United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/010 Apr. 1985
SERA Project Summary
The Vertical Redistribution of a
Pollutant Tracer Due to
Cumulus Convection
J. A. Ritter and D. H. Stedman
Mathematical formalisms that incor-
porate the physical processes respon-
sible for the vertical redistribution of a
conservative pollutant tracer due to a
convective cloud field are presented.
Two modeling approaches are pre-
sented differing in the manner in
which the cloud fields are forced. In
the first or implicit approach, the ver-
tical cloud development is limited by
the satellite observed value, and cloud
forcing is determined from synoptic-
scale heat and moisture budgets. In
the explicit approach, the vertical
development is similarly limited, but
the forcing functions are obtained by
explicitly incorporating the vertical
distribution of cumulus cloud cover,
thereby dynamically incorporating the
influences of sub-synoptic scale
phenomena. The two approaches give
internally consistent results and give
similar results for the convective mass
flux. The manner in which the upward
mass flux is apportioned to the
various cloud classes, however, dif-
fers as consequence of the different
vertical profile of forcing functions
used. The explicit model gave more
reasonable profiles but the predictions
are highly sensitive to input condi-
tions. The implicit model, was
somewhat less sensitive to its input
parameters if the data are prepared
judiciously. This study shows that the
concentration increase in the cloud-
layer due to the venting action of
cumulus clouds can be as, if not more
important than, the in-situ production
and this process should therefore be
incorporated in regional-scale
transport models.
This Project Summary was
developed by EPA's Atmospheric
Sciences Research Laboratory,
Research Triangle Park, NC, to an-
nounce 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 U.S. EPA is undertaking the devel-
opment and validation of a regional scale
photochemical model (ROM). Extensive
experimental field studies have been car-
ried out to provide the experimental bases
for the complex processes that the ROM
must treat, and to provide a suitable
model validation observational data base.
This was accomplished as part of the
NEROS (Northeast Regional Oxidant
Study). In this model approach, it was
recognized that transport and transforma-
tion will occur as a result of dynamic ac-
tivity of cumulus cloud convection. Pro-
visions were made to develop a computer
module that would address this process,
which heretofore has been relatively ig-
nored in transport and dispersion models.
This report describes the model develop-
ment program that addresses the trans-
port aspect of the cloud venting process.
The procedure utilizes satellite derived
cloud top height distribution data. Two
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alternative approaches were investigated
and their strengths and shortcomings
identified. The results of either approach
provides a prediction of the vertical
distribution of cloud flux. Initial results in-
dicate the effect of venting by cumulus
clouds is comparable to chemical transfor-
mation rates.
Approach
Statistical data of total cloud amount,
cumulus cloud amount, and cumulus
cloud top height for certain regions and
dates are generated and prepared as input
to the regional-scale photochemical oxi-
dant model of air pollution from NEROS II
digitized satellite data. These statistics are
used to parameterize the presence of
sunlight for photochemical reactions and
to diagnose vertical transport of
pollutants. Digital satellite data transmit-
ted from the Geostationary Operational
Environmental Satellite (GOES) were
analyzed with the help of Colorado State
University's (CSU) Interactive Research
Imaging System (IRIS) to generate the re-
quired diagnoses of total cloud amount,
cumulus cloud amount, and cumulus
cloud-top height. Synoptic rawinsonde
data also were used to translate cloudtop
temperatures to cloud-top heights. The
IRIS also produced the quantitative cloud
field statistics necessary to manipulate the
digital data. These data along with the
synoptic temperature and moisture field
and surface meteorological data were
used as input for the cloud venting
module. The frequency distribution of
cumulus cloud-top heights, provided by
CSU in terms of IR pixel counts for each
500 feet layer from 500 to 20,000 feet,
was transformed into a fractional cloud-
cover distribution as a function of height
using a regressional analysis technique.
This analysis related the observed frac-
tional cumulus cloud cover for a given
grid cell to the vertically integrated IR pix-
el count for that cell in terms of a second-
order equation.
The rawinsonde data were truncated at
the 500-mbar level and objectively inter-
polated in space; a scale-dependent filter-
ing technique was employed for the scalar
fields of temperature and moisture, and a
variational analysis technique was
employed to compute the vector wind
fields. All of the subsequent fields were
then linearly interpolated in time to ob-
tain, for each grid cell, a vertical profile
for each of the required variables on an
hourly basis.
Standard hourly meteorological data
were used to calculate cloud-base heights
for each grid cell. The reported
temperature and moisture values, given at
3-hour intervals, were spatially inter-
polated using a scale-dependent filtering
technique and temporally interpolated
with a standard cubic spline routine.
The two modeling approaches
presented in the study are both based on
the same framework; however, the first
approach (implicit method) requires that
the cloud forcing functions be determined
from the synoptic-scale heat and moisture
budgets and that the maximum cumulus
cloud-top height for each grid cell be
specified. The second approach (explicit
method) determines the cloud forcing
functions for each grid cell from the fre-
quency distribution of cumulus cloud-top
heights corresponding to that cell.
Assuming mass conservation within a
given grid cell the following calculations
were carried out using both the implicit
and explicit formulations:
a) Vertical mass flux distribution
resulting from cumulus updrafts, the
induced mass flux in the cloud en-
vironment that compensates for the
updraft mass flux, and the synoptic-
scale mass flux.
b) The contribution to the total cloud-
base mass flux as a function of
cloud-top height.
c) The increase in concentration as a
function of pressure-height of a
passive tracer in the cloud layer due
to cloud venting.
Test cases were run for circumstances
of weak, moderate, and strong, but non-
precipitating convection. The results in-
dicated a reasonable degree of internal
consistency between both the implicit and
explicit modules in that more mass flux is
obtained for larger and more extensive
cloud fields in both approaches. These
model analyses reveal that the rate of
concentration increase in the cloud layer
of a passive tracer due to the cloud vent-
ing process can be more important than
in situ photochemical production and
should, therefore, not be ignored. The
results also show that satellite data can be
used to close the "spectral gap" that is
present when only synoptic-scale rawin-
sonde data are used.
This report includes a documented test
case organized and presented together
with the code modules to facilitate its
use.
Because it is very difficult to test a
cloud venting module quantitatively, it is
particularly important to perf
diagnostic type sensitivity analyses.
Parameter
Pertinent Mo
1.
2.
3.
4.
5.
6.
7.
8.
Fractional updraft
area (5)
Cloud half-life
parameter (TMN)
Ratio of entrain-
ment (AH/Zi) zone
depth to mixed
layer depth
Radiative heating
(On)
Sensible heating
rate (Q2)
Latent heating rate
Vertical distribution
of cloud liquid
water content (qf)
Vertical gradient of
the entrainment
rate (d X D/dp)
explicit
explicit
explicit
implicit
implicit
implicit
explicit/
implicit
explicit/
implicit
The sensitivity of the explicit mo
prediction was found to be highly de
dent on the fractional updraft area of
, convective element. The validity of
assumption of a constant ratio of up<
to total cloud area for all clouds of di
ing vertical extents, has not been
amined. Other important issues that r
to be addressed, which involve both
implicit and explicit formulations, con
the appropriateness of employing the
dimensional entraining plume model
proach and, as in all subgrid-s
models, the proper' treatment of
meteorological data to give meanir
subgrid-scale parameters. The use
satellite data has helped to bridge
spectral gap.
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J. A. Ritter is presently with NASA Langley Research Center, Hampton. VA 23665,
and D. H. Stedman is with the University of Denver, Denver, CO 80208.
Jason K. S. Ching is the EPA Project Officer (see below).
The complete report, entitled "The Vertical Redistribution of a Pollutant Tracer
Due to Cumulus Convection," (Order No. PB 85-172 971 /AS; Cost: $16.00,
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
U.S. GOVERNMENT PRINTING OFFICE: 1985-559-016/27037
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