The Planetary Boundary Layer in Complex Terrain

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The Planetary Boundary Layer in Complex Terrain

• John Horel
• NOAA Cooperative Institute for Regional
  Prediction
• Department of Meteorology
• University of Utah
• jhorel_at_met.utah.edu

Photo J. Horel
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What is CIRP?

• CIRP NOAA Cooperative Institute for Regional
  Prediction at the University of Utah
• Mission Improve weather and climate prediction
  in regions of complex terrain
• People
• Staff John Horel, Jim Steenburgh, Mike Splitt,
  Judy Pechmann, Will Cheng, Bryan White, Brian
  Olsen
• Students Justin Cox, Jay Shafer, Ken Hart, Dave
  Myrick, Dan Zumpfe, Erik Crossman, Greg West

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References

•  
• Barry, R., 1992 Mountain Weather and Climate.
  Rutledge
• Blumen, W., 1990 Atmospheric Processes Over
  Complex Terrain. American Meteorological Society,
  Boston, MA.
• Clements, C., D. Whiteman, J. Horel, 2003 Cold
  pool evolution and dynamics in a mountain basin.
  J. Appl. Meteor., 42, 752-768.
• Garratt, J., 1992 The Atmospheric Boundary
  Layer. Cambridge
• Horel, J., M. Splitt, L. Dunn, J. Pechmann, B.
  White, C. Ciliberti, S. Lazarus, J. Slemmer, D.
  Zaff, J. Burks, 2002 MesoWest Cooperative
  Mesonets in the Western United States. Bull.
  Amer. Meteor. Soc., 83, 211-226.
• Kalnay, E., 2003 Atmospheric Modeling, Data
  Assimilation and Predictability. Cambridge
• Kossmann, M., and A. Sturman, 2003
  Pressure-driven channeling effects in bent
  valleys. J. Appl. Meteor., 42, 151-1158.
• Lazarus, S., C. Ciliberti, J. Horel, K. Brewster,
  2002 Near-real-time Applications of a Mesoscale
  Analysis System to Complex Terrain. Wea.
  Forecasting. 17, 971-1000.
• Stull, R. B., 1999 An Introduction to Boundary
  Layer Meteorology. Kluwer
• Whiteman, C. D., 2000 Mountain Meteorology.
  Oxford
• Zhong, S. and J. Fast, 2003 An evaluation of the
  MM5, RAWMS, and Meso-Eta Models at Subkilometer
  resolution using VTMX field campaign data in the
  Salt Lake Valley. Mon. Wea. Rev., 131, 1301-1322.
• Notes Summer School on Mountain Meteorology
  2003. http//www.unitn.it/convegni/ssmm.htm

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Outline

• Part I- Characteristics/impacts of complex
  terrain
• Part II- Resources for observing surface weather
• Part III- Basin boundary layer
• Part IV- Mountain-valley and lake breezes

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Field Programs

• CASES-99 Cooperative Atmosphere-Surface Exchange
  Study. Kansas. Poulos et al., 2002 BAMS, 83,
  555-581.
• MAP Mesocale Alpine Program. Alps. Bougeault et
  al., 2002, BAMS, 82, 433-462.
• VTMX Vertical Transport and Mixing Experiment.
  Salt Lake Valley. Doran et al. 2002, BAMS, 83,
  537-551.

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PBL Issues
www.pnnl.gov/vtmx

• VTMX Science Plan
• Measurement and modeling of vertical transport
  and mixing processes in the lowest few kilometers
  of the atmosphere are problems of fundamental
  importance for which a fully satisfactory
  treatment has yet to be achieved
• Although a general theoretical understanding of
  many of the physical phenomena relevant to
  vertical transport and mixing processes exists,
  that understanding is incomplete, the
  representation of various phenomena in models is
  often poor, and the data needed to test those
  models are lacking.
• The upward and downward movements of air parcels
  in stable and residual layers of the atmosphere
  and the interactions between adjacent layers are
  particularly difficult processes to characterize,
  and significant difficulties also exist in
  describing the behavior of the atmosphere during
  morning and evening transition periods.
• Complications due to heterogeneous land surfaces
  and complex terrain further compromise our
  ability to treat vertical transport and mixing
  processes properly.

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VTMX Science Questions

• What are the fundamental processes that control
  vertical transport for stable and transition
  boundary layers?
• How can momentum, heat, and moisture fluxes be
  modeled and predicted in a stratified atmosphere
  with multiple layers?
• What improvements in numerical simulations and
  forecasts of vertical transport and mixing during
  stable and transition periods are feasible and
  how can they be implemented?
• What formulations are most appropriate for the
  description of vertical diffusion in stable air?
  For example, how rapidly will an elevated layer
  of pollutants mix towards the ground in a stable
  pool trapped within a basin, and how can that
  mixing be modeled?
• What is the sensitivity of current local weather
  forecast and dispersion model predictions to
  variations in the treatment of vertical
  diffusivity and turbulence?
• What limits our ability to forecast vertical
  transport in current numerical prediction models?
• How do traveling weather systems remove stable
  stagnant air out of a basin, and under what
  conditions do these removal mechanisms fail?
• What is the nature of the interaction of
  terrain-induced flows (e.g., drainage winds at
  night, upslope winds during the day, and waves)
  with cold air pools in basins, and how do such
  flows affect the formation and erosion of those
  pools and the dispersion of pollutants in them?

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What are the effects of complex terrain?

• Substantial modification of synoptic or meso
  scale weather systems by dynamical and
  thermodynamical processes through a considerable
  depth of the atmosphere
• Recurrent generation of distinctive weather
  conditions, involving dynamically and thermally
  induced wind systems, cloudiness, and
  precipitation regimes
• Slope and aspect variations on scales of 10-100 m
  form mosaic of local climates
• (Barry 1992)

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Effects of Complex Terrain
Carruthers and Hunt 1990
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Billiard ball analogy

• If the earth were greatly reduced in size while
  maintaining its shape, it would be smoother than
  a billiard ball. (Earth radius 6371 km
  Everest 8.850 km)
• Nonetheless, mountains have a large effect on
  weather. Why is this, if they are so
  insignificant in size?
• Answer the atmosphere, like the mountains, is
  also shallow (scale height 8.5 km) so mountains
  are a significant fraction of atmos depth.
• But, this answer underestimates mountain effect
  for two reasons
• Stability gives the atmosphere a resistance to
  vertical displacements
• The lower atmosphere is rich in water vapor so
  that slight adiabatic ascent brings the air to
  saturation.
• Example flow around a 500-m mountain (ltlt 8.5 km)
  could include 1) broad horizontal excursions, 2)
  downslope windstorm on lee side, and 3)
  torrential orographic rain on windward side.

Smith (1979)
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Distribution of mountains on the globe (Barry
1992)
Total land surface is about 149 million km2.
Oceanic islands covering 2 million km2 are not
included in the listed areas. Plateau mountains
are both included in the tables 1st line.
Louis (1975)
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Energetic Considerations

• Since the atmosphere is heated mainly from the
  ground, cooling effect upon earths surface of
  latent and sensible heat fluxes is nearly double
  that of radiative fluxes
• Since much of the land surface is hilly,
  thermally driven circulations play important role
  in global energy balance

F. Fiedler. Summer School Trento
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Chen, C.-C., D. Durran and G. Hakim(2003) ICAM
Surface Wind and Vorticity Around Isolated
Mountain Interaction with Large-scale flow
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Potential Temp, Vertical Velocity, and Turbulent
Mixing
Chen, C.-C., D. Durran and G. Hakim (2003) ICAM
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Planetary boundary layer
1 km
Height (m)

• Energy and mass exchanges near ground
• ---interactions among soil science, hydrological
  cycles
• (ground and air), ecosystems, and atmosphere.
• Canopy
• Terrain
• Heterogeneous surfaces
• Clouds/fog
• Urban environment, air pollution

D. Lenschow
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Shallow Drainage Flows Mahrt, Vickers,
Nakamura, Soler, Sun, Burns, Lenschow BLM,
101, 2001.
Schematic cross-section of prevailing southerly
synoptic flow, northerly surface flow down The
gully, and easterly flow likely drainage flow
from Flint Hills. Numbers identify the Sonic
anemometers on the E-W transect. E is to the
right and N into the paper.
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Pollutant Transport in Valleys
Nighttime Stable Layer in Valley
After Breakup of Nighttime Stable Layer in Valley
Savov et al. (2002 JAM)
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Daytime vertical mixing processes
Jerome Fast
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Diurnal mountain wind systems
Whiteman (2000)
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Mountain-plain circulation, Rocky Mountains
US radar profiler network, 1991-1995, Jun-Aug,
500 m gate, max3.5 m/s
Whiteman and Bian (1998)
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Alpine pumping
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Mountain-plain circulation in Alps (Vertikator)
Emissions within the area of Alpine Pumping are
transported into the Alps and mixed convectively
to higher levels
Boundary of Alpine pumping synoptic conditions
modify shape
Munich
100 km
Zürich
Graz
Innsbruck
Milan
Lyon
Turin
Lugauer et al. (2003)
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Mountain venting, anti-slope flow
25 July 2001
CBL Height from Lidar
Reuten et al.( 2002) with Steyn
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Valley cross sections
temperature and wind structure layers at a time
midway through the transition
Whiteman (2000)
Whiteman (1980)
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Channeling of synoptic/mesoscale winds
Forced Channeling
Whiteman (2000)
Pressure Driven Channeling
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Bent valley with 45 changes in wind direction
above valley
Kossmann Sturman (2003)
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Dynamic Channeling
Kossman and Sturman 2003
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Western U.S. Terrain(high- darklow-light)
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High terrain (dark) Flat (tan)Mtn. Valleys
(light)A. Reinecke
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Normalized surface-layer velocity standard
deviations for near neutral conditions in the
Adige Valley in the northern Italy alpine region.
a is from Panofsky and Dutton, 1984 b the
average values from MAP e/u2 is the normalized
turbulence kinetic energy (From de Franceschi,
2002).
D. Lenschow
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West DEM Grid Points vs. MesoWest Stations
Green-West Blue-MesoWest
of Total
Valley Flat
Mountain
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Adding Physiographic Information to MesoWest
Land Data Assimilation Systems (LDAS) UMD
Vegetation Types
Exposure? Forested? Nearby Water?
Mountain/Valley? Urban? Slope? Aspect?
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MesoWest land characterization
Sites located disproportionately in urban areas
and near water resources.
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Diurnal Temperature Range
A. Reinecke
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Diurnal fair weather evolution of bl over a plain
Whiteman (2000)
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free ? troposphere
mixed ? layer
surface ? layer
D. Lenschow
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D. Lenschow
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D. Lenschow
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D. Lenschow
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Diurnal evolution of the convective and stable
boundary layers in response to surface heating
(sunlight) and cooling.
D. Lenschow
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Atmospheric structure evolution in valley terrain
Whiteman (2000)
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Roughness Effects

• For well-mixed conditions (near neutral lapse
  rate)
• U2 u1 ln (z2/zo)/ln(z1/z0)
• Roughness length zo.5 h A/S where h height of
  obstacle, A- silhouette area, S surface area A/Slt
  .1
• Zo- height where wind approaches 0

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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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Effects of irregular terrain on PBL structure

• Flow over hills (horizontal scale a few km
  vertical scale a few 10s of m up to a fraction
  of PBL depth)
• Flow over heterogeneous surfaces (small-scale
  variability with discontinuous changes in surface
  properties)
• Inner layer region where turbulent stresses
  affect changes in mean flow
• Outer layer height at which shear in upwind
  profile ceases to be important

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(Kaimal Finnigan, 1994).
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(Kaimal Finnigan, 1994).
47
(Kaimal Finnigan, 1994).
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D. Lenschow
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Effects of horizontal heterogeneity in surface
properties

• Changes in surface roughness
• Rough to smooth
• Smooth to rough
• Changes in surface energy fluxes
• Sensible heat flux
• Latent heat flux
• Changes in incoming solar radiation
• Cloudiness
• Slope

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Summary- Impacts of Complex Terrain

• Terrain affects atmospheric circulation on local
  to planetary scales
• Terrain induced eddies modify and contribute to
  the vertical and horizontal exchange of mass,
  temperature, and moisture in a much stronger
  manner than turbulent eddies over flat terrain

Photo J. Horel
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Problems and possible future directions

• Most theoretical, modeling and observational
  results are applicable to a horizontally
  homogeneous PBL and underlying surface.
• Non-uniform surfaces predominate over land.
• New tools are needed and are becoming available
  to address PBL structure over heterogeneous
  terrain.

D. Lenschow