multi-scale processes involved in dust lifting and transport are captured in state-
of-the-art computer models (see also Chaps. 9 and 10 ) .
While there is no doubt that each dust source region has its own particular local
characteristics as discussed in many other articles and topics, this chapter does not
aim to describe and discuss all these in any detail. Instead it aims at conveying a
general understanding of the meteorological mechanisms of greatest importance
to dust emission and subsequent transport, which are in fact common to many
dust source regions. A certain focus will be placed on the world's most significant
dust source regions in the so called dust belt, reaching from northern Africa across
the Middle East and central Asia to northern India, as well as those in China and
Mongolia (see Chap. 3 for more details). Additional discussion will be devoted to
sources in North America and Australia, while the understudied weaker dust regions
in South America and southern Africa are covered less.
Many of the dust emission regions mentioned above, particularly the single
most important one, the Sahara, have a sparse observational network, as population
density tends to be low in source areas. Recent fieldwork on different continents (see
list in Chap. 1 ) , together with significant advances in dust remote sensing (Chap. 7 )
and dust modelling (Chaps. 9 and 10 ) , has improved our understanding of the role
of the multi-scale meteorological processes involved in dust emission and transport
as well as their relative importance. Given the highly non-linear dependence of
emitted dust mass on peak wind speeds, a better understanding of the meteorological
processes involved is not only key to interpreting observations of dust, but crucially
also to successful modelling of the global dust budget and its climatic impact (see
Chap. 13 ) , as many of the processes described in the following occur at scales below
that of a typical climate or Earth system model.