Download Linking the scales of process, observation and modeling of dust

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Linking the scales of process, observation and modeling of dust
emissions: Workshop in Boulder, Colorado, 21-22 October 2004.
K. E. Kohfeld,1 R. L. Reynolds,2 J. Pelletier,3 B. Nickling,4 and Workshop Participants
1School
of Earth and Environmental Sciences, Queens College of the City University of New York, 65-30
Kissena Blvd, Flushing, NY 11367, USA ([email protected])
2U.S.
Geological Survey; MS 980, Box 25046, DFC; Denver, CO 88025, USA ([email protected])
3Department
of Geosciences, University of Arizona, 1040 E. Fourth St., Tucson, AZ 85721, USA
([email protected])
4Department
of Geography,
([email protected])
University
of
Guelph,
Guelph,
Ontario,
Canada
N1G
2W1
Each year approximately four billion tons of dust is mobilized from dry landscapes, and
remains in the atmosphere from hours to weeks before descending on surfaces such as
ice, forests, and oceans. This enormous amount of atmospheric dust affects the earth in
numerous ways. Radiative properties of airborne dust affect atmospheric dynamics and
global climate [IPCC, 2001]. Emerging medical evidence suggests that dust can have
adverse effects on human health [Plumlee and Ziegler, 2003]. [Kinney and Lippmann,
2000; Mar et al., 2004; Shinn et al., 2003]Dust storms create transportation hazards, and
wind erosion causes loss of soil fertility. Furthermore, dust influences ecosystem
dynamics in the ocean where it fertilizes phytoplankton, and in soil where it influences
hydrology and enhances nutrient status.
Understanding controls on dust emissions is the key to quantifying atmospheric dust
feedbacks in the climate system, as well as assessing its effect on health, agriculture, and
ecosystems. Despite recent progress in elucidating some climatic and transport aspects of
the dust cycle (see supplementary Figure 1), the dominant controls on dust emission at
regional and global scales, and how these controls could change, remain poorly
quantified. Certain key questions remain:

What land-surface and climate conditions give rise to dust emission, and what
geomorphic processes influence the capacities of these settings to generate dust?

What portion of the emissions is attributable to anthropogenic disturbances,
whether direct effects (e.g., land-use practices), direct consequences of climate
change (e.g., climate-induced changes in dust emissions), or indirect
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consequences of climate change (e.g., shifts in land-use practices resulting from
climate change)? [Zender et al., 2004]

How can we forecast changes in the first two factors to assess future dust
emissions, on the scale of seasons to a few decades?
Dust processes span more than twelve orders of magnitude in spatial and temporal scales,
from individual grain-to-grain collisions that take place in fractions of a second, to global
transport of dust over glacial-interglacial cycles (Figure 1a). The methods used to study
dust-cycle processes must then account for this range of scales (Figure 1b).
Traditionally, studies have explored dust emissions, transport, and deposition using either
“top-down” (global modeling) or “bottom-up” (small-scale process) approaches, resulting
in focus on different processes. Global or regional models consider atmospheric
processes (e.g., synoptic patterns and convective regimes) that trigger dust storms and
control dust transport, but lack the ability to predict how sources will change as a result
of small-scale land-surface variability. Local-scale studies focus on relations among
particle movement, wind stress, surface roughness, wind stress, and dust mobilization,
but they are rarely applied to regional-scale or continental-scale distributions of dust.
Moreover, information from geologic-geomorphic investigations of dust sources, along
with detailed mapping of sources from remote platforms, have not yet been folded
comprehensively into regional or global models of dust emission. The integration of
these approaches offers a ripe opportunity to quantify dust-emission potential (winderosion vulnerability) at spatial scales suitable for linking to mesoscale and global climate
models.
A workshop held recently (21-22 October, 2004) in Boulder, Colorado, brought together
dust researchers studying different aspects of the dust cycle with the aims of (a)
developing a dialogue among the dust-modeling, field, and experimental research
communities and (b) outlining strategies to link the approaches that examine dustemission at different spatial and temporal scales. Participants identified the following
critical topics for future research.

Soil-texture characteristics and sediment availability are driving forces for
understanding dust emissions. Surface texture (particle-size distribution) is critical
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for accurate simulation of dust emissions, and remains to be adequately addressed in
most models. A global data set quantifying the proportions of clay, silt, and sand
available in surface soils remains to be developed. Several physical and biological
processes create sand- to silt-sized aggregates of finer particles, thereby changing
their emission potentials. Diverse opinions about which characteristics are most
important to dust emission emphasized that further field research and sensitivity
studies are required.

Vegetation (type, cover, and distribution) influences dust emissions.
Vegetation cover is a primary controlling factor; regions with >15% vegetation cover
emit minimal dust.
In regions of sparse cover, the pattern or distribution of
vegetation can result in vastly different dust emissions, depending on how vegetation
is clustered. Senescent vegetation may also play a role in stabilizing surfaces, For
example, shrubby vegetation may reduce dust emissions even when no leaves are
present. Although vegetation cover is regularly incorporated in dust emissions model
schemes, these latter effects remain to be adequately evaluated.

Crusts inhibit dust emissions. In sparsely vegetated regions, sedimentological,
biological, and salt/precipitation crusts can all inhibit dust emission. Deflation
requires that the energy of impacting grains overcome the binding strength of soil
crusts, and this strength can vary tremendously depending on the conditions under
which crusts form. Crusting processes remain to be incorporated into dust models,
and their importance to both regional and global dust budgets has not been quantified.

Moisture, both precipitation and groundwater, has a strong control on dust
emissions at a given site. Moisture often operates with temporal lags of different
scales, depending on the process that it influences. The effect of moisture upon dust
emissions appears to depend on the frequency and magnitude of precipitation, and the
effect on dust emission can be positive or negative.

Small-scale surface stresses and their interaction with surface topography
are crucial for understanding where dust is mobilized.
Correctly simulating
surface wind shear at smaller scales is perhaps the greatest challenge for reanalysiswinds datasets that are used in global models, and underscores the need for regional,
mesoscale climate models to bridge the gap between the two scales.
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Identification of dust emission “hot spots” from satellite imagery needs more
field work for “ground truth” verification. Despite the importance of the Sahara as
a dust source, the specific geomorphic settings (i.e., playa margins, fan surfaces, wadi
mouths) that are hot spots [Gillette, 1999] are poorly understood.
Workshop participants agreed that field experiments are needed to link small-scale
processes controlling surface emissions to remotely sensed characteristics, at scales
applicable to dust models. A successfully designed field experiment would serve as a
“proof of concept” that field programs are capable of integrating small-scale observations
with regional-to-global scales in dust research. Ideally, the experiment would later be
expanded, applied, and tested in other dust-producing regions around the world. Several
participants agreed to serve on a committee that will design an experiment combining
modeling, field studies, and monitoring.
While many regions would be outstanding locations for integrated field research, the
workshop identified the desert regions of western North America, with the Mojave Desert
as a focal point. The western North America is historically significant because landmanagement practices have dramatically altered its dust emissions. For instance, the
desiccation of Owens Lake, California created a large source of alkaline dust [Cahill et
al., 1996].
The Mojave Desert, the western Great Plains, and northern Mexico
collectively contain several types of dust-emission settings that have generated dust
storms.
Logistically, the Mojave Desert already has extensive documentation of
geological, ecological, and surface characteristics, anthropogenic activities, and climate
changes.
Thus, the proposed field study could take advantage of the monitoring,
instrumentation, and geologic knowledge of the Mojave Desert while expanding to
encompass a larger setting of dust emissions (see supplementary Figure 2).
The
collective capabilities in mesoscale modeling, remote sensing, and field geology will
enable us to examine a wide range of dust emission settings in western North America
and in other deserts of the world.
References
Cahill, T.A., Gill, T.E., Reid, J.S., Gearhart, E.A., and Gillette, D.A., Saltating particles,
playa crusts, and dust aerosols at Owens (dry) Lake, California. Earth Surface
Processes and Landforms, 21, 621-639, 1996.
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IPCC, Climate Change 2001: the Scientific Basis. Contribution of Working Group I to
the Third Assessment Report of the Intergovernmental Panel on Climate Change,
881 pp., Cambridge University Press, New York, 2001.
Gillette, D.A., A qualitative geophysical explanation for “hot spot” dust emitting source
regions, Contr. Atmos. Phys., 72 (1), 67-77, 1999.
Plumlee, G.S., and T.L. Ziegler, The medical geochemistry of dusts, soils, and other earth
materials, in Treatise on Geochemistry, edited by B.S. Lollar, pp. 264-310, 2003.
Zender, C.S., R.L. Miller, and I. Tegen, Quantifying Mineral Dust Mass
Budgets:Terminology, Constraints, and Current Estimates, Eos, 85 (48), 509–512,
2004.
Figure 1: Dust processes span more than twelve orders of magnitude in spatial and
temporal scales (a). The methods used to study dust-cycle processes must address these
ranges of scales (b).
Supplementary Materials
Supplementary Figure 1. Examples of Developments in Dust Research. (a) At
regional and global scales, there has been systematic recognition of paleolake basins as
major dust sources. Shown here is historical satellite image data for aerosol optical depth
(TOMS-AOD) which has a remarkably strong correlation with paleolake depressions
[Prospero et al., 2002]. Some of these areas, especially in the "dust belt" of North Africa,
comprise the most significant source regions in the world. (b) D. Westphal has achieved
great success in predicting dust concentrations in real time. These products are used by to
the U.S. military for operations planning and safety. (c) Obtaining accurate measurements
of controlling surface properties, including texture and surface roughness, at high spatial
resolution over wide areas remains a major challenge. However, Okin and Painter [2004]
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provided promising new results indicating that surface texture can be mapped using
MASTER imagery. This result is important because texture is the primary controlling
factor for threshold velocity. (d) Monitoring of saltation and its controlling
meteorological, hydrological, biological, and geologic factors in dust-emission settings of
the southwestern U.S. has highlighted the dynamic changes that occur and the role of
vegetation and water-table variations in dust production (near Soda Lake, Mojave Desert,
California; [Chavez et al., 2002].
Supplementary Figure 2. The Mojave Desert as a Study Region. Bridging the scales
of the dust cycle can be facilitated with focused studies in an area where the surface
characteristics are well known. The Mojave Desert region, for example, has a unique
wealth of readily available data for surficial geology (i.e., reflecting soil and pavement
development), vegetation, historical dust activity, and remote sensing. We propose a
community-wide effort, informal at first, to focus on the dust cycle in this region.
Supplementary References
Chavez, P., MacKinnon, D., Clow, G., Tigges, R., Urban, F., Fulton, R., Reheis, M.,
Miller, D., Bultman, M., Reynolds, R.L., 2002, Monitoring dust emission in the
southwest U.S.—interannual differences related to climatic variability: Geological
Society of America Abstracts with Program, v. 34, no. 6, p. 246.
Okin, G. S., and T. Painter, Effect of grain size on remotely sensed spectral reflectance of
sandy desert surfaces, Remote Sensing of Environment, 89, 272-280, 2004.
Prospero, J. M., P. Ginoux, O. Torres, S. E. Nicholson, and T. E. Gill, Environmental
characterization of global sources of atmospheric soil dust identified with the Nimbus 7
Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product, Reviews of
Geophysics, 40, Art. No. 1002, 2002.
Supplementary Figure 1.
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Supplementary Figure 2.