Numerical Dust Models - Mineral Dust: A Key Player in the Earth System - page 199

9.1 Introduction

Dust aerosol particles influence the Earth's radiation balance, atmospheric dynam-
ics, biogeochemical processes and atmospheric chemistry (see Chaps. 4 , 11 , 12 , 13
and 14 ) . These effects are only partly understood and remain largely unquantified
to date. In turn, emission, transport and deposition of dust are influenced by
meteorological and climate parameters, such as surface winds and the vegetation
cover of soils (see Chaps. 5 , 6 , 7 and 8 ) .
An assessment of the various effects and interactions of dust and climate requires
a quantification of global atmospheric dust loads and their optical and microphysical
properties (see Chaps. 11 and 12 ) . Dust distributions that are used in assessments
of dust effects on climate usually rely on results of large-scale numerical models
that include dust as a tracer (e.g. Shao et al. 2011 ). One option for performing dust
simulations are the so-called offline models, in which dust emission, transport and
deposition processes are computed using meteorological fields from reanalyses or
climate model simulations as meteorological drivers. Those fields are read into the
model at regular time intervals (e.g. Mahowald et al. 1999 ). This approach allows
multiple model studies of dust processes with identical meteorology, but the need of
large data storage can be problematic. Also, this type of model setup does not allow
taking into account feedbacks of dust radiative forcing on atmospheric dynamics
and climate processes.
Alternatively, dust processes can be computed online by implementing dust as
a tracer in regional or global models, that is, dust processes are computed utilizing
the simulated wind fields, mixing processes and precipitation. To understand the
interactions between dust and climate, dust needs to be described as a radiatively
active tracer in general circulation models (GCMs) (Perlwitz et al. 2001 ; Woodward
2001 ; Stier et al. 2005 ; Miller et al. 2006 ; Mahowald et al. 2006 ; Colarco et al. 2010 ;
see also Chap. 13 , this topic). In such models the changes of the radiative heating
of dust-containing air layers and the cooling of the underlying surface are computed
online, taking into account relevant model fields such as surface albedo and cloud
cover.
In an early study, Coakley and Cess ( 1985 ) estimated the effect of dust aerosol on
temperature and precipitation within the National Center for Atmospheric Research
(NCAR) Atmospheric General Circulation Model (AGCM) through perturbing the
incoming solar radiation above desert regions by a climatologically fixed amount
under perpetual July conditions. Models of the three-dimensional dust transport
first focused on simulating individual dust events (Westphal et al. 1988 ). Joussaume
( 1990 ) developed the first model of dust emission and transport at a global scale
by implementing dust as a tracer in the Laboratoire de Météorologie Dynamique
(LMD) AGCM. Dust emissions were prescribed in relative units as a function of
wind speed in areas with low soil moisture for perpetual February and August
conditions. Tegen and Fung ( 1994 , 1995 ) included dust as a tracer in the offline
Goddard Institute for Space Studies (GISS) tracer transport model, using the
European Centre for Medium-Range Weather Forecasts (ECMWF) assimilated