Geoscience Reference
In-Depth Information
12
CHAPTER TWELVE
Boundary layer
climates
LEARNING OBJECTIVES
When you have read this chapter you will:
n
understand the significance of surface characteristics for energy and moisture exchanges and
thus small-scale climates
n
appreciate how forest and urban environments modify atmospheric conditions and the local
climate
n
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 compari-
son with those for a range of human activities.
Small-scale turbulence, with wind eddies of a
few meters dimension and lasting only for a few
seconds, represents the domain of
micrometeo-
rology
, or boundary layer climates. Small-scale
climates occur within the planetary boundary
layer (see Chapter 5) and have vertical scales in 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 1km thick, but
varies between 20m and several kilometers 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 air
velocity aloft to lower values near the surface, and
ultimately to the zero-velocity
roughness length
height (see Chapter 5).
Diffusion processes within the boundary layer
are of two types:
1
Eddy diffusion
. Eddies involve parcels of air
that transport energy, momentum and mois-
ture from one location to another. Usually,
they can be resolved into upward-spiraling
vortices leading to transfers from the earth's
surface to the atmosphere or from one vertical
