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?

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

Diurnal evolution of the convective and stable boundary layers in response to surface heating (sunlight) and cooling.

surface →

layer

mixed →

layer

free →

troposphere

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 = = ρu*z

where ρ is air density, u* is the friction velocity (= - <u΄w΄>) and z is height above the ground. Integrating this yields

where z0 is the roughness length.

Logarithmic surface-layer profile

zU

u

/

2*

0

* lnz

z

k

uU

Wind profile in stable, neutral and unstable air.

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 country0.03 Level country with low vegetation and isolated obstacles with separations of at least 50 obstacle heights0.10 Cultivated area with regular cover of low crops; moderately open country with occasional obstacles with separations of at least 20 obstacle heights0.25 Recently developed “young” landscape with high crops or crops of varying height and scattered obstacles at relative distances of about 15 obstacle heights0.50 Old cultivated landscape with many rather large obstacle groups separated by open spaces of about 10 obstacle heights; low large vegetation with with small interstices1.0 Landscape totally and regularly covered with similar sized obstacles with interstices comparable to the obstacle heights; e.g., homogeneous cities

(

/

(AND STABLE)

MO Surface-layer formulations:

Φm = (kz/u*)(U/z) - wind shear

Φh = (kz/T*)(θ/z) - thermal stratification

Φw = σw/u* - fluctuations in vertical velocity

Φθ = σθ/|T*| - fluctuations in temperature

Φε = kzε/u*3 - turbulence energy dissipation

Normalized mixed-layerspectra for the 3 velocitycomponents. 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.

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).

Diurnal Evolution and Clouds

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

• Complex terrain complicates structure

Suggestions for Further ReadingMain 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.