United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-86/034 Mar. 1987
Project Summary
Interim User's Guide to the
RADM Scavenging Module
Version 1.0
C. M. Berkowitz and R. C. Easter
The full report is an interim version of
the user's manual for the RADM
Scavenging Module (RSM). The scav-
enging module simulates wet removal
and aqueous-phase chemical processes
as they occur on a regional scale. It is
designed for inclusion into the code of
the Eulerian Regional Acid Deposition
Model (RADM), which does not ex-
plicitly account for processes involving
clouds and precipitation.
Using surface precipitation rates and
vertical profiles of temperature and
moisture for a regional scale grid square,
the scavenging module calculates
steady-state cloud, precipitation and
vertical velocity fields for subgrid areas
representing stratiform, convective and
fair weather cumulus clouds. It then
calculates the effects of aqueous phase
chemistry, precipitation scavenging and
transport on the pollutant fields pre-
sented by the host code for a specified
period (one hour).
The RSM version described in the full
report includes the stratiform and con-
vective storm component; it does not
include the fair weather cumulus (al-
though provisions for including this
component are outlined). A description
of several proposed benchmark tests is
provided; the results of these tests will
be published in future reports.
This Project Summary was developed
by ERA's Atmospheric Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research pro/ecf that Is fully docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
The objective of this work was to
develop a set of generalized subroutines
for NCAR's Regional Acid Deposition
Model (RADM) which would describe wet
removal and aqueous-phase chemical
processes as they occur on a regional
scale. The resulting module was designed
to have sufficient chemical and physical
generality such that alternative pathways
and mechanisms can be evaluated by
numerical experimentation.
The intended host code for the scav-
enging module is assumed not to explicitly
take into account the transport or trans-
formation of pollutants associated with
cloud, rain, or snow. The host code for
the scavenging module is intended to
provide information only on the gas phase
chemistry and associated regional scale
meteorology. Conservation equations for
aqueous phase species are integrated by
a differential equation solver within the
scavenging module. After the scavenging
integration is completed, the aqueous
phase concentration values calculated
within the scavenging module are con-
verted to their clear air equivalents and
these values are returned to the host
code. In its present form, a one-hour
integration period is assumed.
Because the designated host code is
still being developed, the scavenging
module cannot be considered a fixed
product. Assumptions regarding the inter-
face between the RADM Scavenging
Module (RSM) and the RADM should be
recognized as assumptions. Until the RSM
is operational within the RADM, changes
can be expected.
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Major Features
The task of the RSM is to simulate the
effects of clouds and precipitation on
pollutant species for a specified period
(currently one hour) within a "grid-
column" of horizontal dimension 80 x 80
km and vertical dimension of up to 12
km. Depending on whether operator
clouds (representations of the cloud
population) are supported by the environ-
mental conditions, the RSM partitions
the grid-columns into one to four cells.
Pollutant species concentrations are
computed as a function of time, height
and cell by solving a set of conservation
equations for the species.
The three-dimensional conservation
equation for each species relates its mass
concentration evolution (x,y,z,t) to the
horizontal air velocity, vertical air speed,
fall speed for precipitating species, hori-
zontal and vertical turbulence, and gen-
eration and depletion rates for chemical
transformation and physical scavenging.
The meteorological variables are steady-
state and are re-calculated hourly by
concepts of Scott (1978), Scott (1982),
Scott (1986a), Scott (1986b) and Easter
and Hales (1984).
The RSM operates on one RADM grid-
column at a time. All horizontal inter-
actions between grid columns are handled
in the RADM; none occurs in the RSM.
Also, the RSM does not retain the species
and precipitation profiles from the end of
the previous hour. That feature has re-
quired a special initialization procedure
for the time-dependent chemistry in the
RSM.
The final version of RSM will include
stratiform precipitation (Scott, 1986a),
convective precipitation (Scott, 1986b),
and fair-weather cumulus scavenging
(Vukovich, 1986). Each of these cloud
types will be able to exist in the grid-
column at one time. Because the cloud
models are steady-state, the cloud and
precipitation fields do not change during
the one-hour RSM activation although
pollutant fields can change. The non-
convective storm descriptions are one-
dimensional (parameters vary with
height); those for convective storms have
coupled updraft and downdraft cells. The
three cloud models will be used to define
a typical cloud environment in which
aqueous chemistry, scavenging, and wet
removal will be simulated by the RSM.
Figure 1 illustrates the interaction of the
RSM with the meteorological transport
driver MM4 (Anthes, 1985), RADM, and
the three storm/cloud scavenging
models. The precipitation stratiform cloud
(Regional Scale
Meteorology)
(Regional Scale
Meteorology!
Stratiform
Scavenging
Environment
Convective Storm
Scavenging
Environment
Aqueous
Phase
Chemistry
Clear Air
Chemistry
Fair Wh.
Cumulus
Scavenging
Environment
Figure 1. Illustration of calling hierarchy between RADM, and RADM scavenging module
(RSM), MM4, the precipitating stratiform model, the precipitating convective storm
model, and the fair weather cumulus model.
is simulated if a smoothed form of the
RADM/MM4 updraft in conjunction with
RSM microphysics suggests such a cloud
can exist. An overview of the method by
which this is done is included in the full
report; details (including a definition of
the microphysics) are contained in a
companion volume (Scott 1986a).
The code outlined in the stratiform
cloud model user's manual (Scott, 1986a)
is designed to provide profiles of updraft
and cloud parameters consistent with a
specified precipitation rate. This develop-
ment and documentation occurred during
the early stages of the RSM project when
plans called for the RSM to define a
stratiform cloud producing precipitation
at a rate defined by the RADM/MM4
system. This feature has been removed
in the present version of the RSM and
only those sub-routines which calculate
the cloud parameters for a specified up-
draft are used. Complications related to
mass conservation and advection neces-
sitate this removal.
If the RADM/MM4 system specifies
any convectional precipitation, then a
convective storm cloud producing precipi-
tation at a rate equal to that defined by
this specification is simulated. This type
of operator cloud category consists of
two sub-areas: a convective updraft and
an associated downdraft. Precipitation
grows in the former and falls out of the
latter. A set of look-up tables is used to i
define the scavenging environment in
the RSM. An overview of the method by
which the tables were generated is con-
tained in the full report; details are
presented by Scon (1986b).
If no precipitating stratiform cloud can
be supported with the environmental
conditions defined by the host code, then
a second set of subroutines is used to
define an environment characterizing a
fair weather cumulus cloud. The physics
of this operator cloud are defined by
Vukovich (1986).
Depending on which types of clouds
are active, each type occupies a certain
fraction (determined by RSM) of the
horizontal area of the grid-column. The
area occupied by a convective cloud type
is further divided into two cells: updraft
and downdraft cells for the precipitating
convective storm and updraft and sub-
sidence cells for the non-precipitating
convective cloud. As a result, the grid-
column is divided into one to four "zones,"
each having its own cloud, precipitation,
and vertical velocity profiles. The scav-
enging calculations include scavenging,
aqueous chemistry, and vertical transport
within each of these zones and also
transport between zones (dynamic en-
trainment/detrainment effects).
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The RSM includes the following cloud
interconversion processes:
cloud auto-conversion to rain;
accretion of cloud water by rain;
riming of cloud water by snow;
freezing of liquid water;
melting of snow;
evaporation of condensed water; and
aerosol scavenging by rain and snow.
The RSM includes mass transfer
between gas/aerosol and cloud/precipi-
tation forms, mass transfer between dif-
ferent cloud/precipitation forms, and
chemical reactions for the following
species:
gaseous sulfur dioxide and its
aqueous sulfur (IV) forms;
sulfate aerosol, aqueous sulfate;
nitric acid, nitrate aerosol, aqueous
nitrate;
ammonia, ammonium aerosol,
aqueous ammonium;
crustal cation aerosol, aqueous
cation (treated as a lumped mono-
valent cation equal to 2 x [Ca(ll) +
Mg(ll)]);
gaseous and aqueous hydrogen
peroxide;
gaseous and aqueous ozone;
hydrogen ion in cloud and rain
water; and
gaseous and aqueous carbon
dioxide.
Its code, whose detailed characteristics
are described in the report, is in modular
form to accommodate scientific know-
ledge advances, progressive improve-
ments in parameterizations, and inter-
facing with host codes. It is written in
standard FORTRAN 77 code and has not
been prepared for vectorization on an
array-processor. The code contains
diagnostics with software switches to
test removal rates and mass conservation.
The limitations of RSM Version 1.0 and
quality control tests (comparisons of
outputs with PLUVIUS II) are described in
the report. Magnetic tape and hard copy
of the code (and benchmark results when
available) may be obtained from Battelle's
Pacific Northwest Laboratory (PNL) at a
reasonable cost for duplication.
Conclusion
The report on RSM Version 1.0 is an
interim guide for incorporating the code
into the host code RADM. This version of
RSM is currently being revised, but its
general framework is essentially fixed.
Expected changes will be internal and
will not modify the requirements of input
ariables. Future PNL reports will include
nchmark cases and results.
References
Anthes, R.A. (1985) "Summary of
Meteorological Data Sets from PSD/
NCAR Mesoalpha Scale (Regional)
Limited-Area Model." NCARS Ms. 0901-
86-02, National Center for Atmospheric
Research, Boulder, Colorado.
Easter, R.C. and J.M. Hales (1984)
"PLUVIUS: A Generalized One-Dimen-
sional Model for Reactive Pollutant
Behavior, Including Dry Deposition,
Precipitation Formation, and Wet Re-
moval. 2nd Edition." Report No. PNL-
4046. Pacific Northwest Laboratories,
Richland, Washington.
Scott, B.C. (1978). "Parameterization
of Sulfate Removal by Precipitation." J.
Appl. Met. 17,1375-1389.
Scott, B.C. (1982). 'Theoretical Esti-
mates of the Scavenging Coefficient for
Soluble Aerosol Particles." Atmospheric
Environment, 16(7), 1753-1762.
Scott, B.C. (1986a) "User's Manual for
the Cloud and Scavenging Module." In
preparation. Pacific Northwest Labora-
tories, Richland, Washington.
Scott, B.C. (1986b) "User's Guide to
CUVENT." In preparation. Pacific North-
west Laboratories, Richland, Washington.
Vukovich, F. (1986) "User's Guide to
CUVENT." In preparation. Research Tri-
angle Institute, Research Triangle Park,
North Carolina.
C. M. BerkowitzandR. C. Easter are with Battelle Pacific Northwest Laboratories,
Richland. WA 99352.
Jack L. Durham is the EPA Project Officer (see below).
The complete report, entitled "Interim User's Guide to the RADM Scavenging
Module: Version f.O,"(Order No. PB87-102 364/AS; Cost: $ 16.95, 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
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