Geography Reference
In-Depth Information
CHAPTER 5
The Planetary Boundary Layer
The
planetary boundary layer
is that portion of the atmosphere in which the
flow field is strongly influenced directly by interaction with the surface of the
earth. Ultimately this interaction depends on molecular viscosity. It is, however,
only within a few millimeters of the surface, where vertical shears are very intense,
that molecular diffusion is comparable to other terms in the momentum equation.
Outside this
viscous sublayer
molecular diffusion is not important in the boundary
layer equations for the mean wind, although it is still important for small-scale tur-
bulent eddies. However, viscosity still has an important indirect role; it causes the
velocity to vanish at the surface. As a consequence of this
no-slip boundary con-
dition,
even a fairly weak wind will cause a large-velocity shear near the surface,
which continually leads to the development of turbulent eddies. These turbulent
motions have spatial and temporal variations at scales much smaller than those
resolved by the meteorological observing network. Such shear-induced eddies,
together with convective eddies caused by surface heating, are very effective in
transferring momentum to the surface and transferring heat (latent and sensible)
away from the surface at rates many orders of magnitude faster than can be done
by molecular processes. The depth of the planetary boundary layer produced by
this turbulent transport may range from as little as 30 m in conditions of large
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