United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-84-085 Aug. 1984
&EPA Project Summary
A Regional-Scale (1000 km)
Model of Photochemical Air
Pollution: Part 2. Input
Processor Network Design
Robert G. Lamb
Detailed specifications are given for a
network of data processors and sub-
models that can generate the parameter
fields required by the regional oxidant
model formulated in a previous report
(A Regional Scale Model (1000 km) of
Photochemical Air Pollution: Part 1.
Theoretical Formulation, EPA-600/3-
83-035, May 1983). Operations per-
formed by the processor network
include simulation of the motion and
depth of the surface, nighttime radiation
inversion layer; simulation of the depth
of the convective mixed and cloud
layers; estimation of the synoptic scale
vertical motion fields; generation of
ensembles of layer averaged horizontal
winds; calculations of vertical turbu-
lence fluxes, pollutant deposition
velocities, and parameters for a subgrid
scale chemistry parameterization
scheme; and many other functions. This
network of processors and submodels
in combination with the core model
developed in the previous report repre-
sents the EPA's first generation regional
oxidant model.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back}.
Introduction
In a previous report (A Regional Scale
Model (1000 km) of Photochemical Air
Pollution: Part 1. Theoretical Formula-
tion), and hereafter referred to as Part 1, a
theoretical basis was developed for a
model that can simulate all the physical
and chemical phenomena that are be-
lieved to control the fate of photochemical
air pollutants over large time and space
domains. Among the phenomena that the
model was designed to consider are'
1. Horizontal transport
2. Photochemistry, including the very
slow reactions
3. Nighttime chemistry of the products
and precursors of photochemical
reactions
4. Nighttime wind shear, stability
stratification, and turbulence "epi-
sodes" associated with the noctur-
nal jet
5. Cumulus cloud effects, e.g., venting
pollutants from the mixed layer,
perturbing photochemical reactions
rates in the shadows, providing
sites for liquid phase reactions,
influencing changes in the mixed
layer depth, and perturbing hori-
zontal flow
6. Mesoscale eddy effects on urban
plume trajectories and growth rates
7. Terrain effects on horizontal flows,
removal, and diffusion
8. Subgrid scale chemistry processes
resulting from emissions from
sources smaller than the model's
grid can resolve
9. Natural sources of hydrocarbons,
NO,, and stratospheric ozone
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10. Wet and dry removal processes,
e.g., washout and deposition.
In this report, Part 2, detailed plans are
given for transforming the theory of Part 1
iryto an operational model.
To be practicable, the model design
must satisfy several constraints. First, it
must allow continued incorporation of
state-of-the-art techniques without the
need to overhaul the computer code each
time. Second, the architecture of the
model must be amenable to internal
partitioning to facilitate a division of labor
in the model construction and trouble-
shooting operations during model tests.
Third, the design must be capable of
performing simulations with maximal
efficiency. The last requirement is partic-
ularly important due to the unique manner
in which this model is applied. Specifical-
ly, for a given source distribution, the
model is used to simulate the concentra-
tions that would result from each atmos-
pheric flow field in a finite ensemble of
flows; the set of concentration fields that
is produced by this process are used as
the basis for estimating the probability
distribution of concentration values. By
contrast, conventional air pollution
models consider only a single wind field
and the resulting concentration predic-
tions are regarded as the values that one
would expect to observe under the mete-
orological and emissions conditions
considered.
Procedure
The design constraints were met by
structuring the model in a modular form.
Each module contains mathematical de-
scriptions of small groups of individual
physical and chemical processes, and the
modules are interconnected by a network
of communication channels
The principal module, designated
CORE, contains computer language ana-
logues of the differential equations that
describe the governing processes con-
sidered in the development of the model
theory in Part 1. The CORE module is
expressed in a very primitive mathemati-
cal form in the sense that its inputs are
matrices and vectors whose elements are
composites of variables among which are
meteorological parameters, and chemical
rate constants. Module CORE is linked to
a module labeled CHEM, which contains
the analogue of the chemical kinetics
scheme. The communication between
these two modules consists of two
vectors,£ andQ, each of length N, where
N is the total number of chemical species
simulated. The nth element of£ is the net
rate of production of species n due to
source emissions and chemical reactions
among all other species, and the nth
element of Q is the net rate of destruction
of species n due to its chemical interaction
with all other species. Thus, any .chemical
kinetics mechanism can be incorporated
into the model as long as it is expressed in
a form that is compatible with the vector
interfaces that link CORE with the chem-
istry module CHEM.
The remainder of the inputs required by
CORE are prepared by a module desig-
nated BMC (b-matrix compiler). This
module performs essentially the same
task that language compilers perform in
computers. The BMC translates the pa-
rameter fields such as the thicknesses of
each of the model's layers, interfacial
volume fluxes, and horizontal winds into
the matrix and vector elements that are
required by the algorithms in CORE.
The input variables are supplied in turn
by a series of interconnected processors,
several of which are rather complex
models themselves. These processors
generate the wind fields, the interfacial
surfaces that separate the layers, turbu-
lence parameters, source emissions, and
many other variables. Their inputs consist
of information generated by other pro-
cessors in the network and partially
processed raw data.
The processor network consists of both
permanent and interchangeable compo-
nents. The permanent elements are
CORE, which embodies the theory de-
veloped in Part 1, the BMC, and the
communication channels. All other pro-
cessors including the chemical kinetics
module CHEM are interchangeable com-
ponents of the network. Any or all of the
interchangeable elements can be replaced
by other modules as long as they are
compatible with the communication
channel interfaces.
The Project Report on which this Sum-
mary is based provides detailed specifica-
tions for each of the processors in the
network. As new techniques become
available for estimating meteorological,
chemical, and pollutant deposition param-
eters, and as information is acquired on
the accuracy of the procedures that form
the basis of the current processor designs,
the interchangeable components of the
system will eventually be replaced by
more refined methods in the course of
developing a second-generation regional
oxidant model.
Conclusions
The regional oxidant model theory
developed in Part 1 has been implemented
in the form of a network of discrete
processors. All processors except the
central CORE processor can be inter-
changed with other modules that perform
similar tasks. With this design, state-of-
the-art techniques can be incorporated
into the model with minimal effort, errors
can be isolated relatively quickly during
troubleshooting operations, and a high
level of computational efficiency can be
achieved in multiplesimulation studies in
which only a few of the many input
parameter fields vary from one simulation
to another.
Detailed plans are given for each
processor based in part on currently
available techniques for estimating
meteorological and chemical parameters
and in part on newly formulated proce-
dures. Among the latter is a method for
estimating vertical material fluxes in
cumulus clouds, a scheme for handling
lateral boundary conditions in numerical
solution of the shallow water equations, a
procedure for initializing the concentra-
tions of each of the simulated pollutant
species, and a method for generating
ensembles of wind fields. Quantitative
analyses of the performance of individual
processorsandthefull model itself will be
presented in subsequent reports.
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Robert G. Lamb (also the EPA Project Officer, see below) is with Environmental
Sciences Research Laboratory, U.S. Environmental Protection Agency, Re-
search Triangle Park, NC 27711.
The complete report, entitled "A Regional-Scale (1000 KM} Model of Photo-
chemical Air Pollution, Part 2. Input Processor Network Design," (Order No. PB
84-232 651; Cost: $25.00, 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:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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