United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S8-88/004 Mar. 1988
Project Summary
User's Guide to the CTDM
Meteorological Preprocessor
(METPRO) Program
Robert J. Paine
The Complex Terrain Dispersion
Model (CTDM) Is a refined air quality
model for use in stable and neutral
conditions in complex terrain
applications. Its use of meteoro-
logical input data and terrain
information is different than current
EPA models; considerable detail for
both types of input data are required
and are supplied by preprocessors
specifically designed for CTDM. This
User's Guide presents a review of the
structure of the atmospheric
boundary layer and its Implications
for the design of CTDM and its
meteorological preprocessor
(METPRO). The CTDM meteorological
preprocessor calculates required
meteorological variables that are
derived from conventionally available
data. These required variables
include the Monin-Obukhov length,
the surface friction velocity, the
surface roughness length, and the
mixed layer height. The CTDM input
data files contain these values as
delivered by the meteorological
preprocessor.
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 requirement for use of
meteorological information in CTDM is
based upon the current understanding of
the Atmospheric Boundary Layer. A
discussion of the design of the
meteorological preprocessor.METPRO,
and its linkage with CTDM is
accompanied here by a summary of the
features of the boundary layer and how
they relate to plume dispersion
calculations.
The Atmospheric Boundary Layer lies
between the earth's surface and the
geostrophic free atmosphere, in which
surface effects upon the flow are
negligible. The boundary layer can be
considered to contain two distinct layers:
the surface layer near the ground,
capped by a mixed layer.
The nocturnal "mixed" layer features
low turbulence levels and, often, laminar
flow, so the term "mixed" is misleading.
It is used here to mean the layer above
the surface layer, a usage consistent with
the daytime case. The surface layer is
dominated by the frictional force and
horizontal shear stress near the ground.
The horizontal stress is caused by the
drag of friction-retarded air molecules
on faster-moving air molecules at higher
levels. The depth of the surface layer, h,
is defined to be the height above the
ground through which the magnitude of
the shear stress is approximately
constant (varies by no more than about
10%). Other properties of the surface
layer that are useful for modeling
purposes are:
• vertical fluxes of heat and momentum
that are nearly constant with height,
and
• steady-state and horizontally
homogeneous temperature and
velocity fields.
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These assumptions allow the vertical
structure of the surface layer to be
parameterized by similarity theory,
developed for this purpose by Monin and
Obukhov.
Similarity theory assumes that the
vertical fluxes of heat, momentum and
moisture are approximately constant
throughout the surface layer. The
resulting turbulence of the flow within the
surface layer is solely determined by the
mean temperature, T, the friction velo-
city, u* (derived from the vertical mo-
mentum flux), and the sensible heat flux,
H. These parameters are combined into
a length scale referred to as Monin-
Obukhov length, L. This length scale is
composed of quantities that are
approximately constant throughout the
surface layer, and is an important length
scale governing diffusion and profiles of
wind, temperature, and turbulence in the
surface layer. As such, it is a useful
substitute for the discrete stability class
value used by air quality models
developed in the past.
During unstable conditions, the height
of the mixed layer, drive by convection,
can be defined as the layer through
which the potential temperature is less
than that of the heated layer at the
surface, from which convective
"thermals" or updrafts originate. The
updrafts lose their buoyancy when their
potential temperature becomes colder
than that of the surrounding air of the top
of the mixed layer (z\). The daytime
mixed layer can grow rapidly in response
to a steadily increasing surface heat flux.
During the afternoon, z, reaches a
maximum and remains relatively
constant as the surface heat flux attains
its peak value.
Near sunset, an abrupt transformation
of the Atmospheric Boundary Layer
occurs as the heat flux throughout the
entire layer turns negative rapidly. The
surface layer becomes stably stratified
while the mixed layer above remains
relatively unstable, at least initially. In
some models of the boundary layer, the
nocturnal case features a "mixed" layer
above the surface layer with
supergeostrophic wind speeds - the
well-known low-level nocturnal jet
phenomenon. This jet develops when the
mixed layer becomes decoupled from
the surface layer near sunset and surface
friction is then not effective up to as
great a height as during the daytime. The
balance of forces on the stable mixed
layer is then disturbed, and the wind can
accelerate because the pressure force
now has a component in the direction of
the wind (which was directed toward
lower pressure because of the friction
force during the daytime). As a result,
supergeostrophic wind speeds can occur
above the nocturnal surface layer in the
low-turbulence, laminar flow of the
mixed layer. Of course, large-scale
mechanisms such as the influence of low
pressure areas and warm or cold fronts
can dominate and prevent the low-level
jet from appearing. In general, it is
difficult to predict with certainty the onset
and strength of the nocturnal jet.
The predictability of the structure of
the low-turbulence nocturnal mixed
layer is further complicated by the
occurrence of momentum burst
phenomena during conditions that favor
the onset of the nocturnal jet. A useful
parameter for determining the likelihood
of the breakdown of the laminar flow of
the nocturnal mixed layer is the
Richardson number, Ri. Large values of
Ri (>1) are associated with stable
conditions, while low values (<0.15) are
present when mechanical turbulence due
to wind shear overcomes the resistance
to turbulence motion presented by a
thermally-stable atmosphere. A review
of investigations into the critical vatue of
Ri for the breakdown of laminar flow into
turbulent motion yields a value of 0.25.
Observations in wind tunnels and the
atmosphere show that the critical value
varies, with turbulence being certain if Ri
< 0.15 and absent for Ri > 0.5.
During low-level jet periods, the
laminar flow in the stable mixed layer can
break down if the speed shear between
the turbulent surface layer and laminar
mixed layer aloft results in a Richardson
number favoring the breakdown. The
momentum in the mixed layer is then
transported to the surface in a "burst,"
resulting in a temporary absence of the
low-level jet. The Richardson number
then can become large again, perhaps
allowing the laminar flow in the mixed
layer to become re-established. The
low-level jet disappears in the morning
when convective mixing transports
momentum away from the jet and
smooths out the vertical momentum
distribution.
It is evident from the summary
presented above that while the nocturnal
surface layer, like the daytime, surface
layer, is reasonably well-behaved, the
nocturnal mixed layer is highly
unpredictable, even in flat terrain.
Therefore, CTDM relies heavily upon
direct measurements of wind,
temperature, and turbulence in the
nocturnal mixed layer. The role of the
meteorological preprocessor, METPRO,
is two-fold:
• deliver to CTDM observed and4
predicted values of the nocturn
surface layer length, h, and th
daytime mixed layer height, z\;
• compute values of u*. L, and th
surface roughness length, z0, so th
CTDM can supplement direi
measurements in the surface layi
with computed profiles of wim
temperature, and turbulence.
Summary of METPRO
Operation
METPRO can accept inpi
meteorological data from several sourc<
(rawinsondes, National Weather Servk
data, on-site measurements) ar
produce an output file which contaii
hourly values of mixed layer heigr
friction velocity, Monin-Obukhov lengt
and surface roughness length. Dire
observations of the mixed layer heig
are used when available. Otherwis
upper air data are used in the mix<
layer height calculation after initi
processing. METPRO uses si
characteristics (surface moisture [Bow<
ratio], albedo, and surface roughness)
conjunction with the input meteorologic
data to determine a best estimate f
mixed layer height as well as the frictii
velocity and the Monin-Obukhov lengtl
A variety of theoretical and empirk
techniques are used to determine tl
boundary layer variables calculated I
METPRO. During daytime hours, ,
energy balance method (among latei
ground, and air heating) is used
determine the surface heat flux, which
then used in conjunction with wind a
temperature profile data to estimate z,,
and u*. At night, the downward heat fl
into the ground is a function of t
surface wind speed and cloud cov
Estimates of u* and L in stable conditic
are then used to calculate the height
the stable surface layer.
The site characteristics which i
used in the calculation of heat flux, L a
u* vary as a function of season a
direction. These variations are accounl
for in METPRO with the allowance of
to eight different direction sectc
(angular widths are variable, but mi
sum to 360°) and monthly changes in 1
Bowen ratio, albedo, and surfa
roughness.
Organization of the Manual
In the manual, the theory behind
METPRO program is discuss
separately from the operatioi
instructions for running the prograr
Section 2, which contains a discussior
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^^^»»^*
technical basis for METPRO, need
hot be consulted by users who merely
wish to run the program. Section 4
contains user instructions for METPRO. If
mode 3 of METPRO is to be used
(requiring computation of mixing
heights), then Section 3, which describes
the program (READ62) that decodes the
NCDC's upper air data in TD6201 format,
must be referenced.
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Robert J. Paine is with ERT, Inc., Concord, MA 01742.
Peter L Finkelstein is the EPA Project Officer (see below).
The complete report, entitled "User's Guide to the CTDM Meteorological
Preprocessor (METPRO) Program," (Order No. PB 88-162 1021 AS;
Cost: $19.95) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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
Official Business
Penalty for Private Use $300
EPA/600/S8-88/004
0000329 PS
U S
T*U.S. GOVERNMENT PRINTING OFFICE: 1988-548-013/870
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