Observations and Models of BoundaryLayer Processes Over Complex Terrain

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Observations and Models of Boundary-Layer
Processes Over Complex Terrain

• What is the planetary boundary layer (PBL)?
• What are the effects of irregular terrain on the
  basic PBL structure?
• How do we observe the PBL over complex terrain?
• What do models tell us?
• What is our current understanding of the PBL and
  what are the outstanding problems to be addressed?

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What is the Planetary Boundary Layer?

• The PBL is defined by the presence of turbulent
  mixing that couples the air to the underlying
  surface on a time scale of less than a few hours

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Diurnal evolution of the convective and stable
boundary layers in response to surface heating
(sunlight) and cooling.
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free ? troposphere
mixed ? layer
surface ? layer
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Logarithmic surface-layer profile
Dimensional arguments for turbulent exchange in
the surface (or constant flux) layer ( 0.1 zi)
lead us to an eddy diffusivity, or turbulent
exchange coefficient for momentum,
Km ?uz where ? is air
density, u is the friction velocity ( - ltu?w?gt)
and z is height above the ground. Integrating
this yields where z0 is the
roughness length.
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Wind profile in stable, neutral and unstable air.
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Roughness lengths zo for different natural
surfaces (from M. de Franceschi, 2002, derived
from Wieringa, 1993).
zo (m)
Landscape Description ____________________________
____________________________________ 0.0002
Open sea or lake, tidal flat, snow-covered plain,
featureless desert, tarmac,
concrete with a fetch of several km. 0.005
Featureless land surface without any noticeable
obstacles snow covered or
fallow open country 0.03 Level country
with low vegetation and isolated obstacles with
separations of at least 50
obstacle heights 0.10 Cultivated area
with regular cover of low crops moderately open
country with occasional
obstacles with separations of at least 20
obstacle heights 0.25
Recently developed young landscape with high
crops or crops of varying
height and scattered obstacles at relative
distances of about 15 obstacle
heights 0.50 Old cultivated landscape
with many rather large obstacle groups
separated by open spaces of about 10
obstacle heights low large
vegetation with with small interstices 1.0
Landscape totally and regularly covered with
similar sized obstacles with
interstices comparable to the obstacle heights
e.g., homogeneous cities
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(AND STABLE)
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MO Surface-layer formulations

• Fm (kz/u)(?U/?z) - wind shear
• Fh (kz/T)(??/?z) - thermal
  stratification
• Fw sw/u -
  fluctuations in vertical velocity
• F? s?/T - fluctuations in
  temperature
• Fe kze/u3 - turbulence
  energy dissipation

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Normalized mixed-layer spectra for the 3
velocity components. The two curves define the
envelopes of spectra that fall within the z/zi
range indicated. The dashed blue lines
indicate contri- butions to the u and v spectra
due to mesoscale variability, both from
synoptic systems and from surface
hetero- geneity.
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Schematic of energy spectrum within a plant
canopy showing energy from shear production
feeding energy into the spectrum at the integral
scale (?) and energy from wake and waving
production feeding into smaller scales in the
inertial subrange (Kaimal Finnigan, 1994).
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Idealized stable boundary-layer flow regimes as a
function of height and stability. The vertical
dashed line indicates the value of z L
corresponding to the maximum downward heat flux
(Mahrt, BLM, 1999).
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Diurnal Evolution and Clouds

• Daily cycle of solar heating/radiative cooling
  has major impacts on PBL structure
• Complex terrain complicates structure

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Suggestions for Further Reading
Main Reference Sources for these
Lectures Belcher, S.E. and J.C.R. Hunt, 1998
Turbulent flow over hills and waves. Annu. Rev.
Fluid Mech.. 30507-538. Blumen, W., 1990
Atmospheric Processes Over Complex Terrain.
American Meteorological Society, Boston,
MA. Geiger, R., R.H. Aron and P. Todhunter, 1961
The Climate Near the Ground. Vieweg Son,
Braunschweig. Kaimal, J.C. and J.J. Finnigan,
1994 Atmospheric Boundary Layer Flows. Oxford
Univ. Press, New York. Oke, T.R., 1987 Boundary
Layer Climates. Routledge, New York. Venkatram,
A. and J.C. Wyngaard, Eds.,1988 Lectures on Air
Pollution Modeling. American Meteorological
Society, Boston MA. Abstracts from the10th
Conference on Mountain Meteorology, 17-21 June
2002, Park City, UT, American Meteorological
Society, Boston.