72
Air Pollution and Turbulence: Modeling and Applications
air from this back into the BL, contributing to the its vertical growth, a process often
referred to as top-entrainment. In these conditions, a top-entrainment region goes
through the top BL inversion layer.
In the same case of a clear sky BL, the ground cools after sunset by the emission
of longwave radiation. This cooling inhibits surface turbulence leading to the establishment of a low stable layer, which develops vertically less than the mixed layer in
daytime. A remainder of the previous day mixed layer may be present, laying above
the stable BL, and is then called the residual layer. In the absence of convection,
wind shear causes turbulence in the night period. If the convective situation remains
for successive days, the daytime growth of the BL will be enhanced by the presence
of the residual layer.
The clear sky CBL was the subject of study of many fi eld observational campaigns:
Wangara (Clarke et al. 1971), HAPEX (Hydrologic-Atmospheric Pilot EXperiment;
André et al. 1986), Phoenix CBL (Phoenix 78 Convective Boundary Layer fi eld
experiment; Kropfl i and Hildebrand 1980; Young 1988a,b,c), etc. Detailed studies
with large eddy simulation (LES) models were also carried out, including those of
Moeng (1984), Schumann and Moeng (1991a,b), Sorbjan (1996a,b), and Sullivan
et al. (1998), that contributed for a better understanding of the structure and dynamics of the CBL.
Great extensions of the globe, and in particular of the oceans, are covered by
low clouds. Clouds modify the radiative balance of the surface and of the BL. The
exchange of latent heat associated with condensation and evaporation of precipitation in the sub-cloud layers constitutes other important thermal forces. Both are
essential for the dynamics of the BL and for the global circulation.
The clouds that are more frequently present in the BL include shallow cumulus,
stratus, stratocumulus, and nimbostratus. Stratus, stratocumulus, and nimbostratus
are stratiform clouds, characterized by a great horizontal extension and a relatively
small vertical thickness.
Stratocumulus above the oceans are often associated with anticyclonic subsidence
in the subtropical and middle latitudes, and above the land surface in the cold season.
In the subtropical eastern ocean basins, the subsidence associated with the descending branches of the Hadley cell, together with the cold oceanic currents, induces the
persistent presence of stratocumulus, for example, off the coast of California, Peru,
Namibia, and Mauritania (Hanson 1991; Klein and Hartmann 1993; Ma et al. 1996).
Stratocumulus have a great impact on climate and its variability (e.g., Philander
et al. 1996; Clement and Seager 1999). The thermodynamic and turbulent structures
of stratocumulus are known, essentially, due to some large observational campaigns,
for example, FIRE (First ISCCP [International Satellite Cloud Climatology Project]
Regional Experiment, Albrecht et al. 1988), ASTEX (Atlantic Stratocumulus
Transition EXperiment, Albrecht et al. 1995), and other studies (Duynkerke et al.
1987, Hignett 1991, Duynkerke and Teixeira 2000). More recently, LES simulations
have been contributing to the understanding of the mechanism involved in the formation and maintenance of stratocumulus (Moeng et al. 1996; Stevens et al. 1998;
Duynkerke et al. 1999).
A CBL with shallow cumulus is present in the entire globe. On average, 12% of
the oceanic surface is covered by this type of clouds, while the terrestrial surface
© 2010 by Taylor and Francis Group, LLC
Air Pollution and Turbulence: Modeling and Applications
air from this back into the BL, contributing to the its vertical growth, a process often
referred to as top-entrainment. In these conditions, a top-entrainment region goes
through the top BL inversion layer.
In the same case of a clear sky BL, the ground cools after sunset by the emission
of longwave radiation. This cooling inhibits surface turbulence leading to the establishment of a low stable layer, which develops vertically less than the mixed layer in
daytime. A remainder of the previous day mixed layer may be present, laying above
the stable BL, and is then called the residual layer. In the absence of convection,
wind shear causes turbulence in the night period. If the convective situation remains
for successive days, the daytime growth of the BL will be enhanced by the presence
of the residual layer.
The clear sky CBL was the subject of study of many fi eld observational campaigns:
Wangara (Clarke et al. 1971), HAPEX (Hydrologic-Atmospheric Pilot EXperiment;
André et al. 1986), Phoenix CBL (Phoenix 78 Convective Boundary Layer fi eld
experiment; Kropfl i and Hildebrand 1980; Young 1988a,b,c), etc. Detailed studies
with large eddy simulation (LES) models were also carried out, including those of
Moeng (1984), Schumann and Moeng (1991a,b), Sorbjan (1996a,b), and Sullivan
et al. (1998), that contributed for a better understanding of the structure and dynamics of the CBL.
Great extensions of the globe, and in particular of the oceans, are covered by
low clouds. Clouds modify the radiative balance of the surface and of the BL. The
exchange of latent heat associated with condensation and evaporation of precipitation in the sub-cloud layers constitutes other important thermal forces. Both are
essential for the dynamics of the BL and for the global circulation.
The clouds that are more frequently present in the BL include shallow cumulus,
stratus, stratocumulus, and nimbostratus. Stratus, stratocumulus, and nimbostratus
are stratiform clouds, characterized by a great horizontal extension and a relatively
small vertical thickness.
Stratocumulus above the oceans are often associated with anticyclonic subsidence
in the subtropical and middle latitudes, and above the land surface in the cold season.
In the subtropical eastern ocean basins, the subsidence associated with the descending branches of the Hadley cell, together with the cold oceanic currents, induces the
persistent presence of stratocumulus, for example, off the coast of California, Peru,
Namibia, and Mauritania (Hanson 1991; Klein and Hartmann 1993; Ma et al. 1996).
Stratocumulus have a great impact on climate and its variability (e.g., Philander
et al. 1996; Clement and Seager 1999). The thermodynamic and turbulent structures
of stratocumulus are known, essentially, due to some large observational campaigns,
for example, FIRE (First ISCCP [International Satellite Cloud Climatology Project]
Regional Experiment, Albrecht et al. 1988), ASTEX (Atlantic Stratocumulus
Transition EXperiment, Albrecht et al. 1995), and other studies (Duynkerke et al.
1987, Hignett 1991, Duynkerke and Teixeira 2000). More recently, LES simulations
have been contributing to the understanding of the mechanism involved in the formation and maintenance of stratocumulus (Moeng et al. 1996; Stevens et al. 1998;
Duynkerke et al. 1999).
A CBL with shallow cumulus is present in the entire globe. On average, 12% of
the oceanic surface is covered by this type of clouds, while the terrestrial surface
© 2010 by Taylor and Francis Group, LLC