Parameterization of Convective Boundary Layer Turbulence and Clouds
73
presents a mean coverage of 5% (Duynkerke 1998). Shallow cumulus are omnipresent
in the trade-wind region of oceans, where they are known as trade-wind-cumulus.
They are also frequent in the middle latitudes of continents, in summer, where they
often evolve into deep convection, turning into cumulonimbus.
Shallow cumulus directly infl uence the global circulation and the hydrologic cycle
since they enhance the vertical transport of heat, moisture, and momentum, namely,
in the intertropical zone of convergence, contributing to the effi ciency of the transport of moisture and heat in the Hadley circulation (Tiedtke 1987; Siebesma 1998).
The vertical transport of moisture associated with shallow cumulus tends to dry the
BL, limiting the formation of stratiform clouds. Several modeling exercises showed
that the distribution of precipitation and its variability in the tropics are strongly
infl uenced by the presence of cumulus convection (Slingo et al. 1994; Gregory 1997).
Tiedtke (1987) emphasized that the presence of shallow cumulus increases the surface evaporation in the ECMWF (European Center of Medium Weather Forecasts,
Beljaars and Betts 1992) model, up to 50 W m −2 in the subtropical regions.
Additionally, shallow cumulus exert an indirect infl uence in the BL through the
modifi cation of the radiative balance. This type of irregular clouds possess unusual
radiative properties (Ackerman and Cox 1981; Marshak et al. 1995). In general,
BL clouds have a radiative net surface cooling effect, since they have very high
refl ectivities. Studies with one-dimensional models (1D) of convection-radiation
with BL clouds point out that an increase of 1% in the global cloud cover of these
clouds could cool the surface, offsetting an equivalent of increase of 25% of CO 2
concentrations (Van Dorland 1999). Note, though, that such a change would be an
increase in about 10% of the current ocean Cu cloud cover and 20% in the continental regions.
In spite of their importance for weather and climate shallow cumuli have received
less attention than other BL clouds. However, some observational campaigns
had been dedicated to its study, like BOMEX (the Barbados Oceanographic and
Meteorological EXperiment, Kuettner and Holland 1969), ARM (Atmospheric
Radiation Measurement, Brown et al. 2002) and SCMS (Small Cumulus Microphysics
Study, French et al. 1999), leading to a number of relevant contributions (Warner
1977; Jonas 1990; Blyth 1993; Smith and Jonas 1995; Grinell et al. 1996; de Roode
and Duynkerke 1997). On the other hand, LES models have been extensively used for
the study of the BL with shallow cumulus (Sommeria 1976; Cuijpers and Duynkerke
1993; Siebesma and Cuijpers 1995; Siebesma and Holtslag 1996; Stevens et al. 2001;
Brown et al. 2002; Neggers et al. 2004; Siebesma et al. 2004).
The BL with shallow cumulus presents a mean cloud cover of 10%–30%, usually
associated with good weather. Shallow cumuli have a small vertical development,
up to a maximum of 2 km, with a cloud-base between 500 m and 1.5 km. These
clouds are turbulent convective structures, where the vertical velocity can reach the
5 m s −1 .
Over land surfaces, the BLs with shallow cumulus have a diurnal cycle, due to the
large variation of the heat and moisture fl uxes at the surface. A typical cycle of this
type of BL is as follows: at sunrise the BL is cloud free; associated with the surface
heat fl ux the fi rst cumuli appear a few hours later, deepening with time; before sunset
the cumuli start to dissipate, and eventually disappear.
© 2010 by Taylor and Francis Group, LLC
73
presents a mean coverage of 5% (Duynkerke 1998). Shallow cumulus are omnipresent
in the trade-wind region of oceans, where they are known as trade-wind-cumulus.
They are also frequent in the middle latitudes of continents, in summer, where they
often evolve into deep convection, turning into cumulonimbus.
Shallow cumulus directly infl uence the global circulation and the hydrologic cycle
since they enhance the vertical transport of heat, moisture, and momentum, namely,
in the intertropical zone of convergence, contributing to the effi ciency of the transport of moisture and heat in the Hadley circulation (Tiedtke 1987; Siebesma 1998).
The vertical transport of moisture associated with shallow cumulus tends to dry the
BL, limiting the formation of stratiform clouds. Several modeling exercises showed
that the distribution of precipitation and its variability in the tropics are strongly
infl uenced by the presence of cumulus convection (Slingo et al. 1994; Gregory 1997).
Tiedtke (1987) emphasized that the presence of shallow cumulus increases the surface evaporation in the ECMWF (European Center of Medium Weather Forecasts,
Beljaars and Betts 1992) model, up to 50 W m −2 in the subtropical regions.
Additionally, shallow cumulus exert an indirect infl uence in the BL through the
modifi cation of the radiative balance. This type of irregular clouds possess unusual
radiative properties (Ackerman and Cox 1981; Marshak et al. 1995). In general,
BL clouds have a radiative net surface cooling effect, since they have very high
refl ectivities. Studies with one-dimensional models (1D) of convection-radiation
with BL clouds point out that an increase of 1% in the global cloud cover of these
clouds could cool the surface, offsetting an equivalent of increase of 25% of CO 2
concentrations (Van Dorland 1999). Note, though, that such a change would be an
increase in about 10% of the current ocean Cu cloud cover and 20% in the continental regions.
In spite of their importance for weather and climate shallow cumuli have received
less attention than other BL clouds. However, some observational campaigns
had been dedicated to its study, like BOMEX (the Barbados Oceanographic and
Meteorological EXperiment, Kuettner and Holland 1969), ARM (Atmospheric
Radiation Measurement, Brown et al. 2002) and SCMS (Small Cumulus Microphysics
Study, French et al. 1999), leading to a number of relevant contributions (Warner
1977; Jonas 1990; Blyth 1993; Smith and Jonas 1995; Grinell et al. 1996; de Roode
and Duynkerke 1997). On the other hand, LES models have been extensively used for
the study of the BL with shallow cumulus (Sommeria 1976; Cuijpers and Duynkerke
1993; Siebesma and Cuijpers 1995; Siebesma and Holtslag 1996; Stevens et al. 2001;
Brown et al. 2002; Neggers et al. 2004; Siebesma et al. 2004).
The BL with shallow cumulus presents a mean cloud cover of 10%–30%, usually
associated with good weather. Shallow cumuli have a small vertical development,
up to a maximum of 2 km, with a cloud-base between 500 m and 1.5 km. These
clouds are turbulent convective structures, where the vertical velocity can reach the
5 m s −1 .
Over land surfaces, the BLs with shallow cumulus have a diurnal cycle, due to the
large variation of the heat and moisture fl uxes at the surface. A typical cycle of this
type of BL is as follows: at sunrise the BL is cloud free; associated with the surface
heat fl ux the fi rst cumuli appear a few hours later, deepening with time; before sunset
the cumuli start to dissipate, and eventually disappear.
© 2010 by Taylor and Francis Group, LLC