Atmospheric Boundary Layer and
Turbulence
Zong-Liang Yang
liang@jsg.utexas.edu
http://www.geo.utexas.edu/climate
Department of Geological Sciences
Jackson School of Geosciences
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Outline
• Definition and Basic Properties
• Why Study ABL?
• Structure of ABL
• Features of Wind Speed Variation
• Surface Energy Balance
• Summary
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ABL: Definition and Basic Properties
The layer of air near the Earth’s surface, also called the Planetary Boundary Layer. It is that portion of
the lower troposphere that feels the effects of the underlying surface within about 30 minutes or less. The
surface influences ABL by friction and by heat fluxes at the surface.
This layer is turbulent and is well mixed. Turbulence is generated by wind shear (wind is approximately
geostrophic at the top of the ABL but zero at the surface). Temperature gradients can either generate or
suppress turbulence.
Temperatures vary diurnally, unlike the free atmosphere above. Its height evolves with time over the
course of a day. Boundary layer clouds: fair-weather cumulus, stratocumulus, fog.
Maximum height: usually ~1 km, ~3 km over deserts, dry fields and boreal forests; 1–2 km over wetter
surfaces.
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Why Study ABL?
Humans live in the ABL.
Fluxes are mediated here. 50% of the atmosphere’s kinetic energy is dissipated in
the boundary layer.
It is the location of the source and sink of many trace gases (including water
vapor, CO2, ozone, methane) and dusts/pollutants.
It is a reservoir of trace gases and pollutants.
It is important for local forecasting. There is a strong effect on the rest of the
atmosphere. Boundary-layer clouds are very important for climate.
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Structure of ABL (1/2)
During a clear day, it consists of a roughness sublayer (air flows around individual roughness elements
– grass, plants, trees, or buildings), a surface boundary layer, a well-mixed layer and a capping
entrainment layer.
The ABL is capped by a temperature
inversion, which inhibits mixing and
confines pollution below it.
Potential temperature and other
quantities are constant with altitude.
Earth’s rotation becomes important,
and the wind direction veers with
height.
Formerly known the constant flux layer,
~ 100 m thick or 10% of the ABL
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Structure of ABL (2/2)
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Features of the Wind Speed Variation
153 metres
64 metres
• Increase in mean (average)
12 metres
speed with height
Wind speed (m/s)
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• Turbulence (gustiness) at each
25
20
height level
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10
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• Broad range of frequencies in
0
the fluctuations
0
1
2
3
Time (minutes)
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• Similarity in gust patterns at
lower frequencies
Wind speeds from 3
different levels recorded
from a synoptic gale
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Surface Energy Balance
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Earth’s Global Energy Budget
80% of net radiation at the surface is used for evaporation!
Trenberth et al. (2009)
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Air Flow and Turbulent Vortices
Air flow can be imagined as a horizontal flow of numerous rotating
eddies, a turbulent vortices of various sizes, with each eddy having
3D components, including vertical components as well. The
situation looks chaotic, but vertical movement of the components
can be measured from the tower.
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Determine Vertical Fluxes
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Surface Energy Balance
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Surface Energy Balance
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Surface Energy Balance
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Surface Energy Balance
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Surface Energy Balance
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Surface Energy Balance
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Surface Energy Balance
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Surface Energy Balance
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Surface Energy Balance
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Surface Energy Balance
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Surface Energy Balance
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Supplementary Materials
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Reynolds Decomposition and Eddy Covariance
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Reynolds Decomposition and Eddy Covariance
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Bulk Aerodynamic Formulas (Parameterizations)
τ
= ρ CDM Ur2
SH = cp ρ CDH Ur [Ts – Ta(zr)]
LE = L ρ CDE Ur [qs – qa(zr)]
CDN = [κ / ln(zr/z0)]2
CDM = CDN,M fM(RiB)
CDH = CDN,H fH(RiB)
CDE = CDN,E fE(RiB)
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Global Distribution of Sensible Heat Flux
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http://www.cdc.noaa.gov/
Global Distribution of Latent Heat Flux
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http://www.cdc.noaa.gov/
Regional Patterns of The Surface Energy Balance
West Palm Beach, Fl energy balance
(ly/day) West Palm Beach, Florida is
located in a warm and moist climate. Latent
energy transfer into the air is greatest during
the summer time which is the wettest period
of the year, and when net radiation is the
highest. During the summer, sensible heat
transfer decreases as net radiation is
allocated to evaporation and latent heat
transfer.
Yuma, AZ energy balance (ly/day)
At the other extreme is Yuma, Arizona, a
warm and dry climate. The most noticeable
characteristic of this place is the lack of
latent heat transfer. Though ample radiation
is available here, there is no water to
evaporate. Nearly all net radiation is used for
sensible heat transfer which explains the hot
dry conditions at Yuma.
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Lawrence et al., 2011
500+
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Summary
Additional Major References
“The ABL” by Roland Stull
Prof. Zong-Liang Yang
+1-512-471-3824
liang@jsg.utexas.edu
http://www.geo.utexas.edu/climate
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