Parameterization of Convective Boundary Layer Turbulence and Clouds
71
boundary layer (CBL), either dry or with shallow cumulus (Siebesma and Teixeira
2000; Soares et al. 2004; Siebesma et al. 2007). The developed approaches are easily
integrated in models of different scales, and could be extended to the problem of the
deep convection parameterization, aiming the establishment of a unifi ed parameterization scheme for turbulence and convection in the troposphere.
It is clear that, in the future, the computational development will allow carrying out simulations with global circulation and/or mesoscale models with much
higher resolutions than is possible currently. However, it is also certain that the turbulent processes, with and without condensation, will continue outside the range of
direct numerical simulation, justifying an increasing effort in the parameterization
development.
4.2 THE ATMOSPHERIC BOUNDARY LAYER
The troposphere is often divided in two layers: the BL and the free atmosphere
(FA). The BL corresponds to the turbulent region where the surface has a direct
infl uence. The timescales of the adjustment of the BL to the different forces are
relatively short. In this layer of the troposphere exist the great majority of living
creatures and take place most of the human activities, conferring to its study an
enormous importance. The transport of momentum, heat, and moisture between the
atmosphere, the land, and the ocean takes place in the BL, and it directly infl uences
the local and regional weather, and the global circulation of the atmosphere.
The understanding of the phenomenology that occurs in the atmospheric BL has
relevance in many domains, including the dispersion of pollutants, the forecast of
temperature, moisture, and wind near the surface, and the processes of cloud and
storm formation. Some key activities are critically dependent on BL monitoring and
forecast, with emphasis on aeronautics.
In anticyclonic fair weather, the solar diurnal cycle of heating and cooling of the
surface determines the temporal evolution of the vertical structure of the BL. At
sunrise, the land starts to get warmed; heat is transferred to the overlaying air in a
heterogeneous pattern, provoking the turbulent mixing of the air. Turbulent transport depends on the difference of properties between neighboring air parcels, but is
essentially controlled by the velocity fi eld. In a stably stratifi ed environment, such
as the one that exists in the early morning surface layer and most of the time in
the FA, vertical velocity is inhibited, and turbulence must work its way up into the
atmosphere by progressively eroding the temperature profi le through the warming
of the lowest layer. Convective movements, contributing to the vertical turbulent
transport of heat and other properties, will be driven by changes in the surface layer
thermal structure due to the upward surface heat fl ux, and will extend into the upper
boundary layer, and intensify, into the afternoon, leading to BL growth and to the
establishment of quasi-neutral layer, often called the mixed layer, on top of a shallow
unstable surface layer, and beneath a top stable inversion layer.
Turbulent structures in the BL have a spectrum of scales. In the CBL, the eddies
containing more energy have a vertical dimension of the order of magnitude of the
height of the BL itself. They are generally referred to as updrafts or thermals, and
can reach more than 2 km in height. The thermals penetrate into the FA and carry
© 2010 by Taylor and Francis Group, LLC
71
boundary layer (CBL), either dry or with shallow cumulus (Siebesma and Teixeira
2000; Soares et al. 2004; Siebesma et al. 2007). The developed approaches are easily
integrated in models of different scales, and could be extended to the problem of the
deep convection parameterization, aiming the establishment of a unifi ed parameterization scheme for turbulence and convection in the troposphere.
It is clear that, in the future, the computational development will allow carrying out simulations with global circulation and/or mesoscale models with much
higher resolutions than is possible currently. However, it is also certain that the turbulent processes, with and without condensation, will continue outside the range of
direct numerical simulation, justifying an increasing effort in the parameterization
development.
4.2 THE ATMOSPHERIC BOUNDARY LAYER
The troposphere is often divided in two layers: the BL and the free atmosphere
(FA). The BL corresponds to the turbulent region where the surface has a direct
infl uence. The timescales of the adjustment of the BL to the different forces are
relatively short. In this layer of the troposphere exist the great majority of living
creatures and take place most of the human activities, conferring to its study an
enormous importance. The transport of momentum, heat, and moisture between the
atmosphere, the land, and the ocean takes place in the BL, and it directly infl uences
the local and regional weather, and the global circulation of the atmosphere.
The understanding of the phenomenology that occurs in the atmospheric BL has
relevance in many domains, including the dispersion of pollutants, the forecast of
temperature, moisture, and wind near the surface, and the processes of cloud and
storm formation. Some key activities are critically dependent on BL monitoring and
forecast, with emphasis on aeronautics.
In anticyclonic fair weather, the solar diurnal cycle of heating and cooling of the
surface determines the temporal evolution of the vertical structure of the BL. At
sunrise, the land starts to get warmed; heat is transferred to the overlaying air in a
heterogeneous pattern, provoking the turbulent mixing of the air. Turbulent transport depends on the difference of properties between neighboring air parcels, but is
essentially controlled by the velocity fi eld. In a stably stratifi ed environment, such
as the one that exists in the early morning surface layer and most of the time in
the FA, vertical velocity is inhibited, and turbulence must work its way up into the
atmosphere by progressively eroding the temperature profi le through the warming
of the lowest layer. Convective movements, contributing to the vertical turbulent
transport of heat and other properties, will be driven by changes in the surface layer
thermal structure due to the upward surface heat fl ux, and will extend into the upper
boundary layer, and intensify, into the afternoon, leading to BL growth and to the
establishment of quasi-neutral layer, often called the mixed layer, on top of a shallow
unstable surface layer, and beneath a top stable inversion layer.
Turbulent structures in the BL have a spectrum of scales. In the CBL, the eddies
containing more energy have a vertical dimension of the order of magnitude of the
height of the BL itself. They are generally referred to as updrafts or thermals, and
can reach more than 2 km in height. The thermals penetrate into the FA and carry
© 2010 by Taylor and Francis Group, LLC