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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?

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observations and models of boundary layer processes over complex terrain
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?
effects of irregular terrain on pbl structure
Effects of irregular terrain on PBL structure
  • Flow over hills (horizontal scale a few km; vertical scale a few 10’s of m up to a fraction of PBL depth)
  • Flow over heterogeneous surfaces (small-scale variability with discontinuous changes in surface properties)
flow over a hill neutral stability
Flow over a hill (neutral stability)
  • Idealized profile (Witch of Agnesi profile):

(After Maria Agnesi; Milano, Italy, 1748)

regions of flow over hills
Regions of Flow Over Hills
  • Inner layer – region where turbulent stresses affect changes in mean flow. Hunt et al. (1988) obtain the relation for ℓ:
  • Outer layer – height at which shear in upwind profile ceases to be important:
  • For h = 10 m, Lh = 200 m and z0 = 0.02 m, ℓ = 10 m and hm = 66 m
effects of horizontal heterogeneity in surface properties
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
scale of changes in pbl downwind of discontinuity
Scale of changes in PBL downwind of discontinuity
  • Confined to surface layer (10 to 50 m)
  • Entire PBL (10 to 100 km)
  • Mesoscale (geostrophic adjustment; > 100 km)
changes in surface roughness
Changes in surface roughness
  • Characterized by change in roughness length –
  • , where upwind

roughness length and downwind roughness length

surface layer internal boundary layer
Surface-layer internal boundary layer

We define internal BL by (subscript θ for temperature and c for other scalars). The simplest formulations for are of the form

(analogous to BL growth on a smooth flat plate in wind tunnel experiments.)

,

surface layer internal boundary layer1
Surface-layer internal boundary layer

A more sophisticated approach is to assume vertical diffusion then,

With at

With this gives reasonable agreement

With observations. (Works best from smooth to

rough).

slide19

z02=1

z02= 0.1

z02=0.01

z02=0.001

the surface energy budget
The Surface Energy Budget

The thermal energy balance at the bottom of the surface layer

is conventionally written as

Rn = H + λeE + Gs ,

where Rn is the net radiation: short- and long-wave incoming

minus outgoing, H is the sensible heat flux, λeE is the latent

heat flux, and Gs is the heat flux going into storage in the soil

or vegetation.

slide26

(a)

Rn

λeE

  • Surface energy budget terms
  • for clear skies over a moist, bare
  • soil in the summer at mid-lati-
  • tudes. (b) Temperatures at the
  • surface, at 1.2 m height in the air,
  • and at 0.2 m depth in the soil
  • (from Oke, 1987 after Novak and
  • Black, 1985).

Gs

H

slide31

Diurnal variation of direct-beam solar radiation

On surfaces with different angles of slope and

aspect ratio at 40 ° N latitude for:

(a) the equinoxes (21 March and 21 September)

(b) summer solstice (22 June)

(c) winter solstice (22 December)

(Oke, 1987)

slide33

Total daily direct-beam solar

Radiation incident upon

Slopes of differing angle and

Aspect ratio at 45 ° N at the

times of the equinoxes

(21 March and 21 September).

Oke, 1987

slide34

Time sequence of valley inversion destruction along with potential temperature

profile at valley center (left) and cross-section of inversion layer and motions

(right).

(a) nocturnal valley inversion (b) start of sfc. warming after sunrise

(c) shrinking stable core & start of slope (d) end of inversion 3-5 hrs. after

breezes sunrise (Oke, 1987, based on

Whiteman, 1982)

slide37

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/u*2is the normalized turbulence kinetic energy

(From de Franceschi, 2002).

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.. 30:507-538.

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.

Suggestions for Further Reading