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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