Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
United States
Environmental Protection
Agency
Research and Development
EPA/600/S3-86/028 June 1986
&EPA Project Summary
Further Case Studies on the
Impact of Mesoscale
Convective Systems on
Regional Ozone and Haze
Distributions
Walter A. Lyons and Rebecca H. Calby
This paper represents a continuation
of an earlier project to study the impact
of mesoscale convective precipitation
systems upon distributions of aerosol
and photochemical oxidant pollutants
in the planetary boundary layer (PBL).
In the original study, using data col-
lected during Persistent Elevated Pollu-
tion Episode/Northeast Regional Oxi-
dant Study (PEPE/NEROS-80), analyses
of surface visibility and ozone data re-
vealed a dramatic response in the
boundary layer pollutant patterns to
the passage of two very large convec-
tive storm systems: a mesoscale con-
vective complex and an intense squall
line. Regional visibilities, at times less
than 5 km, increased dramatically to as
high as 80 km over a multistate area. A
technique was developed to estimate
the total amount of aerosol, presumed
to be primarily sulfate, that was dis-
placed from the PBL. Approximately
38 x 106 kg of sulfate was apparently
redistributed by the convective system.
The resultant clean air region was
termed a convective aerosol removal
event (CARE).
In this study, a search of the existing
PEPE/NEROS-80 data base was ini-
tiated to discover additional CAREs. A
well-defined CARE was found in satel-
lite imagery off the Georgia coast on 14
August 1980. Extensive mesoanalyses
revealed that the clear air pocket em-
bedded within a large surrounding
PEPE originated from a mesohigh sys-
tem formed by a cluster of thunder-
storms over northern Florida and Geor-
gia the previous day. While the increase
in regional visibility and the volumetric
depletion of sulfate aerosol was of the
same magnitude as in the mesoscale
convective complex squall line case,
the total mass of displaced aerosol was
7.7 x 10s kg, fully 50 times less. The re-
sponsible air mass thundershowers, far
smaller than the systems first studied,
clearly can result in less widespread but
still significant impacts on PBL pollu-
tion.
In addition, the phenomenon of the
pseudo-CARE was discovered. "Clear
spots" embedded within large quasi-
homogeneous PEPEs were found not to
be associated with specific thunder-
storms. In one case, large-scale subsi-
dence over the Great Lakes associated
with lake breezes was hypothesized to
have induced CARE-like clear zones
which drifted as far south as the Ohio
River. These observations have impor-
tant implications for mesoscale deposi-
tion modeling and for the interpreta-
tion of field program aerosol
measurements.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research 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).
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Introduction
In the early 1970s, meterological
satellites began providing graphic im-
ages of the long-range transport of
aerosol pollutants. High resolution
Landsat imagery tracked discrete
plumes from power plants and steel
mills for over 200 km. There was specu-
lation that large haze areas seen over
Europe from polar orbiting satellites
were in fact synoptic-scale air pollution
events. By 1976, the visible sensors of
geostationary satellite systems pro-
vided circumstantial evidence that large
areas of haze over the eastern United
States were linked to synoptic-scale
episodes of sulfate pollution. This phe-
nomenon, eventually termed a persis-
tent elevated pollution episode (PEPE),
has since been extensively studied.
Given the frequent recurrence of highly
polluted boundary layers extending
over multi-state areas, questions soon
arose as to the fate of these pollutants.
Transport through the atmosphere
results not only from horizontal advec-
tion, but from vertical motions due to
mesoscale convective processes. Initial
attention was paid to non-precipitating
cumulus congestus clouds. Speculation
as to the impact of deep three-
dimensional cumulonimbus-scale verti-
cal convective motions soon arose
along with the hypothesis that the im-
pact of large thunderstorm systems
should be clearly visible in the haze air
masses when viewed by geostationary
satellite systems. As part of the PEPE/
NEROS-80, an active effort was
launched to use the GOES satellites to
detect mesoscale convective system
(MCS) thunderstorm impact upon
boundary layer aerosol patterns.
The authors previously presented an
extensive case study that indicated
massive displacement of sulfate aerosol
and ozone from the boundary layer over
a mid-Atlantic multi-state area due to
the passage of a mesoscale convective
complex and a related squall line sys-
tem. A large clear area, subsequently
termed a convective aerosol removal
event (CARE), was detected both in the
GOES visible satellite imagery and by
the analysis of surface visibility reports.
It is hypothesized that thunderstorms
are an important sink for sulfate aero-
sols and other pollutants, not only due
to wet removal to the surface, but more
importantly, because of the injection of
massive amounts of pollutants into the
middle and upper troposphere as a re-
sult of thunderstorm convective up-
drafts. Strong downdrafts replace the
near-surface air with air from mid-
tropospheric levels. The perturbations
induced in the antecedent polluted
boundary layer may last for one or more
days until horizontal mixing processes
diffuse the CARE out of existence.
These preliminary results raised a
number of intriguing questions. First,
are CAREs formed by smaller storms,
i.e., the more typical air mass thunder-
showers which dominate the southern
and eastern United States during the
warm season? Second, were the events
of 2 August 1980 a unique occurrence,
not likely to be repeated with enough
frequency to invalidate current trans-
port and deposition modeling efforts?
Finally, are all such turbidity inhomo-
geneities detected in GOES satellite im-
agery the result of thunderstorms or are
there other mechanisms that could
come into play? These questions are ad-
dressed in the report.
Conclusions
The primary purpose of this project
was to determine if a CARE could be
associated with small air mass thunder-
storm systems. This situation contrasts
the extremely large CARE generated by
a massive mesoscale convective com-
plex and squall line on 2 August 1980.
On 13 August 1980, a cluster of small
local thunderstorms developed over
northern Florida and southern Georgia.
Detailed mesoanalyses showed that
several smaller mesohighs eventually
congealed into one larger system,
which ultimately covered an area of
172,000 km2. By the next morning, an
area of improved visibility moved
across eastern Georgia and extreme
southern South Carolina. Visibility at
the affected stations improved to an
average of 18 km, for a total increase of
7.6 km. The estimated mean volumetric
decrease of sulfate aerosol concentra-
tion within this CARE was on the order
of 15 (xg/rn3. These changes were of the
same order as those associated with the
MCC and squall line of 2 August 1980,
indicating little significant difference in
the basic efficiency of the process. How-
ever, the total mass of sulfate estimated
to have been displaced from the CARE
was only 7.7 x 105 kg, or approximately
1/50 of that in the first case study. Thus,
evidence is presented that smaller thun-
derstorm clusters can produce CAREs
of similar intensity, but in smaller geo-
graphical areas, and consequently, with
lower sulfate mass (and presumably ox-
idant) displacement from the polluted
boundary layer.
Since, with some exceptions, the up-
drafts of convective precipitation sys-
tems originate in and draw much of
their mass flux from the polluted plane-
tary boundary layer, this leads to specu-
lation as to the ultimate fate of the dis-
placed aerosol. (A simple schematic
model is presented in the report.) Some
of the aerosol is, of course, removed to
the surface by wash-out and wet depo-
sition mechanisms. The radially out-
ward spreading mesohigh, acting as a
plow, undercuts the polluted boundary
layer air, physically uplifting some of
the material in homogeneous layers to
altitudes as high as 3000 m AGL Given
the intense vertical transport within a
storm's convective tower, large quanti-
ties of boundary layer pollutants are
transported to great heights. Some of
the material is detrained from the cumu-
lonimbus tower into the middle tropo-
sphere. It is suspected that most of the
material is eventually evaporated from
the anvil cloud debris near the base of
the tropopause. Some material may be
actually injected into the stratosphere
by overshooting turrets associated with
the more intense thunderstorms. Since
cumulonimbi are noted for their rain-
producing capability, some of the pollu-
tants must certainly become involved in
the water-phase chemistry and con-
tribute to acidic precipitation. What re-
mains to be quantitatively evaluated is
the percentage of the original boundary
layer pollutants vented into the anvil re-
gion of the cloud versus those that
rained out to the surface. Since the
more intense supercell thunderstorms
are known to have comparatively low
precipitation efficiencies, these are pos-
tulated to be the most efficient venters.
The probable role that thunderstorms
play as sinks of boundary layer pollu-
tion, not to the surface, but to the upper
troposphere, may need to be seriously
considered as components of regional
deposition models.
One totally unexpected result of the
project was the discovery of the
pseudo-CARE. Areas of lower turbidity
or "holes" within the haze PEPE
air mass, originally thought to be
thunderstorm-induced CAREs, were un-
able to be related to specific mesoscale
precipitation systems. During a two-day
period in June, 1975, there was intense
subsidence associated with lake
breezes over the Great Lakes. These in
turn deformed the polluted boundary
layer and appear to have significantly
depressed the top of the aerosol layer.
Upon cessation of the lake breeze, these
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"aerosol holes" drifted with the north-
easterly flow into the Ohio Valley, main-
taining themselves for 18 to 24 hours.
Other apparent pseudo-CAREs were
discovered, including one long, narrow
swath of reduced cumulus convection
and apparent increased turbidity that
was associated with a synoptic-scale
mid-tropospheric subsidence event.
Recommendations
After completion of two case studies,
we have found evidence that three
major classes of thunderstorms gener-
ate CAREs. The mesoscale convective
systems studied were an air mass
thunderstorm cluster, a mesoscale con-
vective complex, and a squall line. At
least one major type of thunderstorm
(the isolated supercell thunderstorm)
has not yet been studied. In any case,
additional studies for all types of
thunderstorms are required for a better
understanding of the relationship be-
tween the storm type, total rainfall, and
sulfate displacement from the polluted
boundary layer.
While the use of surface meterologi-
cal data allows reasonable estimates to
be made of the total aerosol displaced
during a CARE, these data, in turn, pro-
vide virtually no information about the
ultimate fate of these aerosols. A key
question is how much material is deliv-
ered to the surface by wet deposition
compared to how much is injected into
the free troposphere? Field observa-
tions are necessary to support the vari-
ous hypotheses of numerical cloud-
venting modelers. An intensive field
program is needed during which sev-
eral different approaches should be em-
ployed. First, the mass budget for sul-
fate and other aerosols during the life
cycle study of an isolated intense thun-
derstorm should be determined. This
study should attempt to determine the
partitioning of aerosol mass to the sur-
face. It should also determine how
much remains at mid-levels and at the
base of the tropopause once it is in-
jected into the free troposphere, and
ideally, if any is injected into the strato-
sphere. Tracers other than sulfate can
be employed. Ozone, which is less solu-
ble than many gases and does not par-
ticipate directly in the precipitation
process, should be measured. However,
it should be noted that extensive intru-
sions of stratospheric ozone into a tall
cumulonimbus cloud circulation could
complicate the interpretation of the re-
sults. Tracers such as the new genera-
tion of hydrocarbons could prove highly
useful. By "spiking" the updraft around
an isolated and identifiable thunder-
storm and measuring its total final re-
distribution, insight would be gained
into the vertical transport and disper-
sion properties of a thunderstorm. The
upcoming National STORM Program
provides an ideal setting for such an ex-
periment.
Additional efforts should be made to-
ward the collocation of meteorological
wet and dry deposition and air quality
monitoring sites for both routine and
special networks. It is extremely diffu-
cultto interpret the results of changes in
aerosol and other pollutant concentra-
tions if corresponding on-site meteoro-
logical information is not available. Fur-
thermore, whenever possible,
measurements, particularly of sulfate,
should be made on shorter than 24-hour
intervals. Mesoscale systems are inher-
ently short-period phenomena. Long
sampling periods eliminate the high fre-
quency fluctuations that are critical to
the understanding of the mechanisms
and impacts of thunderstorm aerosol
displacement. Interpretation of aerosol
patterns as seen in satellite imagery,
provides a valuable overview of atmos-
pheric processes. However, they are
very difficult to interpret, as are on-site
measurements of aerosols and other
pollutants, without the appropriate sup-
porting meteorological information. It
must be understood that the data collec-
tion and archival systems of the nation's
meteorological services are extremely
crude and do not provide (at least cost-
effectively) most of the information re-
quired for the proper analysis of satel-
lite and pollutant measurements. High
resolution radar data, real-time light-
ning stroke data, and high resolution
animated GOES imagery are, in gen-
eral, not archived in convenient formats
as part of routine government opera-
tions. This must be done on a real-time
basis by using operational facilities cur-
rently available from both the govern-
ment and private sectors. Attempts to
acquire such data after the fact are
costly and will almost always produce
lower quality and less useful informa-
tion to support field program data
analyses.
The unexpected discovery of the
pseudo-CARE phenomenon suggests
that the air quality community should
further interface with dynamic meteor-
ologists. There appear to be a number
of dry atmospheric processes that lead
to mesoscale subsidence phenomena
that significantly alter boundary layer
pollution profiles. The intrusion of large
amounts of mid-tropospheric air into
the boundary layer, unless recognized
as such, could seriously bias the inter-
pretation of measurements made dur-
ing aerosol transport and transforma-
tion experiments.
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Walter A. Lyons and Rebecca H Calby are with MESOMET. Inc.. Chicago, IL
60601.
Francis S. Binkowski is the EPA Project Officer (see below).
The complete report, entitled "Further Case Studies on the Impact of Mesoscale
Convective Systems on Regional Ozone and Haze Distributions," (Order No. PB
86-194 453/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, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-86/028
0000329 PS
AGENcy
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