Geoscience Reference
In-Depth Information
Height
Free atmosphere
Inversion layer
Residual
mixed
layer
Mixed
layer
Stable
boundary
layer
Figure 18.2
Typical diurnal
evolution of the ABL over
land under clear sky
conditions.
Stable surface layer
Unstable surface layer
06:00
Sunrise
12:00
18:00
Sunset
00:00
06:00
Sunrise
12:00
heating and partly as a result of the downward mixing of the warmer air from the
free atmosphere above. On the other hand, the moisture input from the surface is
mainly used to moisten the drier air that is being captured from above, so the
diurnal cycle in humidity content in the ABL is usually limited. At night, the
surface cools and a stable surface layer develops. This may acheive a height of
several hundred meters and is largely uncoupled by a stable boundary layer from
the decaying remnants of the previous day's mixed layer. At sunrise the surface
layer again becomes unstable, and the process of mixed layer growth is reinvigorated.
Daytime ABL profiles
It is instructive to consider the equations describing conservation of heat and
moisture in the ABL during the day in clear sky conditions over uniform, flat ter-
rain. In these conditions, horizontal advection of heat and moisture, and the diver-
gence of horizontal turbulent fluxes can be neglected. Consequently, terms which
involve the partial derivative with respect to
x
and
y
can be neglected in the con-
servation equations. It is also reasonable to assume there is negligible subsidence
over the flat surface so terms involving the mean velocity of
w
can also be neglected.
Net radiation divergence can arguably be neglected in daytime conditions, there
are no chemical sources of water and, if there is no boundary layer cloud, terms
describing phase changes in atmospheric moisture are zero. With these several
simplifying assumptions, the equations describing the mean flow for moisture and
temperature in Table 17.5 simplify dramatically and become respectively:
q
∂
∂
(
wq
′ ′
)
(18.1)
=−
∂
t
∂
z
∂
q
∂′ ′
=−
(
w
θ
)
(18.2)
∂
t
∂
z
