METHOD

1
Influence of mid-latitude cyclones on trace gas
transport from North America to the western North
Atlantic Ocean O. R. Cooper and J. L.
Moody Department of Environmental Sciences,
University of Virginia, Charlottesville D. D.
Parrish, M. Trainer, T. B. Ryerson, J. S.
Holloway, G. Hübler, and F. C. Fehsenfeld Aeronomy
Laboratory, NOAA, Boulder, Colorado M. J.
Evans Division of Engineering and Applied
Sciences, Harvard University, Cambridge,
Massachusetts Andreas Stohl Lehrstuhl für
Bioklimatologie und Immissionsforschung,
Technical University of Munich, Germany

INTRODUCTION The North Atlantic Regional
Experiment (NARE) is a multi-year,
multi-institutional NOAA research initiative.
The programs primary objective is to understand
the chemical processing of trace gases
transported to the atmosphere of the western
North Atlantic Ocean (WNAO) from the surrounding
continents. NARE has produced aircraft-based
trace gas measurements from mid-latitude cyclones
during 1993, 1996, 1997 and 1999. Previous NARE
studies have determined that mid-latitude
cyclones export the bulk of North American trace
gases to the WNAO atmosphere. Despite numerous
case studies of trace gas transport to the WNAO,
the literature lacks a comprehensive
understanding of the typical trace gas signatures
of mid-latitude cyclones. This study explores
the NARE data in terms of cyclone airstreams and
presents a coherent picture of trace gas export
to the WNAO in the form of a conceptual cyclone
model. The model separates the meteorological
influences on airstream trace gas signatures from
the influence of surface emissions heterogeneity.
The analysis focuses on the spring 1996 and
autumn 1997 NARE campaigns. The flight tracks
and cyclones tracks from these study periods are
shown to the right.
Autumn 1997 (black lines) and spring 1996 (yellow
lines) cyclone tracks
Autumn 1997 (black lines) and spring 1996
(yellow lines) flight tracks


Mid-latitude cyclones are composed of four major
airstreams and their trace gas signatures are
influenced by air-mass origin and the associated
meteorological processes. This figure depicts
the airstreams of an idealized mid-latitude
cyclone tracking from North America to the WNAO,
showing the warm conveyor belt (WCB), cold
conveyor belt (CCB), dry airstream (DA) and post
cold front airstream (PCF). The center of the
cyclone is indicated (L) and the scalloped lines
demarcate the border of the comma-cloud formed by
the airstreams. The numbers on the warm and cold
conveyor belts indicate the pressure at the top
of these airstreams, while the numbers on the dry
airstream indicate the pressure at the bottom of
this airstream. The PCF flows beneath the dry
airstream.
This panel provides an example of the airstreams
within a mid-latitude cyclone sampled by the NOAA
WP-3D aircraft during spring, 1996. The cyclone
formed above the coast of the Carolinas and by 00
UTC, April 2, the fronts and cloud features were
clearly visible in the GOES infra-red channel
(above left). The warm conveyor belt flows
northward, parallel to the cold front and rises
quasi-isentropically from the lower troposphere
above the Yucatan Peninsula to the upper
troposphere above the warm front (above right).
Similarly, the cold conveyor belt rises into the
mid-troposphere north of the warm front. The
post cold front airstream flows behind the cold
front from central Canada to the WNAO in the
lower and mid-troposphere. The dry airstream
flows above the post cold front airstream and
descends isentropically behind the cold front
advecting air from the upper troposphere and
lower stratosphere into the mid-troposphere.
The dry airstream is the location of
stratosphere-troposphere exchange which occurs
via tropopause folding within every mid-latitude
cyclone. The magnitude of the exchange is
roughly proportional to the strength of the
cyclone.
METHOD
To develop a conceptual model of the trace gas
signatures of a typical mid-latitude cyclone
tracking from North America to the WNAO, the
flight tracks from each season were classified
according to the airstreams they intersected.
Airstreams were identified using 1)
meteorological data from the aircraft, 2) back
trajectories, and 3) in-situ, modeled and
remotely sensed meteorological data from the
University of Wisconsins Unidata/Mcidas data
stream. Once the flight-track segments were
classified by airstream, the corresponding
chemical data were amalgamated. Ozone, CO, NOx
and NOy were measured from a NOAA WP-3D Orion
aircraft at 1-second time resolution, which at
the speed of the aircraft corresponds to an
average along 0.1 km of flight path.
Here the flight tracks are plotted relative to
the center of the cyclones, showing the spatial
coverage of the four airstreams in the lower,
mid- and upper troposphere. The numbers in each
plot indicate the number of flights that sampled
each type of airstream. The data coverage is
quite good for all airstreams except the CCB.
Note that the DA does not penetrate into the
lower troposphere likewise the CCB and PCF do
not reach the upper troposphere. Data coverage
during spring 1996 was similar.
This figure shows the eleven flight tracks from
late-summer/early-autumn, 1997, and the
concurrent locations of the cyclone centers
(yellow L).
The flight tracks were re-plotted, with latitude
and longitude relative to the center of the
cyclones rather than the intersection of the
Prime Meridian and Equator. As shown above,
three quadrants of the cyclones were well sampled
while the NW quadrant was not.
2
RESULTS
Data from Olivier, J. G. J, J. P. J. Bloos, J.
J. M. Berdowski, A. J. H. Visschedijk and A. F.
Bouwman, A 1990 global emission inventory of
anthropogenic sources of carbon monoxide on 1 x
1 developed in the framework of EDGAR/GEIA,
Chemosphere Global Change Science, 1, 1-17,
1999.
Surface emissions heterogeneity has a strong
impact on the quantity of anthropogenic emissions
that enter cyclone airstreams. This map shows
the annual CO emissions from all anthropogenic
sources in North America. Clearly, cyclone
airstreams that draw from the eastern United
States will contain greater levels of emissions
than those that draw from northern Canada.
These panels show the paths most commonly
traversed by the airstreams sampled over the WNAO
during late-summer/early-autumn 1997, with warmer
colors indicating regions of greater influence.
The WCB had a strong influence from continental
and marine regions, while the CCB and PCF were
mainly influenced by the continent.
Similarly, these panels show the airstream paths
during spring, 1996. Not only does the WCB have
a stronger continental influence during spring,
but the WCB, CCB and PCF have a stronger
influence from the high emissions regions of the
USA. The more southerly cyclone tracks in
spring account for nearly 50 of the increase of
lower troposphere CO from autumn to spring.
C


Median ozone mixing ratios
These figures convey the typical structure and
location of mid-latitude cyclones tracking from
N. America to the WNAO, and also show the median
O3 and CO mixing ratios. O3 is greater in spring
due to a stronger influence from the
stratosphere, the longer lifetime of ozone during
the previous winter months and springtime
photochemistry. CO is greater in spring due to
reduced destruction by the OH radical during
winter. In both seasons the PCF has the smallest
O3 mixing ratios in the lower troposphere due to
transport from central Canada. But the greatest
ozone mixing ratios are found immediately above
in the DA, the result of stratosphere/troposphere
exchange. In both seasons the greatest O3 mixing
ratios in the lower troposphere are found in the
WCB. The WCB forms on the western side of
surface anticyclones where warm and stagnant
conditions favor photochemical ozone production.
In addition, the western sides of surface
anticyclones are the most likely regions for
stratospheric ozone to eventually mix down to the
surface. The CCB has the greatest CO mixing
ratios in the lower troposphere but the
meteorology is not as favorable for photochemical
O3 production. The /- symbols indicate the
airstreams with significant O3/CO slopes.
Median CO mixing ratios
D A
WCB
CCB
PCF
D A
WCB
CCB
PCF
D A
WCB
CCB
PCF
Autumn 1997 Distributions of relative humidity,
O3, CO, NOy, and the NOx/NOy ratio in the lower,
mid- and upper troposphere for each airstream.
The data are expressed as percentiles 50th
percentile (), 25th and 75th percentiles (open
circles), 5th and 95th percentiles (squares), and
the minimum and maximum values (solid circles).
The mean (horizontal line) plus or minus the
standard deviation (shaded bar) is also
shown. Ozone has the greatest variation between
airstreams, while relatively little NOy is
transported out of the lower troposphere.
Spring 1996 O3 vs. CO for all four airstreams.
The lighter (darker) shading corresponds to
relatively higher (lower) data density. The range
of the autumn 1997 data for each airstream is
outlined in red. Linear regression lines for each
airstream are shown for spring 1996 (gray lines)
and autumn 1997 (red dashed lines) with the slope
and r2 values in black (spring) and red (autumn).
O3 and CO background mixing ratios are shown for
spring (blue dot) and autumn (red dot). Autumn
photochemical O3 production occurs in the lower
troposphere PCF and all levels of the WCB.
Significant springtime photochemical O3
production is not evident in any airstream, with
the lower troposphere CCB associated with O3
destruction. The negative DA slopes are the
result of stratosphere/troposphere exchange.

CONCLUSIONS Mid-latitude cyclone airstreams
govern the transport of trace gases to the WNAO.
Composite cyclones for other regions would
undoubtedly show similar results. An airstream's
source region has a strong influence on the mean
or median mixing ratios of its constituent trace
gases. However, the ozone/CO slopes and
ozone/NOy slopes for a particular airstream
remain constant in a given season, regardless of
air mass origin. Stratosphere/troposphere
exchange occurs in every dry airstream of every
cyclone, the intensity proportional to the
strength of the cyclone. However, the seasonal
cycle of ozone in the lowermost stratosphere
allows greater quantities of ozone to enter the
troposphere during spring. The more southerly
cyclone tracks that occur in spring account for
nearly 50 of the observed increase of lower
troposphere CO from autumn to spring. The
remainder of the variation is due to the seasonal
CO cycle. Wet deposition of the water soluble
constituents of NOy occurs in the warm conveyor
belt and cold conveyor belt but not in the post
cold front airstream or the dry
airstream. During autumn photochemical ozone
production is evident in the warm conveyor belt
at all levels and in the lower troposphere post
cold front airstream. During spring net ozone
production appears absent from all airstreams,
with the cold conveyor belt influenced by ozone
destruction. Ozone and CO are greater in spring
but the relative mixing ratios between airstreams
are roughly the same in spring and
autumn. NOx/CO emissions ratios vary across the
mid-latitudes according to socio-economic
factors. It is expected that the emissions
variation influences the ozone production
efficiency of the cyclone airstreams that draw
from these regions. This conceptual cyclone
model has established the fundamental
relationships between synoptic/regional-scale
chemical transport and mid-latitude cyclone
structure. These results can provide critical
tests for the output of those chemical transport
models with the ability to resolve the structure
of cyclone airstreams.
This study has focused on spring and early autumn
data above the WNAO, the two seasons that roughly
coincide with the seasonal maximum and minimum of
tropospheric background O3 and CO. However
neither season contained a pollution episode
comparable to those that occur during summer over
the eastern USA. To illustrate the differences
and similarities between the conceptual cyclone
model and such an episode we compare the present
results to a NARE flight on August 28, 1993 .
Autumn 1997 WCB
Spring 1996 WCB
Autumn 1997 DA
Autumn 1997 WCB
Autumn 1997 DA
Autumn 1997 WCB
Data from all flight altitudes on August 28, 1993
are shown (gray dots) with data in the lower,
mid- and upper troposphere highlighted (yellow
dots). The August 28 data in the lower
troposphere are far greater than anything
measured in spring or autumn but they still fall
along the slope of the early-autumn WCB,
indicating that the slope is a better trace gas
signature than mean or median mixing ratios. The
high O3 mixing ratios measured in the upper
troposphere fall within the domain of the autumn
DA, supporting the meteorological observations
that this air mass partially originated in the
stratosphere.
The flight (white line) occurred in the warm
sector of a cyclone just ahead of the WCB (green
arrow). In the lower troposphere the aircraft
intersected an air mass that had stagnated over
the eastern USA for two days (red trajectory).
In the upper troposphere the aircraft intersected
a remnant of the DA from the previous cyclone
(blue trajectory).
Funding for this research was provided by NOAA
Award Nos. NA96GPO409 and NA76GPO310 to the
University of Virginia. Real-time meteorological
data provided by UNIDATA internet delivery and
displayed using McIDAS software.