Since the adiabatic behavior of air parcels is a function of the external pressure,
potential temperature Eqs. (1.1–1.5) gives temperatures of air parcels at different
pressures.
The virtual potential temperature is defined as the potential temperature dry air
must be equal to the density of moist air, which is smaller at a given pressure so that
moist air is relative buoyant. For unsaturated air with a mixture ratio r, defined as
the ratio of masses of water vapor and dry air, the virtual potential temperature is
given by
h v ¼ ð1 þ 0:61rÞ
ð 1:7Þ
The diurnal boundary layer, also known as the convective or mixed layer, has an
unstable surface layer in which ∂h /∂z < 0, a convective mixing zone in which
vertical gradients are absent, where ∂h /∂z = 0 and an upper inversion zone where
∂h /∂z > 0. The latter is a buffer that dampens the ascending atmospheric motions
(Figs. 1.4 and 1.5).
This dampening effect has a bearing on several phenomena including convective
retention involving the descending recirculation of hot air parcels, mechanical
mixing and conservation within the boundary layer; sensible and latent heat, and
dispersion of pollutants.
The ground surface adjacent to the atmosphere is the zone where greater diurnal
absorption and nocturnal dissipation of thermal energy take place and these effects
decrease as a function of distance from the surface. The daily variation of the air
1.5
0.5
z/z
i
0
0
5
10
300
u (ms
-1 )
(K)
d (deg)
330 290
290
1.0
Speed
Direction
Potential
Temperature
Inversion Layer
Fig. 1.4 Mean vertical profiles of wind speed, wind direction, and potential temperature in the
convective boundary layer (height z is presented as a ratio z/z i where z i is the height of the daytime
boundary layer) (after Kaimal and Finnigan 1994)
6
1 General Characteristics of the Atmospheric Boundary Layer
potential temperature Eqs. (1.1–1.5) gives temperatures of air parcels at different
pressures.
The virtual potential temperature is defined as the potential temperature dry air
must be equal to the density of moist air, which is smaller at a given pressure so that
moist air is relative buoyant. For unsaturated air with a mixture ratio r, defined as
the ratio of masses of water vapor and dry air, the virtual potential temperature is
given by
h v ¼ ð1 þ 0:61rÞ
ð 1:7Þ
The diurnal boundary layer, also known as the convective or mixed layer, has an
unstable surface layer in which ∂h /∂z < 0, a convective mixing zone in which
vertical gradients are absent, where ∂h /∂z = 0 and an upper inversion zone where
∂h /∂z > 0. The latter is a buffer that dampens the ascending atmospheric motions
(Figs. 1.4 and 1.5).
This dampening effect has a bearing on several phenomena including convective
retention involving the descending recirculation of hot air parcels, mechanical
mixing and conservation within the boundary layer; sensible and latent heat, and
dispersion of pollutants.
The ground surface adjacent to the atmosphere is the zone where greater diurnal
absorption and nocturnal dissipation of thermal energy take place and these effects
decrease as a function of distance from the surface. The daily variation of the air
1.5
0.5
z/z
i
0
0
5
10
300
u (ms
-1 )
(K)
d (deg)
330 290
290
1.0
Speed
Direction
Potential
Temperature
Inversion Layer
Fig. 1.4 Mean vertical profiles of wind speed, wind direction, and potential temperature in the
convective boundary layer (height z is presented as a ratio z/z i where z i is the height of the daytime
boundary layer) (after Kaimal and Finnigan 1994)
6
1 General Characteristics of the Atmospheric Boundary Layer