United States
Environmental Protection
Agency
Environmental Sciences Research T, .
Laboratory /'
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-82-011 Sept. 1982
Project Summary
Estimating Cloud
Paramters for Neros I
F. M. Vukovich and D. P. Erlich
Geosynchronous Orbiting Earth
Satellite infrared and visible imagery
were combined with surface and
upper-air meteorological observations
to determine cloud amounts and
cloud-top heights over the Northeast
Regional Oxidant Study grid for 1200,
1500, and 1800 EOT, on August 3,4,
and 13, 1979. Cloud amounts were
determined for cumulus clouds alone
and for all clouds. Cloud-top heights
were determined specifically for
cumulus clouds.
A study was begun to develop a
model that could be used to estimate
the parameters of the cloud ozone
flux. Several models were developed
to estimate the average maximum
cloud vertical velocity; the best model
developed was a multiple linear
regression model. The model input
parameters were the cloud-top height
and the cloud amount, which were
derived from satellite imagery. This
model yielded an average correlation
coefficient of -0.78 and a root mean
square difference of ±0.8 m/s~1. On
the average, with the use of the multi-
ple linear regression model, there was
a 24% error in the estimated average
cloud vertical velocity. However, the
modeling results were not statistically
significant because of the limited data
available for developing the model.
The total number of data points was
nine, but only seven were useful.
This Project Summary was devel-
oped by EPA's Environmental Sci-
ences Research Laboratory. Research
Triangle Park, NC, to announce 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
In July and August of 1979, the U.S.
Environmental Protection Agency (EPA)
conducted the first phase of the North-
east Regional Oxidant Study (NEROS).
The primary purpose of the study wasto
measure concentrations of oxidant and
oxidant precursor on a regional scale in
the boundary layer. From these data,
physical processes could be parameter-
ized in numerical models and numeri-
cal model simulations evaluated.
Solar radiation is a significant factor
in the formation of oxidants from oxi-
dant precursors; the concentration of
pollutants within a layer of the atmos-
phere can be influenced significantly by
the vertical flux via cumulus cloud vent-
ing. To estimate the amount of solar
radiation penetrating the boundary
layer, imagery from the Geosynchro-
nous Orbiting Earth Satellite (GOES)
was used to estimate cloud parameters
over grid squares approximately 20 km
by 20 km in the Northeastern United
States. The GOES imagery also was
used to derive a physical relationship
between cumulus cloud-top growth and
vertical velocity within cumulus clouds
so that the vertical flux of oxidants and
oxidant precursors could be estimated.
Another principal objective of this
research project was to determine cloud
parameters (spatial and vertical extent)
that could be used in modeling the pro-
duction of ozone in the boundary layer
during specified periods in the NEROS
program. The study area was bounded
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by 38° and 45°N latitude and 69° and
84°W longitude. Major objectives real-
ized during the research project were:
1. To determine the fractional cover-
age of cumulus clouds (when
these clouds alone existed) in each
of the 1/4° longitude by 1/6° lati-
tude NEROS grid squares, using
GOES imagery and synoptic mete-
orological observations for 1 200,
1500, and 1800 EOT on August 3,
4, and 13(1979).
2. To determine the average height
of cumulus cloud tops across each
NEROS grid square using the
GOES infrared imagery and avail-
able upper-air temperature profiles.
3. To determine the fractional cover-
age of all clouds (other than cumu-
lus or multilayer clouds that might
include cumulus) for each NEROS
grid square.
A secondary objective of the project
was to study the period during which
the so-called "cloud buster" experi-
ments were performed to determine
cloud parameters such as those dis-
cussed above; study of those parame-
ters relative to in situ measurements of
vertical velocity made in cumulus
clouds would help to determine if a
functional relationship could be devel-
oped between the satellite cloud param-
eters and the cloud vertical velocities.
This relationship could then be used to
model the vertical ozone flow in cumulus
clouds. The specific approach is out-
lined below.
1. Available GOES infrared and vis-
ible imagery in or very near the
periods of the "cloud buster"
experiments (i.e., 1430 to 1530
EOT on 22 August 1979, and 1400
to 1700 EOT on 28 August 1979)
were used to determine the frac-
tional coverage of cumulus clouds
and the average height of the
cumulus clouds within a fixed 20-
km grid squared along aircraft
transects in southeastern Pen-
nsylvania and New Jersey.
2. In situ measurements made from
the aircraft of the peak vertical
velocity in the cumulus clouds, the
average vertical velocity in the
clouds, the ozone concentration in
and around the clouds, and the
upward- and downward-looking
radiometer temperature in the vic-
inity of the clouds were deter-
mined along the transects from
the data provided by EPA.
3. The vertical velocity data provided
from the in situ measurements
were compared with the cloud
parameters obtained from the
GOES data to determine if func-
tional relationships exist.
Determination of Cloud
Parameters for the
NEROS Grid
The principal data used were the
GOES infrared and visible images ob-
tained for August 3, 4, and 13 (1979).
GOES visible images were available
over the region of interest on these days
at 1130, 1430, and 1730 EOT. The
GOES data were collected in hard copy
image form and on magnetic tape by the
Research Triangle Institute's Satellite
Receiving Station in North Carolina.
Synoptic weather data for 1200,
1500, and 1800 EOT from the first-
order, surface-synoptic weather sta-
tions in the region were used, as well as
upper-air data for 0200, 0800, and
1400 EOT from the National Weather
Service Upper-Air Stations The 1400
EOT upper-air data were used exten-
sively because they fell within the
period 1200 to 1800 EOT. These
weather data were obtained either from
the National Climatic Center in Ashe-
ville, NC or from EPA.
The steps employed to determine
cloud amounts and cloud-top heights
over the NEROS grid are as follows:
1. To facilitate interpretation of the
. satellite data, the gray scale of the
GOES infrared and visible images
was enhanced (i.e , the gray scale
was confined to a range of temper-
ature and reflected radiation that
gave the most useful information),
using the data on magnetic tape
and the facilities available at the
RTI satellite receiving station.
2. The enhanced GOES visible and
infrared (IR) images were photo-
graphically enlarged uniformly to
further facilitate interpretation of
the satellite data.
3. A NEROS grid overlay was devel-
oped on transparent Mylar for the
GOES imagery.
4. The cloud amounts, cloud types,
and cloud-base heights from sur-
face synoptic data were plotted on
another transparent Mylar over-
lay.
5. An analysis delineating areas of
clear skies, cumulus alone, and
multiple cloud layers or clouds
other than cumulus were devel-
oped using the GOES visible and
IR imagery and the plotted cloud
data from the surface synoptic
stations
6 Analyses of cloud cover in areas of
cumulus only, and of multiple
cloud layers or clouds other than
cumulus, were performed using
the GOES visible and IR imagery
and the surface synoptic cloud
data
7. An analysis of cumulus cloud-top
temperature was performed using
the GOES infrared imagery and
calibration data available from the
GOES User's Guide.
8 Cumulus cloud-top heights were
derived using the cloud-top tem-
peratures combined with the radi-
osonde data.
9. The cumulus only cloud amounts
(ac), cloud amounts for conditions
other than cumulus alone (cra), and
the cumulus-top heights (Hc) were
selected at each of the NEROS grid
points, formatted, and punched on
computer cards.
Cloud parameters from the surface
synoptic data were plotted on transpar-
ent Mylar overlays using the GOES vis-
ible imagery The plotted cloud data and
the GOES visible and infrared images
were then used to define regions of
cumulus alone, clear skies, and multiple
cloud layers. The visible images were
used to interpolate in areas between
synoptic weather stations. The infrared
images were examined along with the
visible imagery to determine if there
was a change in cloud structure
between synoptic stations that might
indicate a change in cloud type. Sim-
ilarly, an analysis of cloud amounts was
performed using the GOES visible imag-
ery, the plotted cloud data, and the anal-
ysis delineating areas of cumulus, clear
skies, and multiple cloud layers. Once
again, the visible imagery data were
used to interpolate in areas between the
synoptic weather stations.
To obtain the cloud-top heights for the
cumulus clouds, the gray scale for each
of the GOES infrared images was cali-
brated with respect to temperatures,
using data available from the GOES
User's Guide. For each GOES infrared
image, patterns of shades of gray were
analyzed on transparent Mylar overlays,
and the patterns of gray scale were
assigned temperatures using the cali-
brated gray scale. The satellite tempera-
ture in the area of cumulus clouds (Ts)
and the temperature in the area of clear
skies (Ta) nearest the cumulus clouds
were determined. The cumulus cloud-
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top temperature can be estimated using
the following formula
Tc = Ta - -1 (Ta - Ts)
Cc
where ac is the amount of cumulus
clouds in tenths, and Tc is the cumulus
cloud-top temperature.
The cumulus cloud-top height (Hc)
was determined using the cumulus
cloud-top temperature and the temper-
ature profile obtained from the radio-
sonde stations nearest the cloud of
interest. In all cases, the 1400 EOT radio-
sonde data were used. The cumulus
cloud-top temperature and cloud-top
height were also estimated using the
1400 EOT radiosonde data and standard
techniques (i.e., a parcel of air was lifted
dry adiabatically to the lifting condensa-
tion level, then moist adiabatically to the
level of free convection and the equili-
brium level — the level of neutral
buoyancy; the cloud-top height was
assumed to be the height of the equili-
brium level). These values were com-
pared with those values determined
using the satellite data. When major
discrepancies (differences > 1000 m)
were found, attempts were made to jus-
tify the differences. About 25 percent of
the data given satellite analysis could
not be verified using the calculations
from the soundings. These discrepan-
cies were generally due to the develop-
ment of isolated large cumulus con-
gestus and cumulonimbus clouds.
Afterwards, a transparent Mylar over-
lay of the NEROS grid was placed on the
analysis of cloud amounts and the
cloud-top height to determine those
parameters at the grid points. These
data were formatted and punched on
computer cards, which were delivered
to EPA with an explanation of the
format.
Modeling Cloud
Vertical Velocity
A second objective of this project was
to develop a model which would esti-
mate the average maximum-cloud-
vertical velocity using parameters
obtained from satellite data: the cloud
amount and cloud-top height. The
model was to be used to parameterize
the cloud vertical flux of oxidants and
oxidants precursors. Since this was the
first attempt to develop such a relation-
ship, and since the data resources were
limited, the result of this study may be
considered asa guideline for more com-
prehensive studies in the future. The
vertical velocity data were obtained by
aircraft as a part of the so-called "cloud
buster" experiment. The purpose of this
experiment was to collect data for study-
ing the vertical flux of ozone in clouds.
The principal data set utilized to deter-
mine the cloud amounts and cloud-top
heights for the cloud buster experi-
ments was the GOES infrared and vis-
ible imagery for 22 and 28 August 1979.
Since the aircraft data for the cloud
buster experiment were sampled from
1430 to 1530 EOT on 22 August and
from 1400 to 1700 EOT on 28 August,
the GOES visible and infrared images
for 1430 and 1500 EST, respectively,
were used for this study. The GOES data
were collected in hard copy image form
and on magnetic tape by RTI
Synoptic weather data from the first-
order surface weather stations were
also used in the region and for the time
period defined by the flight tracks (over
southeastern Pennsylvania and New
Jersey). The 1500 EOT weather data
were used in all cases, and the 1400
EOT upper-air data were also used to de-
termine cloud-top heights. Cloud
amounts and cloud-top heights were de-
termined in 20-km x 20-km squares
centered along the flight track of the air-
craft, using the methodology discussed
earlier.
Various in situ measurements were
made by an aircraft along the transects.
The aircraft data were used to deter-
mine the average maximum vertical
velocity in cumulus clouds over a 20-km
portion of the flight track coinciding with
the 20-km x 20-km region where cloud
parameters were determined using
satellite data (i.e., the peak vertical
velocity was determined for each cloud
in the 20-km portion of the flight track
and an average was computed over all
clouds in the cell). The upward- and
downward-looking radiometer temp-
eratures were used to verify the exist-
ence of clouds.
The data reduction yielded nine inde-
pendent data points from the various
transects over the two days. Two data
points were obtained along a transect
that bordered two distinct regions of
cloud amounts and cloud-top heights.
For that reason, it was difficult to specify
cloud amount or cloud-top height in
these cases. The values given are asso-
ciated with a cloud system with low
cloud-top height and an approximate
cloud amount of 0.3. The other cloud
system had cloud-top heights on the
order of 5,000 m and cloud amounts of
approximately 0.6. Because of the
problem of selecting proper cloud
amounts and cloud-top heights in this,
case, it was decided that these two data
points would be ignored in the analysis
that follows. Discarding these two data
points left only seven data points for the
analysis.
Results
The following models were used with
the seven data points to develop a rela-
tionship between the average maxi-
mum vertical velocity in the cloud and
the cloud amount and top height derived
from satellite data: a multiple linear
regression model; polynomial models in
which the average maximum cloud ver-
tical velocity was related to the cloud
amount; and the polynomial models in
which the average maximum cloud ver-
tical velocity was related to the cloud-top
height. The analysis indicated that
increasing the degree of the polynom-
ials to a value greater than three did not
significantly improve the models. Table
1 lists statistics on the various models
including the correlation coefficient and
the root mean square difference (RMSD)
between the estimated and the observed
average maximum cloud vertical veloc-
ity. The coefficients of the models were
determined via a standard regression
algorithm developed for the Tektronix
Model 4051 computer.
The data in Table 1 indicate that both
the cloud-top height and the cloud
amount are negatively correlated with
the average maximum cloud vertical
velocity: as the cloud amount increases
or the cloud-top height increases, the
average maximum cloud vertical veloc-
ity decreases. As the degree of the poly-
nomial models increased for both the
cloud amount and the cloud-top height,
the magnitude of the correlation coeffi-
cient increased and the magnitude-of
the RMSD decreased. The statistical
data in Table 1 suggest that the multiple
linear regression model yielded the best
relationship between the average maxi-
mum cloud vertical velocity and the
cloud amount and the cloud-top height.
The specific form of the multiple lin-
ear regression model for the average
maximum cloud vertical velocity (with
seven data points) is
We = 7.8 - 4.4 ac - 0.0011 HT
where we is the estimated average max-
imum cloyd vertical velocity, ac is the
cloud amount, and HT is the cloud top
height. Table 2 gives a comparison of
the estimated and observed average
maximum cloud vertical velocities. Also
given is the residual and the RMSD. On
the average, the error in the estimated
ft US GOVERNMENT PRINTING OFFICE. 1982-559-017/083Z
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Table 1.
Statistical Analysis Of The Various Model Types Yielding Estimates Of
The Average Maximum Cloud Vertical Velocity (We) As A Function Of The
Cumulus Cloud-Top Height (H-r) And The Cumulus Cloud Amount
Which Were Derived From Satellite Data.
Model
R*
RMSD**
We
We
We
We
We
We
= 7.8 -4.4 ac -0.0011 H-r
= 7.7 -0.0014 «T
= 4.5-6.1 ac
= 0.05 WT - 0.000009 t-fi - 74.2
= 6.1 - 18.1 ac+ 17.4al
= 131 -0.15 Hi + 0 00006 H$ - 0.000000007 H?
= / - 19 ac + 20.8 al - 3.3 al
-0.78
-0.56
-0.67
-0.72
-0.67
-0.75
-0.75
±0.8
±1.1
±1.0
±1.0
±1.0
±0.9
+0.8
*R is the correlation coefficient
**RMSD is the root mean square difference between the observed and estimated
average maximum cloud vertical velocity fm/s'1).
average cloud vertical velocity is 24%
and the error decreases as the vertical
velocity increases. However, it should
be noted that these results are not sta-
tistically significant because of the
limited data available for the develop-
ment of the model.
Conclusion
The fact that the cloud amount and
cloud-top height were negatively corre-
lated with the average maximum cloud
vertical velocity was surprising and may
be a result of the small data set. How-
F. M. Vukovich and D. P. Erlich are with Research Triangle Institute, Research
Triangle Park, NC 27711.
Terry L. Clark is the EPA Project Officer (see below).
The complete report, entitled "Estimating Cloud Parameters for NEROS I,"
(Order No. PB 82-186 552; Cost: $7.50, subject to change) 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:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
Table 2. A Comparison Of The Ob-
served (We) And Estimated
(we) A verage Maximum
Vertical Velocities (m/s~*).
Wc
fm/sj
1.5
4.4
2.9
1.8
4.5
4.5
4.4
We
fm/sj
1.2
3.8
3.8
3.1
4.1
3.6
4.5
Residual
0.3
0.6
-0.9
-1.3
0.4
0.9
-0.1
RMSD = ±0.8 m/s.
ever, there is a possibility that the
results may be real. As clouds develop,
both the horizontal and vertical dimen-
sions increase (the cloud amount and
the cloud-top height). Eventually, the
cloud reaches a mature state where the
vertical velocity begins to decrease and
approaches zero or begins to become
negative if hydrometers fall. Therefore,
one can hypothesize that the maximum
vertical velocity in isolated cumulus
would be reached when the cloud is rel-
atively small (i.e., in its development
stage and not in its mature stage). How-
ever, such speculation can be verified
only if the data set were to be increased
substantially.
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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