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motions from above or by radiative cooling from below. Frontal surfaces are slanted
like IBL tops, but they are moving with the synoptic pressure systems and are not
linked to changes in surface properties.
If several subsequent changes in the surface properties occur in streamwise
direction multiple IBL can form. They all grow with distance from the respective
boundary in surface type. In some larger distance to the initiating change in surface
properties, the single IBL looses its identity and multiple IBLs can no longer be
distinguished from each other. This height is also called blending height.
2.2.2 Convective Boundary Layer
The thermal stratification of the ABL is rarely found t
o be r
eally neutral. In most
cases, there is a non-vanishing virtual potential heat flux
v
w
at the ground (the vir-
tual potential temperature,
v
includes the modifying influence of the atmospheric
humidity on the static stability of the air), which leads to a thermal stratification of
the ABL. From this surface heat flux and the friction velocity
u
∗
, a length scale,
L
∗
,
the Obukhov length, can be formed,
u
3
∗
v
w
κ
L
∗
=
.
(2.22)
g
The heat flux is counted positive if it is directed from the atmosphere towards the
ground (cooling the atmosphere) and negative if it is towards the atmosphere (heat-
ing the atmosphere). Thus an unstable
ABL
is characterized by a negative Obukhov
length. The virtual potential heat flux
v
w
can be separated into a heat flux and a
humidity flux,
v
w
=
w
+
q
w
.
0.61
(2.23)
The ratio of the turbulent heat flux and the humidity flux is called Bowen ratio,
c
p
w
L
v
q
w
B
=
,
(2.24)
where
q
is specific hum
idity.
The buo
yanc
y exerted by the vertical heat and humidity
gradients is given by
g
0.61
gq
w
. The ratio of these two contributing terms
is called the buoyancy ratio,
BR
, which is inversely proportional to the Bowen ratio:
w
+
q
w
w
0.61
c
p
L
v
0.61
1
B
.
BR
=
=
(2.25)
This unstable type of the ABL is usually found during daytime over surfaces
heated by insolation and over waters that are warmer than the air above. The degree
of instability is described by the non-dimensional ratio of the height
z
over the
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