Donner, L. J., 1993: A cumulus parameterization including mass fluxes,
vertical momentum dynamics, and mesoscale effects. Journal of the
Atmospheric Sciences, 50(6), 889-906.
Abstract: A formulation for parameterizing cumulus convection, which
treats cumulus vertical momentum dynamics and mass fluxes consistently,
is presented. This approach predicts the penetrative extent of cumulus
updrafts on the basis of their vertical momentum and provides a basis for
treating cumulus microphysics using formulations that depend on vertical
velocity. Treatments for cumulus microphysics are essential if the water
budgets of convective systems are to be evaluated for treating mesoscale
stratiform processes associated with convection, which are important for
radiative interactions influencing climate.
The water budget (both condensed and vapor) of the cumulus updrafts is
used to drive a semi-empirical parameterization for the large-scale effects
of the mesoscale circulations associated with deep convection The parameterization
for mesoscale effects invokes mesoscale ascent to redistribute vertically
water detrained at the tops of the cumulus updrafts. The local cooling
associated with this mesoscale ascent is probably larger than radiative
heating of the mesoscale anvil clouds, and the mesoscale ascent may be
in part a response to such radiative heating.
The parameterization was applied to two tropical thermodynamic profiles
whose diagnosed forcing by convective systems differed significantly. A
spectrum of cumulus updrafts was allowed. The deepest of the updrafts penetrated
the upper troposphere, while the shallower updrafts penetrated into the
region of the mesoscale anvil. The relative numbers of cumulus updrafts
of characteristic vertical velocities comprising the parameterized ensemble
corresponded well with available observations. However, the large-scale
heating produced by the ensemble without mesoscale circulations was concentrated
at lower heights than observed or was characterized by excessive peak magnitudes.
Also, an unobserved large-scale source of water vapor was produced in the
middle troposphere. When the parameterization for mesoscale effects was
added, the large-scale thermal and moisture forcing predicted by the parameterization
agreed well with observations for both cases.
The significance of mesoscale processes, some of which may depend in part
on radiative forcing, suggests that future cumulus parameterization development
will need to treat some radiative processes. Further, the long time scale
of the mesoscale processes relative to that of the cumulus cells indicates
a possible requirement for carrying some characteristics of the convective
system in time as cumulus parameterizations are incorporated in large-scale
models whose resolutions remain too large to capture explicitly the mesoscale
processes. A formulation for parameterizing cumulus convection, which treats
cumulus vertical momentum dynamics and mass fluxes consistently, is presented.
This approach predicts the penetrative extent of cumulus updrafts on the
basis of their vertical momentum and provides a basis for treating cumulus
microphysics using formulations that depend on vertical velocity. Treatments
for cumulus microphysics are essential if the water budgets of convective
systems are to be evaluated for treating mesoscale stratiform processes
associated with convection, which are important for radiative interactions
influencing climate. The water budget (both condensed and vapor) of the
cumulus updrafts is used to drive a semi-empirical parameterization for
the large-scale effects of the mesoscale circulations associated with deep
convection. The parameterization for mesoscale effects invokes mesoscale
ascent to redistribute vertically water detrained at the tops of the cumulus
updrafts. The local cooling associated with this mesoscale ascent is probably
larger than radiative heating of the mesoscale anvil clouds, and the mesoscale
ascent may be in part a response to such radiative heating. The parameterization
was applied to two tropical thermodynamic proiles whose diagnosed forcing
by convective systems differed significantly. A spectrum of cumulus updrafts
was allowed. The deepest of the updrafts penetrated the upper troposphere,
while the shallower updrafts penetrated into the region of the mesoscale
anvil. The relative numbers of cumulus updrafts of characteristic vertical
velocities comprising the parameterized ensemble corresponded well with
available observations. However, the large-scale heating produced by the
ensemble without mesoscale circulations was concentrated at lower heights
than observed or was characterized by excessive peak magnitudes. Also,
an unobserved large-scale source of water vapor was produced in the middle
troposphere. When the parameterization for mesoscale effects was added,
the large-scale thermal and moisture forcing predicted by the parameterization
agreed well with observations for both cases. The significance of mesoscale
processes, some of which may depend in part on radiative forcing, suggests
that future cumulus parameterization development will need to treat some
radiative processes. Further, the long time scale of the mesoscale processes
relative to that of the cumulus cells indicates a possible requirement
for carrying some characteristics of the convective system in time as cumulus
parameterizations are incorporated in large-scale models whose resolutions
remain too large to capture explicitly the mesoscale processes.