United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-85/056 Dec. 1985
&EPA Project Summary
Atmospheric Diffusion
Modeling Based on Boundary
Layer Parameterization
J. S. Irwin, S. E. Gryning, A. A. M. Holtslag, and B. Sivertsen
The conclusions of a work group are
presented in this project summary,
outlining methods for processing me-
teorological data for use in air quality
diffusion modeling. To incorporate the
proper scaling parameters, the discus-
sion is structured in accordance with
the current concepts for the idealized
states of the planetary boundary layer.
A number of diffusion models are
recommended, the choice of which
depends on the actual idealized state of
the atmosphere. Several of the models
characterize directly the crosswind in-
tegrated concentration at the surface,
thus avoiding, whenever justified, the
assumption of a Gaussian distribution
of material in the vertical. The goal of
this study was to characterize the
meteorological conditions affecting the
diffusion for transport distances of 10
km or less. Procedures are suggested
for estimating the fundamental scaling
parameters. For obtaining the meteoro-
logical data needed for these estima-
tions, a minimum measurement pro-
gram to be carried out at a mast is
recommended. If only synoptic data are
available, methods are presented for
the determination of the scaling param-
eters. Also, methods are suggested for
estimating the vertical profiles of wind
velocity, temperature, and the variances
of the vertical and lateral wind velocity
fluctuations.
This Project Summary was developed
by EPA's Atmospheric 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
The purpose of this discussion is to
outline a set of methods for processing
meteorological data for use in diffusion >
modeling. The emphasis is on those
methods considered both physically real-
istic and numerically efficient.
Most of the early attempts to estimate
the diffusion of air pollutants were based
on the Gaussian-plume model. These
models gave simple rules for obtaining
the lateral spread based on wind-direction
trace data and suggested that the effects
of thermal stratification in the lower
atmosphere be represented in broad
categories of stability, defined in terms of
meteorological data routinely available in
surface weather observations.
By the early 1970s, air quality simula-
tion models were viewed as a means of
estimating the relative magnitudes of
concentration distributions from various
sources and thereby providing a rational
basis for strategies leading to air quality
improvement or maintenance. (Most of
the air quality simulation models devel-
oped in response to these modeling
requirements were based on the general
Gaussian-plume model.) Invariably the
models were constructed assuming that
the dispersive characteristics of the at-
mosphere were vertically and horizontally
homogeneous.
Current concepts regarding the struc-
ture of an idealized boundary layer are
briefly reviewed, and the basic meteoro-
logical variables of interest to diffusion
modeling are identified. Methods are
proposed for specifying each of these
basic variables. The goal is to characterize
the meteorological conditions affecting
the diffusion for transport distances on
the order of 10 km or less over reasonably
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flat and homogeneous terrain. In this
description of turbulence, the effects of
clouds and fog are only considered in
regard to radiation and surface energy
balance. The relationship and importance
of the variables to diffusion processes are
reviewed.
Results and Discussion
Atmospheric diffusion is controlled by
the turbulence in the air. The parameters
describing the scales of turbulence are
therefore of fundamental importance in a
description of atmospheric diffusion. Most
air pollution sources emit into the bound-
ary layer, which can be defined as the
lower layer of the atmosphere, where the
influence of the surface is present due to
friction. The character and turbulent state
of the boundary layer is strongly affected
by the diurnal heating and cooling cycle.
The unstable boundary layer is directly
affected by solar heating of the ground.
This layer has a very pronounced diurnal
variation and typically reaches a height of
1 -2 km over land in the summer. For the
characterization of turbulence within this
layer, three length scales are important:
(1) the mixing height, z,; (2) the height
above the ground, z; and (3) the Monin-
Obukhov stability length, L. The mixing
height, z,, defines the height above the
surface in which pollutants are mixed by
presence of turbulence. From these three
parameters, two independent dimension-
less parameters can be formed. The
unstable boundary layer can be divided
into several layers defined in terms of
these parameters:
surface layer (z0 ^z < Min-(0.1z,;-L));
free convection layer (-L < z< 0.1 z,);
mixed layer (0.1 z, < z < 0.8z, and z/-L
>D;
entrainment layer (0.8z, < z < 1.2z,);
near neutral upper layer (0.1z, < z <
0.8z, andz/|L| <1).
Traditionally, the boundaries between the
individual layers have not been expressed
in the same set of dimensionless param-
eters. Here, some of the detailed structure
that arises from the traditional way of
describing the boundaries has been sacri-
ficed. The proposed simplified version in
some cases slightly deviates from the
viewpoints traditionally held of the bound-
aries.
The stable boundary layer is created by
cooling of the air adjacent to the ground.
The depth of the inversion layer, h, is
often taken as the height at which the
(negative) heat flux has fallen to a certain
(low) proportion of its surface value. The
study of the stable boundary layer is
much less advanced than its unstable
counterpart. As buoyancy forces suppress
the turbulence under stable conditions,
the magnitude of fluctuations generally is
very low and consequently difficult to
measure. Also the structure of the turbu-
lence is masked by other physical pro-
cesses, such as gravity waves, drainage
and slope flows, intermittent turbulence,
and radiation divergence, that are sup-
ported in a stable atmosphere. The co-
existence of these processes and turbu-
lence complicate the interpretation of the
data. No simple relation exists between
the depth of the surface inversion, h, and
the mixing height, z,, through which the
turbulence exchange processes take
place. The depth, z,, of the turbulent
stable boundary layer can become pro-
gressively smaller at the same time as the
depth of the surface based inversion h
grows. The stable boundary layer can be
divided into two layers:
surface layer (z0 •=€ z, Min (0.1z,;L));
and
local and z-less scaling region (Min
(0.1 z,;L) < z < z, and z/L < 10).
Methods are discussed for specifying
the surface roughness length, the surface
fluxes of sensible heat and momentum,
the Monin-Obukhov length, the mixing
height, and the inversion height. Follow-
ing the discussion on the basic state
parameters of the atmospheric boundary
layer, methods are discussed for esti-
mating the profile of horizontal wind, the
profile of temperature, and the profiles of
horizontal and vertical standard devia-
tions of the wind speed fluctuations. For
each of the seven regimes identified for
the unstable and stable boundary layer,
methods are presented for estimating the
surface concentrations from nonbuoyant
releases of nondepositing material.
Conclusions and
Recommendations
An outline of methods for processing
meteorological data for use in diffusion
modeling is presented. To incorporate the
proper scaling parameters, the discussion
was structured in accordance with cur-
rent concepts for the idealized states
(regimes) of the planetary boundary layer.
The goal was to characterize the meteoro-
logical conditions affecting the diffusion
for transport distances of 10 km or less.
The distance to the maximum ground-
level concentration is often within this
range.
The diffusion is routinely found to be
other than Gaussian in the vertical.
Therefore, whenever justified, the rec-
ommended techniques characterize di-
rectly the crosswind integrated concen-
trations at the surface. For elevated
releases within the near neutral upper
layer or in the stable local scaling region,
use of the Gaussian-plume model is
recommended, where the dispersion
parameters are estimated using statistical
methods. Little is known regarding diffu-
sion within the entrainment layer.
A special complication in the use of the
suggested methods arises when the
proper state of the diffusion process is at
the border line between two regimes, in
which, there is a jump in estimated
concentration dependent on which re-
gime is chosen. No procedures have been
devised to avoid these jumps in calculated
concentration.
For obtaining the meteorological data
needed for estimating the fundamental
meteorological scaling parameters, a
measurement program is presented. The
measurements should be carried out at a
mast. The upper measurement level
should be at a height 100z0, but not less
than 10 m. The lower measuring level
should be at 20z0, but not less than 1 m^
Here, z0 is the effective surface roughness!
length. Also discussed in this report are
methods for determining the fundamental
scaling parameters if ony synoptic mete-
orological data are available.
It is anticipated that the suggested
characterizations of the diurnal variation
of the wind speed and direction profiles
are too simplistic and will need further
development. Furthermore, the turbu-
lence profiles are highly idealized as the
assumption is made that above the mixed
layer, the turbulence is negligible. To
complete the development of a meteoro-
logical processor, the methods outlined
should be tested using available data.
With the test results, future research can
focus on those estimation methods re-
quiring the greatest improvement and on
developing methods to characterize the
spatial variations in the meteorologica
variables.
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The EPA author J. S. Irwin (also the EPA Project Officer, see below) is with the
Atmospheric Sciences Research Laboratory. Research Triangle Park, NC
27711; S. E. Gryning is with the Ris6 National Laboratory, Roskilde, Denmark;
A. A. M. Holtslagis with the Royal Netherlands Meteorological Institute. Debilt,
The Netherlands; and B. Silversten is with the Norwegian Institute for Air
Research, Lillestrdm. Norway.
The complete report, entitled "Atmospheric Diffusion Modeling Based on
Boundary Layer Parameterization," (Order No. PB 86-103 660/AS; Cost:
$11.95, 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, NC27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-85/056
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