Atmospheric Boundary Layer and Turbulence

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Atmospheric Boundary Layer and Turbulence

• Zong-Liang Yang
• liang_at_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 Earths 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 12 km
over wetter surfaces.
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Why Study ABL?
Humans live in the ABL.
Fluxes are mediated here. 50 of the atmospheres
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. Earths 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

• Increase in mean (average) speed with height

• Turbulence (gustiness) at each height level

• Broad range of frequencies in the fluctuations

• 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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Earths 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)
t ? CDM Ur2 SH cp ? CDH Ur Ts
Ta(zr) LE L ? CDE Ur qs qa(zr) CDN ?
/ 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
http//www.cdc.noaa.gov/
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Global Distribution of Latent Heat Flux
http//www.cdc.noaa.gov/
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Regional Patterns of The Surface Energy Balance
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.
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.
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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_at_jsg.utexas.edu
• http//www.geo.utexas.edu/climate