Geoscience Reference
In-Depth Information
12
Boundary layer climates
Learning objectives
When you have read this chapter you will:
Understand the significance of surface characteristics for energy and moisture exchanges and thus small-
scale climates,
Appreciate how urban environments modify atmospheric conditions and the local climate,
Know the characteristics of an urban heat island.
Meteorological phenomena encompass a wide range of
space and timescales, from gusts of wind that swirl up
leaves and litter to the global-scale wind systems that
shape the planetary climate. Their time and length scales,
and their kinetic energy, are illustrated in Figure 12.1 in
comparison with those for a range of human activities.
Small-scale turbulence, with wind eddies of a few
metres' dimension and lasting for only a few seconds,
represents the domain of
micrometeorology
, or boundary
layer climates. Small-scale climates occur within the
planetary boundary layer (see Chapter 6) and have
vertical scales of the order of 10
3
m, horizontal scales of
some 10
4
m, and timescales of about 10
5
seconds (i.e.
one day). The boundary layer is typically 1 km thick, but
varies between 20 m and several kilometres in different
locations and at different times in the same location.
Within this layer mechanical and convective diffusion
processes transport mass, momentum and energy, as
well as exchanging aerosols and chemicals between the
lower atmosphere and the earth's surface. The boundary
layer is especially prone to nocturnal cooling and diurnal
heating, and within it the wind velocity decreases
through friction from the free velocity aloft to lower
values near the surface, and ultimately to the zero-
velocity
roughness length
height (see Chapter 6A).
Diffusion processes within the boundary layer are of
two types:
1
Eddy diffusion
. Eddies involve parcels of air that
transport energy, momentum and moisture from one
location to another. Usually, they can be resolved
into upward-spiralling vortices leading to transfers
from the earth's surface to the atmosphere or from
one vertical layer of air to another. These eddies can
be defined by generalized streamlines (i.e. resolved
fluctuations). They range in size from a few cen-
timetres (10
-2
m) in diameter above a heated surface
to 1 to 2 m (10
0
m) resulting from small-scale con-
vection and surface roughness, and grade into dust
devils (10
1
m, lasting 10
1
to 10
2
s) and tornadoes (10
3
m, lasting 10
2
to 10
3
s).
2
Turbulent diffusion
. These are apparently random
(i.e. unresolved) fluctuations of instantaneous veloc-
ities having variations of a second or less.
