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| Titel |
The Low-Level Jet Phenomenon in North-African Deserts: An Idealized Large Eddy Simulation Study |
| VerfasserIn |
B. Heinold, P. Knippertz, S. Cowie, S. Fiedler, K. Schepanski |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250068160
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| Zusammenfassung |
| Wind-blown dust emitted from desert regions is a major contribution to the global
aerosol load. It influences the climate system by changing the atmospheric radiation
budget through direct and indirect effects. Dust aerosol also plays an important
role in the biogeochemical and hydrological cycle and can affect human health.
Quantitative estimates of the spatial-temporal distribution of mineral dust and its manifold
effects largely base on model simulations. However, due to the highly nonlinear
dependence on peak winds, dust emissions are often underestimated by current dust
models.
Various studies have suggested that the breakdown of nocturnal low-level jets (LLJs),
which generates peak surface winds usually from morning to midday, is an important
meteorological driver of desert dust emissions. LLJs are distinctive maxima in the wind
profile of the lowest ~1.5km of the atmosphere. They primarily form at night as a
result of frictional decoupling of air layers above nocturnal inversions. The wind
maximum can be explained by an inertial oscillation due to the perturbation of the
geostrophic-antitriptic balance. In summer, the large pressure gradient related to the
Saharan heat low and strong night-time radiation temperature inversions provide
ideal conditions for LLJ formation over the Sahara desert. Global and regional dust
models generally match the synoptic-scale dynamics well, but the typical peak in
surface wind speeds caused by the LLJ erosion is often not reproduced, as turbulence
parameterization and vertical resolution may be insufficient to describe the small-scale
processes.
This study is a contribution to the “Desert Storms” project funded by the European
Research Council. The project aims at improving the representation of dust-generating
meteorological processes in numerical dust models. We present idealized model simulations
of LLJs using the Large Eddy Model (LEM) of the UK Met Office. The model is initialized
with either idealised diurnal cycles of sensible heat flux or observed surface temperatures, as
well as characteristic profiles of potential temperature and wind speed from analysis data.
Sensitivity studies are performed to investigate the influence of surface roughness,
latitude/Coriolis force and baroclinicity on the formation and decay of LLJs. The model
results are used to identify optimal latitude/roughness configurations for maximum LLJ
enhancement for a given wind profile. Ideal conditions are found in regions between
10Ë N and 25Ë N with roughness lengths > 0.01m providing a combination of
long oscillation periods and strong ageostrophic wind components. Typical LLJ
enhancements range from about 3 to 6m/s, when a geostrophic wind speed of 10m/s is
assumed. Maximum values are reached in the Sahel region. The results ultimately serve
to develop a LLJ parameterization for usage in global and regional dust models,
which allows assessing the importance of the LLJ breakdown for dust emissions. |
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