United States
                    Environmental Protection
                    Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
                     Research and Development
EPA/600/S3-85/010 Apr. 1985
SERA          Project  Summary

                    The  Vertical  Redistribution  of  a
                     Pollutant  Tracer  Due  to
                     Cumulus  Convection
                    J. A. Ritter and D. H. Stedman
                      Mathematical formalisms that incor-
                     porate the physical processes respon-
                     sible for the vertical redistribution of a
                     conservative pollutant tracer due to a
                     convective cloud field are presented.
                     Two  modeling  approaches  are  pre-
                     sented  differing in the manner in
                     which the cloud fields are forced. In
                     the first or implicit approach, the ver-
                     tical cloud development is limited by
                     the satellite observed value, and cloud
                     forcing is determined from synoptic-
                     scale heat and  moisture budgets. In
                     the  explicit approach,  the  vertical
                     development is similarly limited, but
                     the forcing functions are obtained by
                     explicitly incorporating  the  vertical
                     distribution  of  cumulus cloud cover,
                     thereby dynamically incorporating the
                     influences  of  sub-synoptic  scale
                     phenomena. The two approaches give
                     internally consistent results and  give
                     similar results for the convective  mass
                     flux. The manner in which the upward
                     mass  flux  is   apportioned  to  the
                     various cloud classes,  however, dif-
                     fers as consequence of the  different
                     vertical profile  of  forcing  functions
                     used. The explicit model gave  more
                     reasonable profiles but the predictions
                     are highly sensitive to  input condi-
                     tions.  The  implicit  model,  was
                     somewhat less sensitive to  its  input
                     parameters  if the data  are  prepared
                     judiciously. This study shows that the
                     concentration increase in the cloud-
                     layer  due to  the venting action of
                     cumulus clouds can be as, if not more
                     important than, the in-situ production
 and  this  process should therefore be
 incorporated  in  regional-scale
 transport models.
  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 U.S. EPA is undertaking the devel-
opment and validation of a regional scale
photochemical model (ROM). Extensive
experimental field studies have been car-
ried out to provide the experimental bases
for the complex  processes that the ROM
must treat, and  to provide  a  suitable
model validation  observational  data base.
This  was  accomplished as  part of  the
NEROS  (Northeast  Regional  Oxidant
Study).  In this model  approach, it was
recognized that transport and transforma-
tion will occur as a result of dynamic ac-
tivity of cumulus cloud  convection.  Pro-
visions were made to develop a computer
module  that would address this process,
which heretofore has been  relatively ig-
nored in transport and dispersion  models.
This report describes the model develop-
ment program that addresses  the trans-
port aspect of the cloud venting process.
The  procedure  utilizes  satellite  derived
cloud top  height distribution data. Two

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alternative  approaches were investigated
and  their  strengths and  shortcomings
identified. The results of  either approach
provides  a  prediction   of  the  vertical
distribution of cloud flux.  Initial results in-
dicate  the  effect of venting by cumulus
clouds is comparable to chemical transfor-
mation rates.

Approach
  Statistical data of total  cloud amount,
cumulus  cloud  amount,  and cumulus
cloud top  height for certain regions and
dates are generated and prepared as input
to the  regional-scale photochemical oxi-
dant model of air pollution from NEROS II
digitized satellite data. These statistics are
used  to  parameterize  the  presence of
sunlight  for photochemical reactions and
to  diagnose  vertical   transport  of
pollutants.  Digital satellite data transmit-
ted from the  Geostationary  Operational
Environmental   Satellite   (GOES)   were
analyzed with the help of  Colorado State
University's (CSU)  Interactive  Research
Imaging  System (IRIS) to generate the re-
quired diagnoses of total  cloud amount,
cumulus  cloud  amount,  and cumulus
cloud-top  height.  Synoptic  rawinsonde
data also were used to translate cloudtop
temperatures  to  cloud-top heights.  The
IRIS also produced the quantitative cloud
field statistics  necessary to manipulate the
digital  data.  These  data  along with  the
synoptic temperature and moisture  field
and  surface  meteorological  data  were
used  as input  for  the  cloud  venting
module.  The  frequency  distribution of
cumulus cloud-top  heights,  provided by
CSU in terms of IR pixel counts for each
500 feet layer from 500 to 20,000 feet,
was transformed into a  fractional  cloud-
cover distribution as a function of height
using  a regressional analysis  technique.
This analysis  related the  observed frac-
tional  cumulus  cloud  cover for a  given
grid cell to the vertically integrated  IR pix-
el count for that cell in terms of a second-
order equation.
  The  rawinsonde data were truncated at
the 500-mbar  level and  objectively  inter-
polated in space; a scale-dependent filter-
ing technique  was employed for the scalar
fields of temperature and moisture, and a
variational  analysis  technique   was
employed  to  compute  the  vector wind
fields.  All  of  the subsequent fields were
then linearly interpolated  in time  to  ob-
tain, for each  grid cell,  a vertical profile
for each of the  required variables on an
hourly basis.
  Standard  hourly  meteorological  data
were used to  calculate cloud-base heights
for  each  grid  cell.   The   reported
temperature and moisture values, given at
3-hour  intervals,  were  spatially  inter-
polated using a scale-dependent filtering
technique  and   temporally  interpolated
with a standard  cubic spline routine.

  The  two  modeling  approaches
presented in the study are both based on
the same framework;  however,  the  first
approach  (implicit method) requires  that
the cloud forcing functions be determined
from the synoptic-scale  heat and moisture
budgets and that the maximum cumulus
cloud-top height  for each  grid cell be
specified.  The second  approach (explicit
method)   determines the cloud  forcing
functions  for each grid cell from the fre-
quency distribution of cumulus cloud-top
heights   corresponding  to   that   cell.
Assuming  mass  conservation  within  a
given grid cell  the following calculations
were  carried out  using  both the implicit
and explicit formulations:

  a) Vertical  mass  flux  distribution
     resulting from cumulus updrafts, the
     induced mass flux  in the  cloud en-
     vironment that  compensates for the
     updraft mass flux,  and the synoptic-
     scale mass  flux.
  b) The  contribution to the total cloud-
     base  mass  flux  as  a  function of
     cloud-top height.
  c) The  increase in concentration  as a
     function  of  pressure-height  of  a
     passive  tracer in the cloud layer due
     to cloud venting.

  Test cases were run  for circumstances
of weak,  moderate,  and strong, but non-
precipitating  convection.  The  results in-
dicated a  reasonable degree  of internal
consistency between both the implicit and
explicit modules in that more mass flux is
obtained  for larger  and more extensive
cloud  fields  in  both approaches.  These
model analyses  reveal  that the rate of
concentration increase in the cloud layer
of a passive tracer due  to the cloud vent-
ing process can be  more important than
in  situ  photochemical   production  and
should, therefore,  not  be ignored.  The
results also show that satellite data can be
used  to  close the "spectral gap" that is
present when only  synoptic-scale  rawin-
sonde data are used.
  This report includes a documented test
case  organized  and presented together
with  the code  modules to facilitate its
use.
  Because it is  very  difficult  to test a
cloud  venting module quantitatively,  it is
 particularly   important  to   perf
 diagnostic type sensitivity analyses.
       Parameter
Pertinent Mo
1.

2.

3.



4.

5.

6.
7.


8.


Fractional updraft
area (5)
Cloud half-life
parameter (TMN)
Ratio of entrain-
ment (AH/Zi) zone
depth to mixed
layer depth
Radiative heating
(On)
Sensible heating
rate (Q2)
Latent heating rate
Vertical distribution
of cloud liquid
water content (qf)
Vertical gradient of
the entrainment
rate (d X D/dp)
explicit

explicit

explicit



implicit

implicit

implicit
explicit/
implicit

explicit/
implicit

   The  sensitivity of the  explicit  mo
 prediction was found to be highly de
 dent on the fractional updraft area of
, convective element. The  validity of
 assumption of a constant  ratio of up<
 to total cloud area for all clouds of di
 ing vertical  extents, has  not been
 amined. Other important issues that r
 to be addressed, which involve both
 implicit and explicit formulations, con
 the appropriateness of employing the
 dimensional  entraining plume model
 proach  and,   as  in   all  subgrid-s
 models,  the  proper' treatment  of
 meteorological  data to give  meanir
 subgrid-scale  parameters.  The   use
 satellite data has helped  to  bridge
 spectral gap.

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      J. A. Ritter is presently with NASA Langley Research Center, Hampton. VA 23665,
       and D. H. Stedman is with the University of Denver, Denver, CO 80208.
      Jason K. S. Ching is the EPA Project Officer (see below).
      The complete report, entitled "The Vertical Redistribution of a Pollutant Tracer
       Due to Cumulus Convection," (Order No. PB 85-172 971 /AS; Cost: $16.00,
       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:
             Atmospheric Sciences Research Laboratory
             U.S. Environmental Protection Agency
             Research Triangle Park, NC 27711
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