Geoscience Reference
In-Depth Information
Dust emission fluxes are reduced when the soil is wet, as the soil water increases
the cohesive forces between soil particles. However, in the field the strong winds
required for dust emission increase evaporation and typically cause rapid drying
of the thin soil top layer. It has been observed that dust emission can start shortly
after a precipitation event (Gillette 1999 ), when the uppermost part of the soil has
dried up, even though the lower layers of the soil may still be wet. Fécan et al. ( 1999 )
derived an empirical relationship for the dependency of u *tr for dust emission on soil
moisture from wind tunnel experiments, which is used in many regional and global
dust emission models to describe the influence of soil moisture on dust emission
fluxes (see Chap. 5 ) .
9.4
Role of Meteorology in Dust Emission and Transport
A prerequisite for realistic modelling of dust emissions is the correct simulation of
the meteorological processes that initiate dust emission and mix dust aerosol within
the atmosphere. The activation of dust sources and prediction of dust emission fluxes
resulting from model simulations can only be realistic with accurate representation
of the meteorology in dust source regions.
Strong surface winds are required to initiate and sustain dust emission from soils.
As described in detail in Chap. 6 , typical meteorological phenomena that may cause
dust emission are: (1) large-scale monsoon-type flows, (2) synoptic-scale systems,
(3) gust fronts from moist convective storms and (4) dry convection events like
dust devils and dust plumes. Models at sufficient spatial resolution can well resolve
the synoptic-scale meteorological events such as cyclones and frontal passages and
associated high surface winds leading to dust emissions.
Dry convective events like dust devils are mainly of local importance (Balme
et al. 2003 ). So far the dust emissions from such events are not taken into account
in dust emission models. Thunderstorms forming in unstable conditions may cause
vigorous surface winds causing dust emission, but these are generally difficult to
reproduce (e.g. Marsham et al. 2011 ). A case study of an observed dust mobilization
event in southern Morocco caused by cold outflow from convection originating
over the Atlas Mountains (Knippertz et al. 2007 ) shows the dependence of dust
mobilization on model parameterization of convection (Fig. 9.3 ). Evidently the
model using parameterized convection cannot reproduce the dust emission event,
while the higher-resolution run simulates much more realistic emission fields. The
model resolution that can be used for computing dust emissions is usually limited
by the available computational resources and is typically much coarser than the
example shown in Fig. 9.3 .
Another important meteorological mechanism for dust emission is the morning
breakdown of low-level jets that frequently form above the stable nocturnal
boundary layer in arid areas (see Chap. 6 ) . A prominent example for a strong
low-level jet influence on dust emission is the Bodélé Depression in Chad, where
the jet formation is locally enhanced through orographic channelling (Washington
Search WWH ::




Custom Search