United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-88/011 May ^ 988
&ERA Project Summary
Parametric Methodologies of
Cloud Vertical Transport for
Acid Deposition Models
F. M. Vukovich and R. C. Haws
A CUmulus VENTing (CUVENT)
cloud module has been develbped that
calculates the vertical flux of mass
between the boundary layer and the
cloud layer by an ensemble of nonpre-
cipitating cumulus clouds. This model
has been designed to be integrated into
the Regional Acid Deposition Model
(RADM) to establish the effect of cloud
venting in that model. In the first phase
of this project, using data obtained
during the VENTEX field piogram. a
parameterization scheme was devel-
oped for the cloud model so that it may
be incorporated directly into the
RADM. This parameterization scheme
uses basic meteorological data to
predict the convective cloud amount at
cloud base and cloud distribution
parameters. In the second j phase, a
number of improvements and changes
were made to the CUVENT algorithm
so that it may interface with the RADM
by being integrated into the Regional
Scavenging Module (RSM). The
changes included the incorporation of
sidewall detrainment and the develop-
ment of limiting conditions for the
existence of nonprecipitating cumulus
clouds relative to certain seasons, of
a realistic cloud liquid water profile, and
of a simplified model to estimate cloud
base cloud amount. In order to meet
the requirements of the RSM, CUVENT
provided information about a single
cloud, the ' 'processor cloud," that
represented the ensemble of nonpre-
cipitating cumulus clouds. In the final
phase, a "table look-up" version of
CUVENT was developed in order to
significantly reduce the amount of the
computer execution time. The new and
improved version of CUVENT that was
developed in the second phase was
used to establish the tables of parame-
ters needed for the processor cloud.
The tables were developed for five
characteristic atmospheres and ten
cloud amounts.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory. Research Trian-
gle Park, NC, to announce key findings
of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The primary purpose of this research
project was to develop a cloud model
(CUVENT) that calculated the vertical flux
of mass out of the planetary boundary
layer into the cloud layer due to an
ensemble of nonprecipitating, subgrid
scale, air mass convective clouds.
CUVENT, by design, is a submodule to
the Regional Acid Deposition Model
(RADM), which is being developed by the
National Center for Atmospheric
Research/State University of New York,
and will serve to establish the effects of
cloud venting for the RADM.
Approach
CUVENT was established in three
separate phases. These are summarized
below:
a. Phase 1:
The first phase consisted of two major
steps:
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1. the selection and modification of a
model to calculate the vertical trans-
port of mass due to an ensemble of
nonprecipitating, subgrid scale, air
mass convective clouds; and
2. development of a closure procedure
for that cloud model that is consist-
ent with the RADM.
The cloud model that was selected for
CUVENT determines acceptable cloud
classes that are defined by entrainment
rates which are functions of atmospheric
state. This model assigns the entrain-
ment rate to a particular cloud size, which
is represented by cloud depth and by a
fractional cloud amount. The greatest
entrainment rate is assigned to those
clouds having the least depth, and the
smallest entrainment rate, to clouds
having the greatest depth. The percent
of area covered by each cloud class is
also determined. G iven these parameters
and the primary forcing functions, heat
and moisture convergence, the model
computes the vertical flux of mass due
to the ensemble of nonprecipitating
cumulus clouds.
This model, called CUVENTI, was
originally developed to be applied using
observed data. Radiosonde data were
used to define the moist static energy
distribution, and satellite data were used
to determine the cloud parameters.
However, RADM operates in a predictive
mode and is self-consistent. Incorpora-
tion of observed data will produce
unacceptable errors. Since CUVENTI is
to be integrated into the RADM, it also
must be self-consistent in the sense that
it can only use data provided to it by the
RADM. Therefore, a procedure had to be
developed to adapt the cloud model to
the predictive mode; i.e., the closure
procedure, RADM provides data through
which the moist static energy distribution
can be calculated. Therefore, the basic
aspects of the closure procedure con-
sisted of three major parts:
1. A test for moist convergence;
2. The estimation of the cloud base
convective cloud amount;
3. The development of the convective
cloud amount as a function of cloud
classes.
The test to determine whether moist
convection existed at a particular time
consisted of two parts:
1. A test to determine whether the
forcing function, the surface heat
flux, was consistent with convection;
and
2. A test to determine whether mois-
ture conditions were consistent with
moist convection.
A fundamental part of this test cen-
tered around the relative location of the
entrainment zone and the lifting conden-
sation (LCL) zone.
If the atmosphere supports moist
convection based on tests discussed in
the above paragraph, then the convective
cloud amount was determined. A statis-
tical model was developed which esti-
mates the convective cloud amount at
cloud base using basic meteorological
parameters. The data that were used to
develop the statistical model were
obtained during the National Acid Pre-
cipitation Assessment Program (NAPAP)
venting experiment (VENTEX) that took
place near Lexington, Kentucky, in the
summer of 1984. Surface, upper air,
sodar, laser, and aircraft measurements
were made to characterize the lower
atmosphere during current cumulus
convection and to characterize the
cumulus clouds in terms of the cloud
amount, the cloud depth, and the cloud
width. The measurement program was
a joint effort by Battelle Pacific Northwest
Laboratories, Argonne National Labora-
tory, and the Research Triangle Institute.
The statistical model for the cloud
amount consisted of terms that charac-
terized the major forcing function for air
mass convective clouds: the surface heat
flux and the moisture parameters in the
boundary layer. The major parameters
were the dew-point depression at the
surface and at the top of the boundary
layer, the surface heat flux and the time
rate of change of the surface heat flux,
the height of the lifting condensation
level, and the height of the top of the
boundary layer. One hundred and five
separate comparisons were used to
construct the model; the resulting corre-
lation for these comparisons was 0.81.
As previously discussed, the cloud
classes were defined by an entrainment
rate, as well as the cloud depth. In order
to establish the cloud amount for a given
cloud class, it was necessary to assume
that the cloud depths are continuous over
the spectrum of cloud classes, that the
cloud depth is a function of the cloud
width for nonprecipitating convective
clouds. Utilization of these assumptions
permitted development of a mathemat-
ical expression that established the cloua
amount as a function of the cloud depth.
The system of equations that defined
thermodynamics and dynamics of the
cloud processes and defined the parame-
ters necessary to solve the cloud model
produced a self-consistent cloud flux
model (CUVENT) which defined the
vertical mass flux due to an ensemble
of nonprecipitating air mass convective
clouds. The establishment of CUVENT in
this matter made it consistent with the
predictive mode of the RADM.
b. Phase 2:
In the second phase of this research
project, a number of improvements and
changes were made to CUVENT. These
included:
1. the incorporation of sidewall
detrainment;
2. the development of a test which
limits conditions for the existence of
an ensemble of nonprecipitating air
mass clouds to certain seasons;
3. the development of a more realistic
cloud liquid water profile for
CUVENT; and
4. the development of a simplified
model to estimate cloud amount at
cloud base.
In CUVENTI, the first generation ver-
sion of CUVENT, detrainment took place
at the cloud top only. Under these
conditions, the maximum predicted
vertical velocity in the cloud was found
near cloud top and the vertical velocity
went rapidly to zero at cloud top. Most
observations of the vertical velocity
distribution in cumulus clouds have
indicated that the maximum vertical
velocity is generally found midlevel in the
cloud. Such vertical velocity distribution
can be obtained by incorporating sidewall
detrainment in the cloud model.
Cloud liquid water is needed in
CUVENT in order to calculate the updraft
moist static energy, and it is essential
in the solution for each cloud class. In
this model, the vertical distribution of
cloud liquid water was provided as a data
set that was derived from the literature.
The initial statistical model which was
used to estimate the convective cloud
amount at cloud base for CUVENTI used
two terms which might have complicated
the integration of CUVENT into the
RADM. One of these terms was the time
rate of change of surface heat flux. In
order to calculate this term, it was
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required that the surface heat flux from
the previous hour is saved in memory
at each RADM grid point. Furthermore,
the precision and accuracy needed for
the term that involved the difference in
height between the top of the entrain-
ment zone and the bottom of the lifting
condensation zone was questionable
using RAOM meteorological data. In
order to remove the complication of
requiring a memory array for the surface
heat flux and in dealing with the ques-
tionable accuracy of the term involving
the height difference, it was decided that
both these terms should be removed from
the model and a new model be developed
whose terms are physically consistent
with fair weather cumulus, but are more
practical and have no need for storage
capabilities. Such a model was developed
and the terms in the model were the
dewpoint depression at the top of the
boundary layer, the surface heat flux, the
height of the lifting condensation level,
and the free convection scaling velocity.
This model provided a correlation coef-
ficient with observations of 0.79.
The air mass cumulus clouds which
are accounted for by CUVENT have a
definite warm season preference. This
preference was used to limit utilization
of CUVENT. In order to develop this warm
season preference and to determine
restriction limits, a frequency analysis
was established which matched the
existence of nonprecipitating cumulus
clouds with the surface dry bulb temper-
ature and the surface dewpoint temper-
ature. Based on this analysis, it was
decided to use the 80 percentile from the
cumulative probability distribution to
define the restriction limit for cumulus
clouds. The 80 percentile value provided
a surface temperature of 10 degrees and
a surface dewpoint temperature of 0°C
(i.e., the restriction limit states that there
is an 80 percent probability that nonpre-
cipitating cumulus clouds will exist if the
surface temperature isgreaterthan 10°C
and the surface dew point is greater than
0°C).
CUVENTI was designed to estimate the
vertical mass flux by nonprecipitating
clouds. This configuration for CUVENTI
was developed for its integration with the
RADM. Subsequently, CUVENT was
assigned to be a subroutine to the
Regional Scavenging Model (RSM). In
order to meet the requirements of the
RSM, CUVENT had to provide informa-
tion about a single cloud that could be
used to represent the ensemble of
subgrid scale nonprecipitating clouds.
This single cloud was hereafter referred
to as the "processor cloud." CUVENTI
was modified to develop the information
about the processor cloud that is required
by the RSM. That information included
the updraft vertical velocity distribution,
the updraft temperature distribution, the
updraft specific humidity distribution, the
cloud amount distribution, the cloud
liquid water distribution in the updraft,
the cloud liquid water distribution in
areas outside the updraft, and the
precent area covered by updraft.
After all changes and improvements
were made, the new version of the
CUVENT algorithm was called CUVEN-
TIA. CUVENTIA underwent various sen-
sitivity analyses to examine the behavior
of algorithm as the vertical distribution
of temperature and dewpoint (moist
static energy) change from one atmos-
pheric state to another. Various temper-
ature and dewpoint profiles were used
and results were presented in terms of
the vertical velocity distribution in the
processor cloud.
c. Phase 3:
In the final phase of this research
project, a "Table Look-Up" version of
CUVENT, CUVENTIIA, was developed in
order to reduce significantly the compu-
ter execution time of the algorithm.
CUVENTJftJwas used to develop the
tables of significant parameters for the
processor cloud. The tables were devel-
oped for five characteristic atmospheric
categories and ten cloud amounts. The
distinction between atmospheric cate-
gories was based on the lapse rate and
moisture in the cloud layer. Fifteen
atmospheric states that were character-
ized by their temperature and dewpoint
profiles were examined. The reduction
from fifteen to five categories was based
on the change in cloud parameters for
the processor cloud across the range of
atmospheric states defined by the cate-
gories. These within-category variations
were small for the five chosen categories.
However, compromises had to be made
because there was a restriction on the
acceptable storage requirement that
CUVENT could demand from the RADM
system. These two factors limited the
number of atmospheric categories to five.
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F. M. Vukovich and R. C. Haws are with Research Triangle Institute, Research
Triangle Park, NC 27709.
Jason K. S. Ching is the EPA Project Officer (see below).
The complete report, entitled Parametric Methodologies of Cloud Vertical
Transport for Acid Deposition Models," (Order No. PB88-191 374/AS; Cost:
$32.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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