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| Titel |
Wind Shear Effects on the Structure and Dynamics of the Daytime Atmospheric Boundary Layer |
| VerfasserIn |
Armin Haghshenas, Juan Pedro Mellado |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250148580
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| Publikation (Nr.) |
 EGU/EGU2017-12849.pdf |
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| Zusammenfassung |
| The daytime atmospheric boundary layer (ABL), in which the positive buoyancy flux at the
surface creates convective instability and generates turbulence, has been a subject of
extensive research during the last century. However, fewer studies have considered wind
shear in detail and most of them are single-case studies. So most of the available
theories and parameterizations have not been sufficiently tested over a wide range of
atmospheric conditions. Moreover, since previous numerical studies were mostly
carried out by large eddy simulation, a complete understanding of the physics of
the problem is still missing due to the lack of information about the small-scale
dynamics. Specifically, despite the consensus in the community that wind shear
enhances the entrainment process, the amount of enhancement is still matter of
contention.
In order to investigate the effects of wind shear on the structure and dynamics of the ABL
in detail, direct numerical simulations are used in this study. Shear is prescribed by a
height-constant velocity in the troposphere and the simulation runs until a fully
turbulent, quasi-equilibrium regime is observed. Despite the simplification of neglecting
the Coriolis force, our configuration reproduces the main features observed in the
previous studies, which had taken the Coriolis force into account. As a novelty
compared to previous single-case studies, we introduce a dimensionless parameter
that allows us to study systematically any combination of surface buoyancy flux,
buoyancy stratification, and wind shear; We refer to this dimensionless number as shear
number. Seven simulations with shear numbers ranging from 0 (no wind) to 20
(moderate wind) are conducted; this range of shear numbers corresponds to wind
strength from 0 to 15 m/s in the free troposphere for typical midday atmospheric
conditions.
In general, we find that shear effects are negligibly small when the shear number is
below 10, and for larger values the effects remain constrained inside the entrainment
zone and surface layer. This critical shear number is justified by scrutinizing the
turbulence regimes (convective and mechanical) within the entrainment zone in the
sense that, for this shear number, the turbulence transport of turbulence kinetic
energy inside the entrainment zone equals the shear-production rate. Following this
analysis a critical flux Richardson number of 0.6 inside the entrainment zone is
found. In particular, we observe the following: First, the mean buoyancy and total
buoyancy flux inside the mixed layer remain invariant under a change of shear number
and they follow the free-convection scaling laws. Second, the height of minimum
buoyancy flux increases due to shear effects, but just moderately (less than 5%).
Nevertheless, this increment represents a growth of entrainment zone’s thickness by 50%
for shear numbers of the order of 20. Third, we observe that for shear numbers
larger than 10, the entrainment flux ratio grows by up to 50% in an early state of
ABL development. We provide explicit parameterizations of all these shear effects. |
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