FURTHER CASE STUDIES ON THE IMPACT OF MESOSCALE CONVECTIVE SYSTEMS
ON REGIONAL OZONE AND HAZE DISTRIBUTIONS
by
Walter A. Lyons and Rebecca H. CaTby
MESOMET, Inc.
35 East Wacker Drive
Chicago, IL 60601
EPA 68-02-4051
Project Officer
Francis S. Binkowski
Atmospheric Science Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
ATMOSPHERIC SCIENCE RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NC 27711
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NOTICE
The information in this document has been funded by the United States
Environmental Protecton Agency under Contract No. 68-02-4051 to MESOMET,
Inc. It has been subject to the Agency's peer and administrative review
and it has been approved for publication as an EPA document.
n
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ABSTRACT
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 collected during PEPE/NEROS-
80, analyses of surface visibility and ozone data revealed a dramatic
response in the boundary layer pollutant patterns to the passage of two
very large convective storm systems; a mesoscale convective complex and
an intense squall line. Regional visibilities, at times less than 5 km,
increased dramatically over a multistate area to as high as 80 km. A
technique was developed to estimate the total amount of aerosol, resumed
to be primarily sulfate, that was displaced from the PBL. On the order
of 38 x 10^ 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
initiated to discover additional CAREs. A well defined CARE was found in
GOES satellite imagery off the Georgia coast on 14 August 1980. Extensive
mesoanalyses revealed that the clear air pocket embedded within a large
surrounding PEPE (Persistent Elevated Pollution Episode) originated from
a mesohigh system formed by a cluster of thunderstorms over northern
Florida and Georgia the previous day. While the increase in regional
visibility and the volumetric depletion of sulfate aerosol was of the
same magnitude as in the MCC squall line case, the total mass of displaced
aerosol was 7.7 x 10^ kg, fully 50 times less. The responsible air mass
thundershowers, far smaller than the systems first studied, clearly can
result in less widespread but still significant impacts on PBL pollution.
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 thunderstorms. In one case, large
scale subsidence over the Great Lakes associated with lake breezes is
hypothesized to have induced CARE-like clear zones which drifted as far
south as the Ohio River. These observations have important implications
for mesoscale deposition modeling and the interpretation of field program
aerosol measurements.
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ACKNOWLEDGEMENTS
Manuscript preparation and graphics support were provided by Lynn
Newman, Jill Bauer, David Lau, Liv Nordem Lyons, and especially, Barbara
Bishel. Their contributions are gratefully recognized.
Satellite imagery and supporting data for several case studies were
kindly provided by Midwest Communications, Inc., Minneapolis, Minnesota.
Guidance and encouragement provided by the Project Officer, Dr.
Francis Binkowski, ASRL, has been greatly appreciated.
iv
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CONTENTS
wle
1.
2.
3.
4.
5.
6.
7.
dgements
Introduction . . . , . . .
Recommendations
Case Study: 13-14 August 1980
Case Study: 29-30 June 1975
Other CAREs and Pseudo-CAREs
....... iv
1
3
5
7
17
39
52
Extended Bibliography 56
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SECTION 1
INTRODUCTION
In the early 1970's meteorological satellites began providing graphic images
of the long-range transport of aerosol pollutants. High resolution Landsat ima-
gery tracked discrete plumes from power plants and steel mills for over 200 km
(Lyons and Pease, 1973). Mohr (1971) speculated that large hazy areas seen over
Europe from polar orbiting satellites were in fact synoptic scale air pollution
events. By 1976, Lyons and Husar (1976), using the visible sensors of geosta-
tionary satellite systems, were able to provide circumstantial evidence that
large areas of haze over the eastern United States were linked to synoptic scale
episodes of sulfate pollution. This phenomenon, eventually termed a Persistent
Elevated Pollution Episode (PEPE), has since been extensively studied (Vaughan
et al., 1982). Given the frequent reoccurrence 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 hori-
zontal advection, but from vertical motions due to mesoscale convective pro-
cesses. Initial attention was paid to non-precipitating cumulus congestus
clouds (Ching, 1982). Speculation as to the impact of deep three-dimensional
cumulonimbus-scale vertical convective motions soon arose (Scott, 1978; Cotton,
1983). Lyons et al. (1978) hypothesized that the impact of large thunderstorm
systems should be clearly visible in the hazy air masses when viewed by geosta-
tionary 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.
Lyons and Calby (1983) presented an extensive case study which indicated
massive displacement of sulfate aerosol and ozone from the boundary layer over a
multi-state area of the mid-Atlantic states due to the passage of a mesoscale
convective complex (Maddox, 1980) and a related squall line system. 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 sur-
face visibility reports. In Lyons, Calby, and Keen (1985) it is hypothesized
that thunderstorms are an important sink for sulfate aerosols and other pollu-
tants, not only due to wet removal to the surface, but more importantly, by the
injection of massive amounts of pollutants into the middle and upper troposphere
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as a result of thunderstorm convective updrafts. 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 thun-
dershowers which dominate the southern and eastern United States during the warm
season? Were the events of 2 August 1980 a unique occurrence, not likely to be
repeated with enough frequency to invalidate current transport and deposition
modeling efforts (NCAR, 1984)? Also, are all such turbidity inhomogeneities
detected in GOES satellite imagery the result of thunderstorms or are there
other mechanisms which could come into play? These questions will be addressed
in the following report.
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SECTION 2
CONCLUSIONS
The primary purpose of this project was to determine if a convective aerosol
removal event (CARE) could be associated with small air mass thunderstorm
systems. This is in contrast to the extremely large CARE generated by a massive
mesoscale convective complex and squall line on 2 August 1980 (Lyons and Calby,
1983). 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 km^. 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 concentration within this CARE was on the order of 15 ug/m^.
These changes were of the same order as those associated with the MCC and squall
line of 2 August, indicating little significant difference in the basic effi-
ciency of the process. However, 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
geographical areas and, consequently, with lower sulfate mass (and presumably
oxidant) displacement from the polluted boundary layer.
Since, with some exceptions, the updrafts of convective precipitation
systems originate in and draw much of their mass flux from the polluted plane-
tary boundary layer, this leads to speculation as to the ultimate fate of the
displaced aerosol. A simple schematic model is presented. Some of the aerosol
is, of course, removed to the surface by wash-out and wet deposition mechanisms.
The radially outward spreading mesohigh, acting as a plow, undercuts the
polluted boundary layer air, physically uplifting some of the material in homo-
geneous layers to altitudes perhaps as high as 3000 m AGL. Given the intense
vertical transport within a storm's convective tower, large quantities of boun-
dary layer pollutants are transported to great heights. Some of the material is
detrained from the cumulonimbus tower into the middle troposphere. 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. As pointed out by Cotton (1983), cumulonimbi are noted
for their rain producing capability. Certainly some of the pollutants become
involved in the water phase chemistry and contribute to acidic precipitation.
What remains to be quantitatively evaluated is the percentage of the original
boundary layer pollutants that are vented into the anvil region of the cloud
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versus what are rained out to the surface. Since the more intense supercell
thunderstorms are known to have comparatively low precipitation efficiencies,
these are postulated to be perhaps the most efficient venters. The probable
role that thunderstorms play as sinks of boundary layer pollution, 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 hazy PEPE air
mass, originally thought to be thunderstorm induced CAREs, were unable to be
related to specific mesoscale precipitation systems. During a two day period in
June, 1975 there was intense subsidence over the Great Lakes associated with
lake breezes. 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 "aerosol holes" drifted with the northeasterly flow into
the Ohio Valley, maintaining themselves for 18 to 24 hours. Other apparent
pseudo-CAREs were discovered, including one long, narrow swath of reduced cumu-
lus convection and apparent increased turbidity that was associated with a
synoptic scale mid-tropospheric subsidence event.
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SECTION 3
RECOMMENDATIONS
With completion of two case studies, we have now found evidence that three
major classes of thunderstorms generate convective aerosol removal events
(CAREs). The mesoscale convective systems studied were: an air mass thun-
derstorm cluster, a mesoscale convective complex, and a squall line. At least
one major type of thunderstorm has not yet been studied, i.e., the isolated
supercell thunderstorm. In any case, additional studies for all types of
thunderstorms are required to better understand the relationship between the
storm type, total rainfall, and sulfate displacement from the polluted boundary
layer.
While the use of surface meteorological data allows reasonable estimates to
be made of the total aerosol displaced during a CARE, these data, in turn, provide
virtually no information as to the ultimate fate of these aerosols. A key
question is what is the partitioning between wet deposition versus injection into
the free troposphere? As highlighted by Cotton (1983), these field observations
are necessary to support the various hypotheses of numerical cloud venting mode-
lers. An intensive field program is needed during which several different
approaches could be employed. First, the mass budget for sulfate and other
aerosols during the life cycle study of an isolated intense thunderstorm shall
be determined. This study should attempt to determine the partitioning of aero-
sol mass to the surface. It should also determine how much remains at mid-
levels and at the base of the tropopause, once injected into the free
troposphere, and ideally, if any is injected into the stratosphere. Other tra-
cers besides sulfates can be employed. Ozone, since it is less soluble than
many gases and does not participate directly in the precipitation process,
should be measured. However, it should be noted that extensive intrusions of
stratospheric ozone into a tall cumulonimbus cloud circulation could complicate
the interpretation of the results. Tracers such as the new generation of hydro-
carbons could prove highly useful. By "spiking" the updraft around an isolated
and identifiable thunderstorm and measuring its total final redistribution,
insight would be gained into the vertical transport and dispersion properties of
a thunderstorm. The upcoming National STORM Program (UCAR, 1983) provides an
ideal setting for such an experiment.
Additional efforts should be made towards the collocation of meteorological
wet and dry deposition and air quality monitoring sites for both routine and
special networks. It is extremely difficult to interpret the results of changes
in aerosol and other pollutant concentrations if corresponding onsite meteoro-
logical information is not available. Furthermore, whenever possible, measure-
ments, particularly of sulfate, should be made on shorter than 24 hour
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intervals. Mesoscale systems are inherently short period phenomena. Long
sampling periods eliminate the high frequency fluctuations which are critical to
the understanding of the mechanisms and impacts of thunderstorm aerosol displa-
cement. Interpretation of aerosol patterns as seen in satellite imagery provi-
des a valuable overview of atmospheric processes. However, they are very
difficult to interpret, as are on-site measurements of aerosols and other pollu-
tants without the appropriate supporting meteorological information. It must be
understood that the data collection and archival systems of the nation's
meteorological services are extremely crude and do not provide (at least
cost-effectively) most of the information required for the proper analysis of
satellite and pollutant measurements. High resolution radar data, real-time
lightning stroke data, and high resolution animated GOES imagery are in general
not archived in convenient formats as part of routine government operations.
This must be done on a real-time basis by using currently available operational
facilities available from both the government and private sectors. The attempts
to acquire such data after-the-fact are costly and will almost always produce
lower quality and less useful information to support field program data analy-
ses.
The unexpected discovery of the pseudo-CARE phenomenon suggests that the air
quality community should further interface with dynamic meteorologists. There
appear to be a number of dry atmospheric processes which lead to mesoscale sub-
sidence phenomena which 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 interpretation of measure-
ments made during aerosol transport and transformation experiments.
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SECTION 4
BACKGROUND
Synoptic scale pollution episodes occur with considerable frequency over the
eastern United States during the summertime (Watson and Saucier, 1979). There
are strong correlations between reduced regional visibility, particulate sulfate
mass, and photochemical oxidants during such PEPE events (Altshuller, 1985).
Since most sulfate aerosol mass is submicron in size, they are excellent scat-
ters of light (White et at., 1976) and are therefore easily visible to the
VISSR scanning system of the GOES geostationary satellite (0.5 - 0.9 micrometer
wavelength). Figure 1 shows a typical GOES medium resolution image over the
eastern United States during an all too rare summertime period in which the
region is dominated by a comparatively "clean" polar anticyclone. Note the
sharp contrast between land and water and the visibility of smaller topographic
features. By contrast, Figure 2, taken during a massive PEPE episode in August,
1976, reveals extensive areas of haziness over large portions of the eastern
United States and southeastern Canada. A sharp discontinuity in the turbidity
also plainly delineates a cold frontal boundary, which is, of course, the demar-
cation separating air masses of different temperature, moisture, and aerosol con-
centrations. Aerosols in fact make an excellent natural atmospheric tracer.
This fact was pointed out by Lyons and Pease (1973) from point source plumes, by
forest fire smoke (Chorowski et at., 1981), and even dust storms (Norton et al.,
1980). Lyons and Olson, (1973) used smoke photography to infer important details
concerning the nature of 3-dimensional lake breeze circulations around the
Chicago metropolitan area. Photography and airborne aerosol measurements were
used to track the Chicago urban plume for long distances downwind (Lyons and
Ruben, 1976).
A PEPE represents an accumulation of secondary aerosol mass distributed over
a wide area for an extended period of time (up to three weeks). Eventually all
material is removed from the atmosphere by a variety of processes. It is
generally assumed that the aerosols are returned to the earth's surface via dry
or wet deposition. The manner by which aerosols are removed and returned to the
earth during precipitation has undergone extensive analysis. Among the earlier
of the larger field programs, the OSCAR experiment (Easter, 1984), the emphasis
was primarily upon removal mechanisms within stratiform clouds. The same is
true of the study of mesoscale variability of urban wet deposition patterns
reported by Patrinos (1985), and in the recently conducted study by Battelle
Pacific Northwest Laboratories (Hadlock, Personal Comm.). Numerous theoretical
studies such as Hegg et al. (1984) and Daum et al. (1983) also have concentrated
on stratiform warm frontal clouds. But as Scott (1978), Hales and Dana (1979),
and Lyons and Calby (1983) have pointed out, stratiform precipitation mechanisms
are profoundly different from those associated with convective storms. There
are orders of magnitude of difference in the vertical motion involved. This
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Figure 1. GOES satellite 2 km resolution visible image, DBS sector over
eastern United States during a period of an exceptionally clean summertime air
mass. Notice the sharp resolution of ground features and the strong land/water
contrasts.
Figure 2. GOES satellite 2 km resolution visible image, during the major PEPE
episode of August, 1976. Note the milky haze and the reduced ground contrast
over much of the eastern United States. Also evident is a sharp discontinuity
of turbidity associated with a cold front stretching from southern Minnesota to
central Quebec.
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realization has prompted studies of vertical ozone transport within cumulus
congestus clouds by Greenhut et al. (1984). Convective clouds range through
their own enormous spectrum of sizes and mass fluxes from the smallest fair
weather cumulus humilis through the typical air mass thundershower (Byers and
Braham, 1949), to the cold frontal squall line (Hane, 1973), the mesoscale con-
vective complex (Maddox, 1980), and the isolated, but intense supercell thun-
derstorm (Browning and Ludlam, 1962; Klemp and Wilhelmson, 1978). As noted by
Auvine and Anderson (1972) and Mack and Wyle (1982), the mass flux from the
boundary layer in an intense thunderstorm can easily approach 10^ g/sec. In
response to this mass evacuation, there is replacement by lateral convergence,
plus downdrafts from mid-tropospheric levels into the surface layers. This
massive exchange of air has profound consequences when considering the fate of
boundary layer pollutants.
The removal of aerosol mass from the polluted boundary layer and its repla-
cement by large quantities of cleaner tropospheric air was graphically
demonstrated by Lyons, Calby, and Keen (1985) in their study of the massive
Mesoscale Convective Complex (MCC) which spread across the mid-Atlantic states
on 1-2 August 1980. Interactive image enhancement techniques, using the McIDAS
system for processing of GOES visible satellite data (Figure 3) showed an
extensive CARE over Virginia and North Carolina, accompanied by a large area of
elevated aerosol layers over portions of Georgia and South Carolina (Shipley et
al., 1983). The CARE'S impact was so extensive that it was plainly visible in
routinely available GOES imagery more than 24 hours after the event had occurred
(Figure 4).
As indicated above, there are many types of thunderstorms. Some are steady
state while others are clearly cyclical in their processes. Some remain as
essentially single cell entities while others are multi-cell clusters (Newton,
1966). Some convective storms lack energy even to reach the tropopause while
others penetrate a substantial distance into the stratosphere. There is little
doubt that precipitation efficiencies within such storms vary widely (Braham,
1952). It is clear that massive amounts of air, water vapor, and potentially,
pollutants are evacuated from the boundary layer into the cloud tower and are
eventually redistributed via several mechanisms including: (1) wash-out and wet
removal processes, (2) transport as cohesive layers to higher altitudes by the
plowing effect of undercutting downdrafts-induced gust fronts, (3) detrainment
into the free troposphere, (4) accumulation near the tropopause upon evaporation
of the cumulonimbus anvil cloud debris, and possibly (5) injection into the
stratosphere through tropopause penetration by intense convective turrets atop
the storm.
It is well known that the dynamics of even a relatively simple supercell
cumulonimbus are extremely complex (Browning, 1964). Figure 5 shows, in schema-
tic form the basic structure and air flow of a typical thunderstorm. While
there are exceptions, most thunderstorm updrafts largely originate from the sur-
face to approximately 2,000 m, which represents the depth of the polluted boun-
dary layer. Hence, these pollutants become an integral part of the mass flux
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Figure 3. McIDAS generated enhanced visible image showing cloud and haze pat-
terns over the eastern United States at 1400 GMT, 2 August 1980. Dense haze
centered in Georgia and South Carolina is enhanced, as is the distinct low tur-
bidity CARE over northern North Carolina, Virginia, and parts eastward (see
Lyons, Calby, and Keen, 1985).
10
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Figure 4. GOES 2 km resolution visible image, DBS sector, 2230 GMT, 02 August
1980 showing a massive CARE stretching from North Carolina and Virginia east-
wards to the Atlantic Ocean.
11
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12
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tive storm event as postulated in Figure 5.
13
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into the storm. It should also be noted that due to the convergent nature of
the low-level cumulonimbus circulation, air is drawn from far beyond the physi-
cal boundaries of the thunderstorm itself to be processed by one of the mecha-
nisms listed above.
Figure 6 suggests the resultant fate of the particulate matter from the ori-
ginally undisturbed planetary boundary layer as the result of a major convective
storm event. Polluted boundary layers can be undercut by the outflowing higher
density air mass behind the thunderstorm gust front. This effect has been docu-
mented in some detail by Shipley et at. (1983), Browell et al. (1983), and
Orndorff (1982). There is less direct evidence of the other processes upon
which we have speculated. Frequent observation of what appears to be layers of
brown aerosol matter at the base of the tropopause when flying in jet aircraft
during the summer, lend additional credence to this speculation. Whether or not
there physically can be an injection of material into the stratosphere by the
penetrative turrets atop a vigorous cumulonimbus cloud is a matter of con-
siderable speculation. If penetration occurs, it suggests the possibility of an
increase in anthropogenic sulfate material in the lower stratosphere, which
would begin accumulating over the course of time due to the extremely small
settling velocities of the submicron particles, as well as the lack of precipi-
tation scavenging mechanisms.
While large amounts of mass are injected upwards, a corresponding compen-
sation mechanism occurs. Penetrative precipitation driven downdrafts produce a
mesohigh pressure system (Fujita, 1959), which radially spreads outwards behind a
gust front, acting as a plow and undercutting boundary layer air. It might be
forced upwards by several hundred to several thousand meters (Goff, 1976). As
Wakimoto (1982) pointed out, gust fronts from even small thunderstorms can
extend more than 50 km from the rain area. These gust fronts are often
marked by so-called "rope clouds" which have been frequently studied by
satellite imagery (Purdom, 1973; Woods, 1983; Haase and Smith, 1984). Figure 7
presents a graphic, but by no means unusual, example of the phenomenon. A highly
divergent mesohigh is spreading air of largely mid-tropospheric origins over a
wide area. A dramatic rope cloud extends from east Texas through western por-
tions of the state and northwards into New Mexico, originating from a large
cluster of thunderstorms more than nine hours earlier in Oklahoma (Figure 8).
If this event were to have occurred during a PEPE, a dramatic change in regional
air quality would have occurred in an area two-thirds the size of our largest
state. The larger fraction of the region affected would never have experienced
cloudiness, much less rainfall.
This paper attempts to continue an on-going series documenting the CARE
.mechanism. In the following sections we will concentrate on much smaller iso-
lated air mass thunderstorm clusters.
14
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August 1984. Notice the extensive rope cloud extending from eastern New Mexico
into east central Texas along with remnants of a dissipated thunderstorm complex
over north central Texas.
15
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SECTION 5
CASE STUDY: 13-14 AUGUST 1980
The major component of this project was performing detailed mesoanalyses of
a cluster of air mass thunderstorms and their resultant CARE over the
southeastern U.S. on 13-14 August 1980. As a preamble to the analyses, a brief
description of the synoptic regime and attendant pollutant patterns is pre-
sented. By 1200 GMT 13 August, a trailing cold front associated with a low
pressure system which had moved across Nova Scotia, was positioned off the
Atlantic coast and across Georgia as shown in Figure 9. This frontal zone moved
northward during the day of the 13th and by 2100 GMT was better characterized as
a stationary front along the South Carolina - North Carolina border. The front
thereafter underwent frontolysis.
The clear air within the high pressure north of the front began to accumu-
late pollutants from local sources and, as aged PEPE air which was undercut by
the shallow front, mixed back to the surface with daytime convection. Thus,
pollutant levels, especially ozone, were increasing during the afternoon of 13
August from south Georgia northwards. This increasing pollutant loading can be
monitored by the degradation of regional visibilities. In the absence of fog,
high relative humidity, or precipitation, visibility can be used as a useful
surrogate for sulfate aerosol measurements. These submicron anthropogenic aero-
sols scatter light highly efficiently (Altshuller, 1985). Visibility plots at
1800 GMT (Figure 9) show the clean (high visibility) air extending from the Ohio
Valley into Virginia, but with distinct hazy regions dominating the Southeast.
A more detailed analysis of the regional visibility pattern is given in
Figures 10a,b using properly screened data from 1800 GMT, 13 and 14 August 1980,
respectively. Only those data that were not concurrent with obstructions to
visibility (rain, fog) or dewpoint depressions of 3° F or less were included.
During mid-day of 13 August, a broad band of low visibility air extended from
Mississippi into the Carolinas, where it was particularly hazy. Visibilities of
3 mi (5 km) or less were noted. A slight thinning of the haze to 10 mi (16 km)
in southern Georgia is believed to be the remnant of an earlier CARE. Only 24
hours later, a dramatic change in the pattern had occurred (Figure lOb). Over
large areas of Georgia and South Carolina, and eastward into the Atlantic, visi-
bilities had improved markedly, reaching 20 mi (32 km) at Savannah, GA. No
synoptic front had traversed this region. Surface winds were essentially light
and variable, with a slight westerly drift.
At Jacksonville, Florida, thunderstorms were reported from 1600-2000 GMT on
13 August 1980. Several cells crossed the Jacksonville area during that time.
The end result was that between 1500-2300 GMT, visibilities improved from 7 mi
(11 km) to 15 mi (24 km), the temperature dropped from 90° to 78° F, and
dewpoint temperature dropped from 78° to 74° F. Atmospheric pressure was
variable. Only 0.03" of rain was reported with these cells.
17
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AUGUST 1980
13 AUGUST 1980
13 AUGUST 1080
Figure 9. Synoptic scale surface winds, ozone, and visibility conditions
observed on 12 and 13 August 1980 in the eastern United States. See Lyons
and Calby (1982b).
18
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Figure lOa. Regional visibility pattern, 1800 GMT, 13 August 1980 over the
southeastern United States. Visibility as low as three miles is present
from Georgia into North Carolina and eastwards over the Atlantic. To obtain
these reports all surface weather reports were screened to remove those cases
where rainfall or fog was reported as an obstruction to visibility or the tem-
perature and dewpoints were closer than 3° F.
Figure lOb. Same as above, except for, 1800 GMT, 14 August 1980,
19
-------
At Alma, Georgia, a thunderstorm began at 2122 GMT. As the gust front
crossed Alma from the east southeast between 2100 and 2200 GMT, the temperature
dropped from 97° to 82° F, and pressure rose almost 2 mb. Since Alma is not a
24 hour reporting station, it is difficult to estimate the changes in dewpoint
and visibility because the observations ended as the rain was ending. At least
0.41" of rain were reported.
At Savannah (Figure 11), a thunderstorm was reported, beginning at 2126 GMT.
Only a trace of rain resulted from the storm at the observing site. Between
2100 and 2200 GMT, the temperature dropped from 89° to 83° F, the dewpoint
dropped from 76° to 72° F, and the pressure rose over 1 mb. Visibility
increased from 7 mi (11 km) to 10 sm (16 km) by 2300 GMT. These storms were
clearly visible in the GOES satellite image. They are small in comparison to an
MCC, but are still fairly large for air mass thundershowers.
The thunderstorms dissipated by the time the weak gust front reached
Beaufort, South Carolina, further to the east, around 2300 GMT. There was a wind
shift, temperature, and dewpoint drop and a pressure rise was reported at 2300
GMT. The outward spreading and advection of the cold air dome apparently slowed
as wind speeds became light and variable during the late evening/early morning
hours.
At Charleston, South Carolina, on 13 and 14 August, no obvious gust front
passages or thunderstorms were apparent. Temperatures did drop in response to
the sea breeze front at 1800 GMT, 13 August, and afternoon cloudiness. Due to
the moist air with dewpoint depressions of approximately 4° F, visibilities
were degraded throughout the evening, and it is difficult to ascertain exactly
when the cleaner air advected into the Charleston area. Perhaps the pool of
cleaner air remained above the nocturnal radiation inversion and mixed to the
surface as solar heating commenced on 14 August. Visibilities increased from 7
mi (11 km) to 15 mi (24 km) between 1100 GMT and 1300 GMT, 14 August 1980.
In order to better interpret these single station reports, we examined radar
and surface precipitation data for indications of the suspected CARE. Figure 12
is a mosaic comprised of hourly digitized weather radar reports which are
transmitted over the NAFAX facsimile circuit. It is apparent that typical
summertime "air mass" convective storms developed over much of the southeast.
These storms were locally intensified by the vigorous sea breeze circulations of
the Gulf and Atlantic coasts. It should be kept in mind that the NAFAX charts
are prepared from MDR (manually digitized radar) gridded data which greatly exa-
ggerate the areal coverage of the precipitation echoes. Regardless, there were
vigorous local storm systems throughout the region. As indicated by radar, echo
tops reached 60,000 ft. in northern Florida and southern Georgia. Rainfall
reports are summarized in Figure 13. Pockets of intense local rainfall were
found. These were of the size and intensity to indicate the probable presence
of organized mesoscale high pressure systems (Fujita, 1959; Purdom, 1973, 1976).
These mesohighs would be comprised largely of air convected downward, possibly
from mid-tropospheric levels, in the evaporatively driven downdrafts.
20
-------
T1IS/1E SECTION
, GEORGIA
80 =
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2180
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00
.90
80 3
60
PBECIPITHTIO
(INCHES)
1500
2100
0300
Figure 11. Time section for Savannah, Georgia, covering the period 0900, 13
August 1980 through 2200 EST, 14 August 1980. Notice the marked improvement in
visibility that begins at 0900 EST on the 14th of August with visibility
approaching 20 miles by 1200 EST.
21
-------
r
\
^-^WEDNESDAY \ X .-~^-~~ _.--'-, WDNESDAY
T&35Z AUG 13. 19.80 RADAR. SUMMARY 1435Z AUG 13, 1980
AR SUMMARY T735Z AUG 13
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?135Z AUG 13. 1980 RADAR SUMMARY 5235Z AUG 13. 1980 RADAR SUMMARY 2335Z AUG 13. 1980 RADAR. SUMMARY
Figure 12. A mosaic comprised of hourly composite MDR (manually digitized
radar) weather radar reports as transmitted over the NAFAX facsimile circuit
from 0635 GMT to 2335 GMT 13 August 1980.
22
-------
.01
.25'
.01 *
-.01
-.25
,.01
RAINFALL
13 AUGUST 1980
NO AVAILABLE DATA
Figure 13. Accumulated rainfall within the project study area from 13 August
1980.
23
-------
Hourly mesoanalyses were prepared using the techniques of classical mesoana-
lysis, as detailed by Fujita (1963). All hourly surface weather reports, radar
summaries, and GOES were analyzed from 2100 GMT 13 August to 0000 GMT 14 August,
and are presented in Figures 14-17. Four parameters were analyzed. These were:
(1) surface wind streamlines with MDR radar echoes superimposed, (2) surface
altimeter analysis (with 0.02" isobars which approximately equal 0.67 mb), (3)
temperature isotherms (every 2° F or 1.2° C), and (4) visibility isopleths in
kilometers.
Other mesometeorological features were present in addition to the thun-
derstorms. A sea breeze front extended the length of the Atlantic coastline and
probably played a role in thunderstorm development in northern Florida. Ocean
temperatures were approximately 83° F (29° C) and inland temperatures were in
the mid-900 F (35° C) range or above. Also, a weak low pressure region was present in
mid-Georgia. This may have been a "heat low" associated with the very high tem-
peratures present in the interior of Georgia on 13 August.
At 2100 GMT (Figure 14a), the streamlines illustrate the overall flow from
the south (south of the stationary front), with perturbations due to the sea
breeze front, heat low, and thunderstorm complexes near Tallahassee and
Jacksonville. The heavy barbed lines indicate analysed locations of gust
fronts. Isobaric analyses (Figure 15a) demarcate the pressure surges associated
with the rain-cooled downdrafts of these thunderstorms. The temperature field
also sharply delineates the mesoscale cold dome produced by the thundershowers
(Figure 16a). Visibilities throughout Georgia and the Tidewater are generally
11 mi (18 km) or less (Figure 17a). The pocket of 16+ km visibility in the
center of Georgia was probably a CARE, produced by a large isolated thunderstorm
in that region during the morning of 13 August.
By 2200 GMT (Figures 14b-17b) the thunderstorms near Jacksonville had
expanded, with a gust front extending north of Alma to near Savannah. The pool
of cooler, drier air produced by the thunderstorm is apparent in Figure 16b.
Due to rain and fog within the active thunderstorm mesohighs, little can be
ascertained at this hour as to any possible CARE formation. Note though, a
region of 16+ km visibility is associated with a weakening thunderstorm cluster
in northwest Georgia.
By 2300 GMT (Figures 14c-17c) the two major mesohighs had merged into an
even larger system. By 0000 GMT (Figures 14d-17d) the cool air had expanded to
cover the area from Beaufort, South Carolina to Tallahassee, Florida. During
the period from 2200 to 2300 GMT, the high visibility air (24+ km) associated
with a newly emerging CARE became evident (Figure 17c). The gust front had now
become the boundary between a CARE and the undisturbed polluted PBL.
In summary we suggest that a CARE resulted from these non-severe, but
vigorous air mass thunderstorms over southeastern Georgia and northern Florida
on 13 August 1983. These storms are located near the lower end of a size
spectrum of mesoscale convective precipitation systems which range from massive
MCCs to small isolated thunderstorms.
24
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Following the procedures of the case studies of an MCC and squall line
(Lyons, Calby, and Keen, 1985), the impact of these air mass thundershowers on
the polluted PBL were examined. Figure 24 shows the change in visibility during
the 24 hour period beginning 1800 GMT, 13 August 1980, and ending 1800 GMT, 14
August 1980. Since a portion of the CARE was over the Atlantic by 14 August,
GOES satellite images (Figures 18-19) were used to estimate the over-ocean visi-
bility contours. The area of visibility change of 5 km (3 mi) or greater was
57,600 km^. The average change in visibility was 7.6 km (4.8 mi). No explicit
measurements of mixing height were available, but sounding data from Athens,
Georgia and Charleston, South Carolina were used to estimate an average mixing
depth of 900 m.
The relationship between atmospheric turbidity, expressed as a backscat-
tering coefficient bscat, and sulfate has been proposed to take the form,
bscat = a + cX (1)
where bscat is expressed in units of lO'^m"^- and X is the total sulfate aerosol
mass concentration (ug/m^). Patterson et al . (1979) proposed values of
a = 1.3 and c = 0.17, while Leaderer et al . (1981) suggested a = 0.48 and c =
0.088. The conversion between regional visibility and bscat is given by the
familiar Koschmieder relationship,
bscat = 24.3 / V (2)
where V is in statute miles. Using the above equations and calculating
visibility/sulfate relationships, neither give particularly satisfying results.
The Patterson formulation disallows any significant sulfate aerosol mass at
regional visibilities greater than about 20 miles, whereas the Leaderer curve
results in concentrations which appear somewhat high, especially at large bscat
values. Since the Leaderer data were developed using largely New York City data
to estimate c, mean data from a variety of sites in the eastern United States
reported in Leaderer et al. (1981) were averaged, providing a c of 0.10 which is
somewhat more in line with the author's experience as to visibility/sulfate
intercomparisons. In all likelihood the proper function is neither constant nor
linear. However, this provides at least a reasonable first approximation as to
the changes in atmospheric sulfate aerosol concentration due to the passage of
the MCS at a site.
Using the above equation, the average volumetric decrease of sulfate aerosol
concentrations within the CARE can be estimated. The average visibility change
and the depth of the CARE (assumed equal to the mixing depth) is employed. The
29
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TABLE 1
COMPARISON OF THE IMPACTS OF VARIOUS SIZED MESOSCALE
PRECEPITATION SYSTEMS: MCC, SQUALL LINE, AND AIR MASS THUNDERSHOWER CLUSTERS
(1)
(1)
Area of analysis (km^)
Area receiving rain
Average rain depth (cm)
Area receiving rain (%)
Total mass of rain (xlO^ kg)
Area with vis. change (km2)
Area with vis. change (%)
Average vis. change (km)
Average vis. after rain (km)
Depth of mesohign (m)
Avg. vol. $04 change(ug/m~3)
Total displaced S04 (xlO6 kg)
Max. $04 cone, in rainwater (mg/1)
Max. 504 wet deposition (mg/m^)
MCC
1-2 Aug 80
650,248
286,700
0.82
47
236
467,527
77
13.3
19.5
1500
38.3
26.8
11.35
93.4
SQUALL LINE '
1-2 Aug 80
636,608
223,000
0.96
35
215
297,657
47
5.9
12 .,6
600
26.9
4.8
2.24
21.6
AIR MASS
13-14 Aug 80
172,800
36,460
0.48
28
17.5
57,600
33
7.6
18.4
900
14.8
7.7
3.38
16.2
(1) From Lyons, Calby, and Keen (1985)
30
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details of the technique are discussed in Lyons and Calby (1984). The average
volumetric sulfate aerosol displacement was calculated to be 14.8 ug/m3. The
total mass of sulfate displaced to form the CARE was 0.8 x 106 kg. This is a
factor of 50 times less than displaced by the MCC of 2 August 1980.
It was not possible to estimate the rainfall volume over the ocean with
these data and no attempt was made to do so. Thus, the total storm precipi-
tation estimates used in this study probably err on the low side. Note, however,
that the isohyets (Figure 13) do indicate that most of the rainfall occurred
just west of the coastline. The average depth of rainfall over land was 0.48 cm
and covered 36,460 km^. if we assume that all particulate sulfate was washed
out in the rain, the average rainwater concentration of $04 would have been 3.38
mg/1. Similarly, the average $04 deposition would have been 16.2 mgm~2.
An examination of Table I shows that the air mass thunderstorm cluster
does represent a smaller perturbation to the polluted PBL than either the MCC or
squall line, as was expected. The area experiencing a visibility increase was
approximately eight times less than that caused by the MCC, and three times less
than the CARE from the squall line. The total displacement of $04 mass from the
polluted PBL, 7.7 x 105 kg, is substantially smaller.
However, it can also be noted that while smaller in area and total mass
displacement, the impact of the air mass thunderstorm cluster was as "intense"
as the large MCS types. The average visibility change of 7.6 km was greater
than for the squall line of 2 August. Similarly the resultant visibility of
18.4 km was only slightly less than for the MCC case. The average volumetric
sulfate aerosol decrease was less than the 2 August case, most likely due to the
substantially less polluted nature of the antecedent air mass.
Regional ozone patterns were also impacted by the CARE. Figures 20-21 show
the maximum hourly ozone concentrations (in ppb) over the southeast on 13 and 14
August respectively. The scarcity of monitoring sites (less than two dozen)
make it difficult to ascertain any strong patterns due to the comparatively
small size of the CARE. An area of substantially reduced maximum 03 levels (10
ppb vs. 80 ppb) is found along the Georgia - South Carolina coast on 14 August.
This is most likely the signature of the eastward portion of the CARE, now
transported offshore.
It is becoming increasingly certain that CAREs of various sizes are
routinely produced by convective storm systems with a considerable variety of
types and magnitudes.
SATELLITE DETECTION OF THE CARE OF 14 AUGUST 1980
The extent and magnitude of the CARE over Georgia, the Carolinas, and adja-
cent ocean areas was evaluated using GOES satellite digital VISRR brightness
values which were archived as part of PEPE/NEROS-80. These brightness values
were obtained using the McIDAS (Man Computer Interactive Data Access System) at
the Space Science and Engineering Center of the University of Wisconsin-Madison
(Wash and Whittaker, 1980). This procedure is discussed in more detail by Lyons
31
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Figure 18. GOES visible image, 2 km resolution, DBS sector, showing cloud and
patterns over the southeast United States at 1430 GMT, 14 August 1980. Notice
the area of reduced turbidity east of the South Carolina coast.
Figure 19. GOES visible image, 2 km resolution, DB 5 sector, 2130 GMT, 14
August 1980, showing convective clouds and turbidity patterns over the southeast
United States. Notice the extensive areas of reduced turbidity east of the South
Carolina coastline.
32
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DAILY MAXIMUM
OZONE READINGS (PPB)
13 AUGUST 1980
Figure 20. Isopleths of maximum one hour surface ozone reports, from monitors
in the study area, 13 August 1980. Contours are in parts per billion (ppb).
33
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60
40
DAILY MAXIMUM
OZONE READINGS (PPB)
14 AUGUST 1980
Figure 21. Same as Figure 23, but for 14 August 1980.
34
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DELTA BRIGHTNESS
13 AUGUST 1980
1400 GMT
Figure 22. Isopleths of delta brightness levels, derived from the McIDAS
system, using digital satellite imagery for 1400 GMT, 13 August 1980. Areas
with cloud cover shown in stipple.
35
-------
10
DELTA BRIGHTNESS
14 AUGUST 1980
1400 GMT
Figure 23. Same as Figure 22, but for 14 August 1980.
36
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2/3.2
6/9.6
8/12.8
4/6.4
24-HOUR VISIBILITY CHANGE
(MILES/KILOMETERS)
13-14 AUGUST 1980
1800 GMT
Figure 24. Change in visibility (statute miles/kilometers) reported by National
Weather Service first order stations in the study area from 1400 GMT, 13 August
to 1400 GMT, 14 August 1980.
37
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and Calby (1983) and Calby (1983). Digital brightness counts were archived for
190 selected areas across the eastern United States at 1400 GMT for each day of
the PEPE field study (15 July - 15 August 1980). The digital brightness count
as determined by the GOES-East sensor is proportional to the square root of the
scene radiance. Conversion of the digital counts to actual radiances was not
attempted in this study. Instead the digital counts were examined for relative
differences. A delta brightness value was determined at each monitoring site by
subtracting the site specific minimum or cloud and haze free minimum brightness
value from the brightness measured on the given day at each site. The lower the
delta brightness value, the less turbid the air mass.
Unfortunately, cloud contamination limited the usefulness of the satellite
data on both 13 and 14 August. But enough data points were obtained to support
the arguments of the previous sections. Figures 18 and 19 show the GOES
visible images for 1430 GMT and 2130 GMT, 14 August 1980. Though cloudiness is
scattered throughout the region, a distinct CARE is visible (especially in the
original) east of the Georgia-South Carolina border. The delta brightness
values obtained at 1400 GMT on 13 and 14 August are analysed in Figures 22 and
23 respectively. On the morning of 13 August note the low delta brightness
values, which indicate the clean air north of the cold frontal zone in North
Carolina and Tennessee. Southern Georgia, however, is still "bright", or hazy,
due to the developing PEPE in the region. By 14 August (Figure 21), much of
southern North Carolina had become increasingly "brighter" as the PEPE continued
to build. The large region of low delta brightness values extending eastward
from the South Carolina-Georgia border is very noticeable. This is the signa-
ture of the CARE from the thunderstorm clusters of the prior afternoon. As can
be seen in Figure 19, this CARE remained largely unchanged in size and shape
while moving northeast during 14 August. The generally stable nature of the
atmosphere over the ocean is in part responsible for the rather long lifetime of
this feature.
38
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SECTION 6
CASE STUDY: 29-30 JUNE 1975
During the last week of June and the first week of July, 1975, a massive
PEPE occurred over the eastern United States. Figure 25 shows the extensive
hazy area associated with this PEPE stretching from the Mississippi River more
than 1500 km east of the Atlantic coast. By 5 July 1975 (Figure 26) the eastern
United States from northern Florida to the southern Great Lakes was still
largely engulfed in anthropogenic haze. This episode has been studied in great
detail by Lyons and Husar (1976), and Lyons (1980). In the midpoint of the epi-
sode, on 30 June, visibility dropped as low as 2.5 mi (4 km) in the St. Louis
area. Since mid-day humidities were in the 40% range, hygroscopicity was not a
question. Nephelometer readings both in urban St. Louis and 30 km away showed
less than 30% variability, indicating no important urban component. Total
sulfate aerosol was measured at approximately 35 ug/m^, 40% of which was found
to be sulfuric acid aerosol. Measurements also showed that the vast majority of
the aerosols were in the .05 to 1.0 micrometer diameter range. Limited sulfate
measurements made across the country on 29-30 June showed that within the area
of haze as seen on the satellite picture (Figure 27), sulfate readings ranged
from 17 to as high as 80 ug/m^. Numerous air quality monitoring sites reported
peak maximum hourly ozone values in excess of 160 ppb.
Meteorological conditions during this period were nearly stagnant. The sur-
face analysis at 1800 GMT on 29 June 1975 (Figure 28), is typical of the pattern
which was maintained during the episode. Most interesting are computer trajec-
tories, at 600 m AGL, which extend for 36 hours, in six hour increments,
beginning at 0000 GMT, 29 June 1975 (Figure 29). Obvious is the slow, but per-
sistent flow from the Great Lakes to the Ohio Valley, then to the Tennessee
and Mississippi River valleys and then northwards into the western Great Lakes
region. The widespread nature of the episode was dramatically illustrated by
analyses of the surface visibility reports made from first order weather sta-
tions at 1800 GMT, 29 June 1975. Only those stations which were not reporting
obstructions to visibility (rain, fog, dust) and which had dewpoint depressions
of 3° F or more were included. The area of lowest visibility clearly
corresponds with the brightest and haziest portions of the satellite image in
Figure 27.
A closer look at Figure 27 reveals that the milky white haze of the PEPE is
not homogeneous. What appears to be thin spots in the haze are visible over
portions of southeastern Missouri, Illinois, Indiana, and along the Ohio River
Valley. It had long been assumed by the authors that these turbidity discon-
tinuities in fact represented CAREs, the result of mesohighs produced by scat-
tered air mass thunderstorms occasionally developing within the otherwise fair
weather high pressure system. An analysis was undertaken to correlate the
suspected CARE with the appropriate mesoscale convective precipitation system.
39
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t £2:00 25JN75 11A-1 01091 18171 DBS
Figure 25. GOES visible image, 2 km resolution, DBS sector, at 2200 GMT, 25
June 1975. A massive area of turbidity marks a PEPE stretching from the
Mississippi River eastward almost 1500 km into the Atlantic Ocean.
40
-------
Figure 26. GOES visible image, DBS sector, 2 km resolution, 1500 GMT, 5 July
1975, showing a large area of turbidity over the eastern United States during
the waning stages of the massive two-week long June/July 1975 PEPE event.
41
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Figure 27. GOES visible satellite image, 2 km resolution, DBS sector, at
2200 GMT, 29 June 1975. The massive PEPE extends from the Central Plains east-
wards to the Atlantic coast. Note in particular the pockets of apparently
cleaner air over portions of Missouri, Illinois, Indiana, and the Ohio River
Valley. Notice also discontinuity in the turbidity pattern over Lake Michigan.
42
-------
Figure 28. Surface analyses, 1800 GMT, 29 June 1975.
Figure 29. Computed trajectories, at 600 m AGL extending at six hour intervals
for 36 hours, beginning 0000 GMT, 29 June 1975.
43
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Figure 30. Analysis of visibility reports (in statute miles), from first order sur-
face weather stations, at 1800 GMT, 29 June 1975. Visibility in portions of Iowa
and Missouri dropped to as low as 2 miles.
44
-------
Archival 10" square GOES image negatives were obtained from National
Environmental Satellite Data Information Service for each hour of the two day
period. Figures 31 and 32 show tracings of the leading edge of the suspected
CARE during the daylight hours. On both days the suspected CAREs were seen to
drift with the low-level wind and in a southwesterly direction.
Paper fascimile copies of the hourly National Summary of Radar Reports (the
SD chart) were obtained from the National Climatic Data Center. These maps are
prepared from manually digitized radar data submitted hourly by each National
Weather Service radar station. While very useful for many purposes, MDR has one
conspicuous failing. Due to the coding technique, the area! coverage of preci-
pitation is greatly exaggerated since even the smallest thunderstorm automati-
cally fills a 22 mile square box completely. For 28-29-30 June 1975 (Figures
33-35), 24 hour composites of the hourly MDR charts were prepared. There was
apparently widespread precipitation indicated in the area. However, indications
of apparent coverage can result only from very brief and widely scattered
showers. On 28 June, precipitation appeared to be quite widespread over the
eastern United States. By 29 June, even in an MDR display, rain had become
quite sparse in the general vicinity of the Great Lakes and the Ohio River
Valley. The source region of the suspected CAREs. The same pattern held for 30
June 1975. Very little thunderstorm activity, other than extremely small, iso-
lated cells developed in the general vicinity of the CAREs on these two days.
Surface precipitation reports (Figure 36) from 27 June 1975 through 0600 GMT 29
June 1975 showed only widely scattered precipitation through the area. In the
areas presumed to be upwind of the CAREs as observed on 29 June 1975, only rela-
tively small and isolated patches of precipitation occurred. It became
increasingly clear that in spite of a most intense scrutiny of the date, these
very distinct CAREs could not be related to specific thunderstorm clusters.
This is quite an unexpected result.
After considerable puzzlement, an interesting observation was made. If one
examines Figure 31 and especially Figure 32, the suspected CARE outline bears a
noticeable resemblance to the shorelines of the Great Lakes which are located
upwind. At this point a new hypothesis is formulated. It appears plausible
that there may exist a pseudo-CARE, i.e., localized intrusion of lower turbidity
air from aloft into an otherwise turbid PEPE region by other than convective
precipitation processes. As noted by Lyons and Cole (1976), the synoptic
situations during the 28-30 June 1975 period was ideal for the development of
intense lake breezes over the Great Lakes. A lake breeze is a highly complex
phenomenon. Among other things it results in substantial transport and disper-
sion of pollutants both on a local and regional scale. This has been studied
with the aid of photography by Lyons and Olson (1973) and numerical models
(Lyons et al. 1983). Lyons (1970, 1971) documented the intense subsidence which
occurs over the Great Lakes during such lake breezes. Thus, while air over land
is being continuously mixed upward due to the development of the daytime heated
45
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Figure 31. Analysis of the turbidity discontinuities observed in the satellite
images on 29 June 1975. Each line indicates the boundary between the clean air
and more turbulent air, on the edge of the apparent CARE. CARE boundaries are
shown at 1 hour intervals.
46
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Figure 32. Same as Figure 31, but for 30 June 1975,
47
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Figure 33. Summary of all areas reporting radar echos via MDR generated radar
reports for the 24 hour period of 28 June 1975. The area of coverages greatly
exaggerates the actual area of coverage since even the smallest thunderstorm 22
mile square area is indicated in blue.
Figure 34. Same as Figure 33, but for 29 June 1975,
48
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Figure 35. The same as for Figure 33, but for 30 June 1975.
Figure 36. Summary of precipitation reports, from 1200 GMT, 27 June 1975 through
0600 GMT, 29 June 1975. Amounts greater than one inch (2.54 cm) are shown in
yellow.
49
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LAKE BREEZE IN/IRACT ON BOUNDARY LAYER AEROSOLS
A SUNRISE
POLLUTED
BOUNDRRV LRVER
fvl ID-AFTERNOON
LflKE BREEZE
FRONT
LRKE BREEZE
FRONT-
Figure 37. Hypothesized mechanism whereby a pseudo-CARE is generated due to
subsidence associated with lake breezes over the Great Lakes. Aerosol matter in
the initial uniformly polluted planetary boundary layer is shown in light "
stippling.
50
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due to the highly divergent surface wind field. The lack of any thermal mixing
over the lake further stabilizes the boundary layer over water. Figure 37
illustrates the hypothesized mechanism whereby a pseudo-CARE can be generated
over the Great Lakes due to the subsidence associated with a lake breeze. An
initially uniformly polluted boundary layer during the early morning hours is
drifting with the synoptic flow across the Great Lakes, in this case from the
northeast to the southwest. By mid-afternoon, with the establishment of a lake
breeze circulation, mixing over land counterbalanced by extreme subsidence over
water causes a marked depression in the depth of the aerosol contamination over
the lake. By evening, with the cessation of the lake breeze circulation, the
aerosol depletion zone drifts inland with the synoptic flow. With the on-set of
convective mixing the following morning, the very thin polluted layer is then
mixed with essentially clean air above, resulting in dramatically reduced aero-
sol loading and turbidity and improved visibilities. Aside from the shape,
other evidence suggests the pseudo-CARE may, in fact, have been formed by the
lake breeze mechanism. The general direction of the boundary layer flow as seen
in the trajectory computations, plus the presence of lake breezes over the Great
Lakes on these days, both suggest this is a plausible explanation.
51
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SECTION 7
OTHER PSEUDO-CARES
In Section 4 it was shown that the development of a CARE as the result of
even smaller scale thunderstorms is a definite and probably frequent occurrence.
However, the apparent discovery of the pseudo-CARE suggests that not all low
turbidity pockets within PEPE air masses are related to precipitation events.
There are a number of phenomena which occur in the dry atmosphere that can
possibly depress the mixed layer. If subsidence brings cleaner mid-tropospheric
air close enough to the surface , it becomes susceptible to ultimate mixing to
the surface itself by thermal convective processes.
Mountain waves are a well known phenomena and appear with considerable regu-
larity over the Appalachian Mountains of the eastern United States (Pecnick and
Young, 1984). In general, intensive standing wave activity may develop in the
lee of mountains when zonal winds increase with altitude, especially if there is
some form of inversion at or above the crest of the mountains in question. It
is not uncommon in GOES imagery to be able to see what appears to be the
rippling effects of mountain waves in the turbidity pattern associated with
PEPEs.
Another commonly occurring phenomena within the boundary layer are parallel,
counter-rotating roll vortices (LeMone, 1973), resulting from inflectional
instability of the wind field which gives rise to phenomena such as cumulus
cloud streets. These have been studied in great detail since the pioneering
paper of Faller and Kaylor (1966). Subsidence may occur in long parallel bands
perhaps 5 or 10 kilometers wide, interspersed between the upward motion zone
capped by clouds. There are many other forms of convective instability in the
heated boundary layer that are only partially understood. Over the oceanic
areas these take on the more familiar patterns of Benard convection which have
been graphically described by Agee (1984) using satellite imagery. Over land
there is occasional evidence of the development of polygonal convection cells.
Cumulus and small thunderstorms form in in the vertices of these open polygons
which surround areas of large scale gentle, but persistent subsidence. These
are rather transient and ephemeral features of the atmosphere, but may well
become involved in the pseudo-CARE generating mechanism.
A more dramatic series of satellite photos shows pseudo-CARE. In Figure 38,
a sequence of images beginning at 1437 and lasting through 2100 GMT, 30 August
1980, shows a widespread PEPE covering the southeastern United States from the
mid-Ohio Valley southwards. What appears to be actual CAREs are seen during the
early morning hours over eastern Alabama although they slowly become obscured by
developing convective clouds as the day wears on. What is more interesting is
the development of a progressively clearer slot that extends from Iowa through
52
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1437
Figure 38. The generation of a pseudo-CARE as indicated by GOES visible
imagery on 19 August 1980. Image sequences begin at 1437 GMT and continue
through 2100 GMT. The last image is an infrared scene.
53
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1830
Figure 38 continued.
54
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northeastern Missouri, central Illinois, southern Indiana, and along the Ohio
River and ultimately goes into western Pennsylvania by the end of the day.
Cumulus clouds are clearly suppressed while the ground contrast is enhanced,
indicative of a lower optical depth of atmospheric aerosols. These features are
clearly unrelated to any precipitation mechanism. Due to the propagation speeds
involved, the clear slot could not be the result of advective phenomena. There
has been considerable research over the past several years concerning mid-
tropospheric circulation systems often associated with frontal boundaries and
jet streams. A very similar, although somewhat smaller version of this type of
clear slot was reported by Koch (1984). In his example what appeared to be a
thermally direct frontogenetical circulation was related to the development of
the major squall line. It also exhibited characteristics of greatly suppressed
cumulus activity and therefore, by inference, strong subsidence in the lower
troposphere. Careful inspection of Figure 38 shows what may be a second
pseudo-CARE developed over northern Georgia and eastern Tennessee at 1830 GMT.
It has become apparent that there may, indeed, be a multitude of mechanisms
resulting in inhomogeneities of PEPE aerosol hazes. This does not in any way
invalidate the theory of precipitation-generated CAREs. However, it underlines
the points made by Lyons and Calby (1983) that, as part of any organized program
to monitor long-range transport and aerosol transformation, extensive supporting
mesometeorological data must be gathered in order to properly interpret those
aerosol measurements. Procedures which allow the gathering of mesoscale infor-
mation in real-time in a convenient format for later analyses (satellite, radar,
or lightning mapping) should be employed.
Furthermore, it should be standard operational procedure wherever possible
during field programs to collocate all pollution and meteorological sensors so
that a better understanding of interrelated phenomena may be gained.
Without an appreciation for the several apparent causes of polluted boundary
layer inhomogeneities, it is possible that aerosol measurements made during
transport and transformation experiments could be improperly interpreted. Lower
aerosol concentrations might be considered as indicative of diminished transfor-
mation rates whereas an incident of entrainment of cleaner air from above may be
the responsible agent.
55
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63
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
FURTHER CASE STUDIES ON THE IMPACT OF THE MESOSCALE
CONVECTIVE SYSTEMS ON REGIONAL OZONE AND HAZE
DISTRIBUTIONS
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
. AUTHOR(S)
Walter A. Lyons and Rebecca H. Calby
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
MESOMET, Inc.
35 EAst Wacker Drive
Chicago, IL 60601
10. PROGRAM ELEMENT NO.
CDWA1A/02-3193 (FY-86)
11. CONTRACT/GRANT NO.
68-02-4051
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
Atmospheric Sciences Research Laboratory - RTP, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report is a continuation of an earlier effort to study the impact of
mesoscale convective precipitation systems upon distributions of aerosol and
photochemical oxidant pollutants in the planetary boundary layer (PBL). Analyses
of surface visibility and ozone data revealed a dramatic response in the boundary
layer pollutant patterns to the passage of two very large convective storm systems.
Regional visibilities, at times less than 5 km, increased dramatically over a
multistate area to as high as 80 km. The resultant clean air region was termed a
convective aerosol removal event (CARE).
In this study, a well defined CARE was found off the Georgia coast on 14 August
1980. Extensive mesoanalyses revealed that the clear air pocket embedded within a
large surrounding PEPE (Persistent Elevated Pollution Episode) originated from a
mesohigh system formed by a cluster of thunderstorms over northern Florida and
Georgia the previous day. While the increase in regional visibility and the
volumetric depletion of sulfate aerosol was of the same magnitude as in the MCC
squall line case, the total mass of displaced aerosol was fully 50 times less
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b.lDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS /This Report/
UNCLASSIFIED
21. NO. OF PAGES
20. SECURITY CLASS /This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (R«». 4-77) PREVIOUS EDITION is OBSOLETE
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