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A.D. Del Genio
possible to determine the role of convective cluster rain processes in determining the radiative
signature of tropical anvils. This information in turn will lead to improved parameterizations
of the microphysical aspects of anvils and the coupling between anvil and convective clouds in
global climate models.
20.5.3 Cumulus Ensemble Model (CEM)
The precipitation distribution measured by TRMM is an indicator of the net vertically integrated latent heating of the atmosphere and as such provides a valuable constraint for model
assessments of the realism of simulated forcing of the general circulation. But the vertical
integral of heating by itself is not sufficient for several reasons. The vertical profile of heating
determines the effect of moist convection on static stability and thus the vigor of large-scale vertical motions as well as the profile of radiative cooling to space. Furthermore, vertical variations
in heating affect cloud formation both directly by altering relative humidity and indirectly by
forcing or suppressing the upward motion that forms clouds. Interactions between convection
and the large-scale circulation are expected to be sensitive to the vertical profile of heating.
Several field studies have indirectly derived vertical profiles of convective heating in a few locations as large-scale residuals using radiosonde-observed temperature tendencies and advection
rates (cf. Reed and Recker, 1971; Thompson et aI., 1979). But a large-scale depiction of variations in heating profile does not yet exist. Furthermore, the heating profile of a convective cluster
differs between the convective and stratiform anvil regions (Johnson, 1984) and hence changes
with time over the life cycle of a convective system. The heating profile may also be different
for different types of convective structures (isolated cells, squall lines, non-squall clusters, etc.),
but information is lacking.
Several TRMM instruments (PR, TMI) will produce information on the vertical profile of hydrometeors of different types. Tao et aI. (1990, 1993) have developed a procedure for estimating
the vertical profile of convective heating from this information. The procedure makes use of a
cumulus ensemble model (CEM), i.e., a model that explicitly resolves individual cumulus clouds
over a limited spatial domain. The CEM is an anelastic model with parameterized Kessler-type
two-category liquid phase microphysics (cloud water, rain) and parameterized three-category
ice microphysics (cloud ice, snow, hailjgraupel). The CEM runs in either 2 or 3 dimensions
with typically 0.5-1.5 km horizontal resolution and 0.2-1.05 km vertical resolution.
The CEM produces its own simulation of both heating profiles and hydrometeor profiles. Based
on these simulations, Tao et aI. have developed a parameterization of the former using the
latter as inputs. The scheme ignores convective-scale heat transport and the vertical and
horizontal transport of hydrometeors. With these approximations, the apparent heat source
due to convection Q1 can be written in terms of the microphysical sources and sinks as
where the terms on the right hand side represent condensation of rain, condensation of cloud
liquid water, deposition of snow and hailjgraupel, deposition of cloud ice, evaporation of rain,
and melting of snow and hailjgraupel, respectively. Each of these is parameterized on the basis
of features of observed hydrometeor profiles (e.g., the vertical gradient of rainwater is related
to rain evaporation, while the surface precipitation rate constrains the net condensation and
deposition) .
An alternative latent heating algorithm makes no use of observed hydrometeor profiles. Instead,
it assumes that a limited number of heating profiles characterize different types of convective
systems in different geographic regions; for each type a convective and stratiform heating profile
A.D. Del Genio
possible to determine the role of convective cluster rain processes in determining the radiative
signature of tropical anvils. This information in turn will lead to improved parameterizations
of the microphysical aspects of anvils and the coupling between anvil and convective clouds in
global climate models.
20.5.3 Cumulus Ensemble Model (CEM)
The precipitation distribution measured by TRMM is an indicator of the net vertically integrated latent heating of the atmosphere and as such provides a valuable constraint for model
assessments of the realism of simulated forcing of the general circulation. But the vertical
integral of heating by itself is not sufficient for several reasons. The vertical profile of heating
determines the effect of moist convection on static stability and thus the vigor of large-scale vertical motions as well as the profile of radiative cooling to space. Furthermore, vertical variations
in heating affect cloud formation both directly by altering relative humidity and indirectly by
forcing or suppressing the upward motion that forms clouds. Interactions between convection
and the large-scale circulation are expected to be sensitive to the vertical profile of heating.
Several field studies have indirectly derived vertical profiles of convective heating in a few locations as large-scale residuals using radiosonde-observed temperature tendencies and advection
rates (cf. Reed and Recker, 1971; Thompson et aI., 1979). But a large-scale depiction of variations in heating profile does not yet exist. Furthermore, the heating profile of a convective cluster
differs between the convective and stratiform anvil regions (Johnson, 1984) and hence changes
with time over the life cycle of a convective system. The heating profile may also be different
for different types of convective structures (isolated cells, squall lines, non-squall clusters, etc.),
but information is lacking.
Several TRMM instruments (PR, TMI) will produce information on the vertical profile of hydrometeors of different types. Tao et aI. (1990, 1993) have developed a procedure for estimating
the vertical profile of convective heating from this information. The procedure makes use of a
cumulus ensemble model (CEM), i.e., a model that explicitly resolves individual cumulus clouds
over a limited spatial domain. The CEM is an anelastic model with parameterized Kessler-type
two-category liquid phase microphysics (cloud water, rain) and parameterized three-category
ice microphysics (cloud ice, snow, hailjgraupel). The CEM runs in either 2 or 3 dimensions
with typically 0.5-1.5 km horizontal resolution and 0.2-1.05 km vertical resolution.
The CEM produces its own simulation of both heating profiles and hydrometeor profiles. Based
on these simulations, Tao et aI. have developed a parameterization of the former using the
latter as inputs. The scheme ignores convective-scale heat transport and the vertical and
horizontal transport of hydrometeors. With these approximations, the apparent heat source
due to convection Q1 can be written in terms of the microphysical sources and sinks as
where the terms on the right hand side represent condensation of rain, condensation of cloud
liquid water, deposition of snow and hailjgraupel, deposition of cloud ice, evaporation of rain,
and melting of snow and hailjgraupel, respectively. Each of these is parameterized on the basis
of features of observed hydrometeor profiles (e.g., the vertical gradient of rainwater is related
to rain evaporation, while the surface precipitation rate constrains the net condensation and
deposition) .
An alternative latent heating algorithm makes no use of observed hydrometeor profiles. Instead,
it assumes that a limited number of heating profiles characterize different types of convective
systems in different geographic regions; for each type a convective and stratiform heating profile