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 z i ) lead us to an eddy diffusivity, or turbulent exchange coefficient for momentum, K m = = ρu * z where ρ is air density, u * is the friction velocity (= - ) and z is height above the ground. Integrating this yields where z 0 is the roughness length. Logarithmic surface-layer profile

Roughness lengths z o for different natural surfaces (from M. de Franceschi, 2002, derived from Wieringa, 1993). z o (m) Landscape Description ________________________________________________________________ Open sea or lake, tidal flat, snow-covered plain, featureless desert, tarmac, concrete with a fetch of several km 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

Wind profile in stable, neutral and unstable air.

( /

(AND STABLE)

MO Surface-layer formulations: Φ m = (kz/u * )(  U/  z) - wind shear Φ h = (kz/T * )(  θ/  z) - thermal stratification Φ w = σ w /u * - fluctuations in vertical velocity Φ θ = σ θ /|T * | - fluctuations in temperature Φ ε = kzε/u * 3 - turbulence energy dissipation

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).

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: 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 the10 th Conference on Mountain Meteorology, June 2002, Park City, UT, American Meteorological Society, Boston.