Download shadoz—a tropical ozonesonde–radiosonde network for the

SHADOZ—A TROPICAL
OZONESONDE–RADIOSONDE
NETWORK FOR THE
ATMOSPHERIC COMMUNITY
BY
ANNE M. THOMPSON, JACQUELYN C. WITTE, SAMUEL J. OLTMANS,
AND
FRANCIS J. SCHMIDLIN
A new observing network reveals complexities of tropical ozone while providing an
international model for climate-related data collection and public archiving.
B
ACKGROUND. The ozonesonde measurement. Although ozone-measuring satellites have
been collecting data since 1970 (Heath et al.
1975), validation of profiles and total column measurements is still performed by relatively lowtechnology instruments. Total column ozone is verified with spectrophotometric data from a ground-based
network (Bojkov et al. 1999). Profiles measured by
balloon-borne ozonesondes are used to evaluate satellite retrievals (WMO 1998a). The ozonesonde is
flown with a standard radiosonde. Designed to measure ozone concentrations from the surface to above
AFFILIATIONS: THOMPSON—NASA Goddard Space Flight Center,
Greenbelt, Maryland; WITTE—SSAI, Lanham, Maryland; OLTMANS—
NOAA/Climate Diagnostics and Monitoring Laboratory, Boulder,
Colorado; SCHMIDLIN—NASA Wallops Flight Facility, Wallops Island,
Virginia
CORRESPONDING AUTHOR: Anne M. Thompson, NASA
Goddard Space Flight Center, Code 916, Greenbelt, MD 20771
E-mail: [email protected]
DOI:10.1175/BAMS-85-10-1549
In final form 2 May 2004
©2004 American Meteorological Society
AMERICAN METEOROLOGICAL SOCIETY
the ozone concentration maximum, the combined
ozonesonde–radiosonde package is flown with a
1200–1500-g balloon that usually bursts at 4–8 hPa.
In an electrochemical concentration cell (ECC)
ozonesonde, air pumped through a pair of cells containing a potassium iodide solution initiates an electric current proportional to the amount of ozone in
the atmosphere. The ozone current is transmitted
back to a ground receiver and the partial pressure of
ozone is recorded with the pressure–temperature–
humidity (PTU) readings of the radiosonde. Figure 1
shows an ozonesonde during its preflight testing
(Fig. 1a) and its assembly with the radiosonde
(Fig. 1b) prior to launch.
Figure 2 shows the raw data (relative humidity,
temperature, and ozone pressure; left panel) and
ozone amount (right panel) in volume mixing ratio,
the unit most commonly used by atmospheric chemists. Note that stratospheric ozone (the “good” ozone
shielding the earth from ultraviolet radiation) is
present at > 1 part per million by volume (ppmv),
whereas tropospheric ozone (sometimes referred to
as “bad” or “smog” ozone) is counted in parts per billion by volume (ppbv). Background surface ozone
ranges from 10 to 40 ppbv, depending on geographiOCTOBER 2004
| 1549
ozone measurements in the 1920s; for a review see
Staehelin et al. (1998). One Dobson unit corresponds
to 2.69 × 10 (16) molecules above 1 cm2 or a thickness of 0.1 mm at standard temperature. The thickness of all ozone molecules is typically 2.5–4.5 cm-atm
or 250–450 Dobson units. The lower values typify a
naturally thinner ozone column over the Tropics. The
highest values occur near the poles, except during
ozone depletion extremes when the column depth
over Antarctica may drop to < 100 DU.
Presently about 50 stations send ozonesonde data
to the World Ozone and Ultraviolet Data Centre in
Toronto, Canada (information online at www.woudc.
org), an archive operated by Environment Canada. In
addition to providing ground truth and climatologies
for satellites (Fortuin and Kelder 1998), ozone profiles are used for the evaluation of chemical models
(Logan 1999a,b; Lawrence et al. 1999; Bey et al. 2001)
and the determination of ozone trends (WMO 1998a;
Randel et al. 1999). Most Northern Hemisphere
ozonesonde stations date from the 1960s and 1970s.
The only tropical station in the Southern Hemisphere
that has operated ozonesondes routinely for more
than a decade is at Natal, Brazil (Logan and Kirchhoff
1986; Kirchhoff et al. 1988).
FIG. 1. (a) Photo of the ECC ozonesonde instrument.
Potassium iodide sensing solutions are placed in the
plastic cells at the left. The inlet (blue tube at right) pulls
air toward the pump; a second tube sends air into the
cells. Behind the upright flat metal piece is the electronics board that transmits the current from the ozone
sensor along with the radiosonde signal. The ozone current is proportional to the ozone partial pressure
(Fig. 2, left box, black). (b) Assembled ozonesonde, inside its foam box, with radiosonde attached on the right.
cal location. On a highly polluted day in the United
States—called “code red” in some areas—surface
ozone may exceed 150 ppbv.
The column-integrated amount of ozone is measured in mm-atm or Dobson units (DU), which is
named after G. M. B. Dobson who began routine
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OCTOBER 2004
Tropical ozone and meteorological issues. Ozone column
amounts in the Tropics vary over a smaller range than
at higher latitudes and normally fall within 225–
300 DU (e.g., Fig. 6 in Thompson et al. 2003a). The
reason is that there is less variability in the stratospheric ozone column in the Tropics compared to the
extratropics. Signatures of the quasi-biennial oscillation (QBO) on ozone have been known since the
1960s, and their investigations have been greatly enhanced by satellite data (e.g., Bowman 1989;
Hollandsworth et al. 1995; Bojkov and Fioletov 1996).
The QBO interactions with ozone are described in
many papers summarized in the Baldwin et al. (2001)
review article (see below). The equatorial Total Ozone
Mapping Spectrometer (TOMS) solar backscatter
ultraviolet (SBUV) total ozone records show evidence
of the cyclical variability attributed to the QBO
(Shotani 1992; Hollandsworth et al. 1995). The Stratospheric Aerosols and Gases Experiment (SAGE) satellite captures changes in the ozone profile as it responds to shifts in midstratospheric winds (Zawodny
and McCormick 1991; Randel and Wu 1996).
Interannual variability in total ozone also correlates
with El Niño–Southern Oscillation (ENSO).
Focused attention on tropospheric ozone at low
latitudes originated in concerns about the impact of
widespread biomass burning in the Tropics and in
1991), although one equatorial cruise found a number of soundings with a tropospheric column depth
< 10 DU (Kley et al. 1996).
Net photochemical destruction of ozone over the
Pacific (Thompson et al.
1993) leads to very low
surface ozone (< 5 ppbv).
Convective mixing of lowozone surface air can lead to
low ozone concentrations
aloft (Piotrowicz et al. 1991)
and a low-tropospheric
ozone column.
The Atlantic–Pacific difference in ozone column
depth was detected by the
TOMS satellite, where a
FIG. 2. Typical profile from the SHADOZ Web site. (left) Data in ozone parzonal view displays a wavetial pressure, relative humidity, and temperature from radiosonde, with (right)
one-like appearance (Fishman
ozone volume mixing ratio determined from the ratio of the ozone and total
and Larsen 1987; Shiotani
atmospheric pressures. Sounding was taken on 17 Oct 2001 from Ascension
1992). The wave-one pheIsland. High ozone peaks below 10 km originate from African regions with
biomass burning.
nomenon refers to a variable ozone depth when the
studies with satellite data in the late 1980s and 1990s. ozone column is viewed zonally. The difference beFor example, a 1980 field study characterized emis- tween the maximum (near 0∞ longitude, over the Atsions of ozone precursors (nitrogen oxides, carbon lantic) and the minimum (near 180∞) varies over the
monoxide, reactive hydrocarbons) from savanna fires course of the year, ranging from ~10–25 DU.
as well as their impact on ozone (Crutzen et al. 1985; Schematically, the wave is similar to the following:
Chatfield and Delany 1990).
In the south tropical Atlantic, a seasonal maximum
in total and tropospheric ozone observed from a satellite was attributed to large-scale savanna burning
(Fishman et al. 1986; 1991). Ozone soundings at
Brazzaville, Congo (4∞S, 15∞E), Ascension Island (8∞S,
15∞W), and Natal (6∞S, 35∞W) confirmed that Atlantic ozone was indeed concentrated in layers trans.
ported from regions of African and South American
burning (Cros et al. 1992; Olson et al. 1996). During
September–October, tropospheric column ozone may
exceed 50 DU over Africa, South America, and the
The fact that the stratospheric ozone column depth
tropical Atlantic, compared to ~30 DU in April or May viewed from SAGE does not show longitudinal vari(Logan and Kirchhoff 1986; Thompson et al. 1996). ability (Fishman and Larsen 1987; Shiotani and
Ozone pollution from African fires frequently crosses Hasebe 1994) suggests that there is no stratospheric
the Indian and Pacific Oceans, where it has been de- wave, but this has been difficult to verify given the
tected in soundings over La Réunion (Baldy et al. 1996), imprecision in lower-stratospheric ozone retrievals.
Java, Indonesia (Komala et al. 1996);Tahiti; French
The aforementioned observations summarize our
Polynesia; Samoa; and Fiji (Oltmans et al. 2001).
knowledge of tropical ozone up to 1998. Only 200–
In contrast to the Atlantic, tropospheric ozone col- 300 soundings from the Tropics were available for
umn amounts over the tropical Pacific were usually analysis by the scientific community. Furthermore,
less than 30 DU (Komhyr et al. 1989; Fishman et al. there was no consistency in launch frequency, instruAMERICAN METEOROLOGICAL SOCIETY
OCTOBER 2004
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mentation, and geographical coverage. Stations
started and stopped with campaigns, so trends and
mechanisms could not be determined with any reliability. Thus, more standardized and comprehensive
data were required for addressing the following:
• How does tropical ozone in the troposphere vary
with time? Which processes are responsible?
• What is the zonal structure of tropospheric ozone
in the Tropics? Can it explain the standing wave-one
pattern in equatorial ozone observed from satellite?
• What is the best way to prescribe an ozone profile
climatology for satellite retrievals?
• How can typical satellite algorithms account for regional and seasonal variability in tropospheric
ozone? Are new ozone products from satellites
accurate—for example, the estimates of tropospheric ozone column depth?
SHADOZ PROJECT AND DATA. In order to
ensure that sufficient numbers of soundings are available throughout the Tropics we initiated the Southern Hemisphere Additional Ozonesondes (SHADOZ)
project in 1998, putting together a network of stations
according to the following criteria:
• That they are operational to avoid startup costs and
to leverage off local support and infrastructure;
• That they contribute to a geographical distribution,
spanning the entire longitude range;
• That there are weekly launches, with the project
sometimes supplying additional sondes (hence, the
name) in exchange for all of the sonde data from
the location; and
• That there is unrestricted Web site access by the
community to the complete SHADOZ dataset.
The nine original SHADOZ stations stretched
from the western Pacific and eastern Indian Oceans
across to South America, Africa, and La Réunion, an
island east of Madagascar. Since 1998, four more stations have joined the network, including two in the
northern Tropics [Surinam and Malaysia (3∞N,
102∞E)] that displays distinct influences from the
Southern Hemisphere (Peters et al. 2004). All stations
are listed in Table 1.
More than 2000 sonde profiles for the twelve stations (• in Fig. 3) are available at the SHADOZ Web
site (online at http://croc.gsfc.nasa.gov/shadoz), including 80 profiles from several campaigns of opportunity (* in Fig. 3). Experimental details of each station and personnel contact information are given in
Thompson et al. (2003a). In addition to the data and
graphical display of the sounding, the air parcel history of each sounding is depicted at the SHADOZ
Web site. These are based on 5-day back trajectories
run with the National Aeronautics and Space Administration (NASA) Goddard kinematic trajectory
model (Schoeberl and Newman 1995) at four standard pressure levels using the 2.5∞ × 2.5∞ National
Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis dataset. For the sounding shown in Fig. 2,
Fig. 4 shows the corresponding trajectories.
HIGHLIGHTS OF SHADOZ OBSERVATIONS. Evaluation of sonde performance. To date,
what do SHADOZ data tell us? First, by having so
FIG. 3. SHADOZ sites (dots) and campaigns of opportunity (stars). Station latitude–longitude information is in
Table 1. SHADOZ campaign data is as follows: Jan–Feb 1999 cruise (Thompson et al. 2000); INDOEX, Jan–Mar
1999 (data summarized in Thompson et al. 2003b); Lusaka, Zambia, Sep 2000 (Thompson et al. 2002).
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OCTOBER 2004
The precision of the
ozonesonde instrument deduced from SHADOZ may
be better than previous
evaluations (WMO 1998b)
because SHADOZ samples
are taken in a fairly uniform
meteorological regime and
with a similar technique.
AMERICAN METEOROLOGICAL SOCIETY
–0.92
San Cristóbal, Galapagos Islands
OCTOBER 2004
–7.60
Watukosek, Java, Indonesia
*positive for North, negative for South latitudes.
**positive for East, negative for West longitudes.
–18.13
–21.06
La Réunion
Suva, Fiji
–1.27
Nairobi, Kenya
–25.25
Irene, South Africa
–2.99
–7.98
Ascension Island
Malindi, Kenya
–5.42
Natal, Brazil
5.81
–18.00
Papeete, Tahiti, French Polynesia
Paramaribo, Surinam
–14.23*
Latitude (∞∞)
Pago, Pago, American Samoa
Station
178.40
112.70
55.48
36.80
40.19
28.22
–14.42
–35.38
–55.21
89.60
149.00
170.56**
Longitude (∞∞)
NOAA/CMDL; S. J. Oltmans
NASDA; T. Ogawa, S. Kawakami; and
Kyoto University; M. Shiotani, M. Fujiwara
CNRS/University of Réunion; F. Posny
Méteosuisse; B. Hoegger, B. Calpini,
G. Levrat
University of Rome; G. Laneve
South Africa Weather Service (SAWS);
G. J. R. Coetzee
NASA/Wallops; F. J. Schmidlin
NASA/Wallops; F. J. Schmidlin
KNMI; H. M. Kelder
NOAA/CMDL; S. J. Oltmans, H. Vömel
NOAA/CMDL; S. J. Oltmans
NOAA/Climate Modeling and Diagnostics
Laboratory (CMDL); S. J. Oltmans
Sponsor; Coinvestigator
University of the Pacific; K. Koshy
LAPAN; S. Saraspriya, N. Komala
University of Réunion; F. Posny, J-M. Metzger
Kenya Meteorological Department; W. Kimani,
W. Ayoma
Project San Marco, M. A. C. Mwasaha
SAWS; N. A. Phahlane, D. Esterhuyse
U.S. Air Force
INPE, V. W. J. H. Kirchhoff, F. da Silva
Meteorological Service of Surinam; C. R. Becker
National Institute of Hydrology and Meteorology
of Ecuador (INAMHI); M. V. A. Reyes
MéteoFrance; P. Simon
NOAA/CMDL
Operating Agency; Station Manager
• The precision of the total
ozone column measured
by a single sounding device
is 5%, realizing that each
sonde launched is essentially a new instrument
(this figure refers to the
reproducibility of an individual sounding).
• Comparison with groundbased instruments (four
Dobson spectrophotometers and one Brewer
spectrometer) showed
agreement between integrated total ozone from
the sondes ranging from
2% to 7%, with the best
agreement at the African
stations of Irene (near
Pretoria, South Africa)
and Nairobi, Kenya.
• Comparison with total
ozone from the TOMS
satellite (version-7 data)
shows a greater degree
of variability (2%–11%)
among stations, with the
satellite measurement
always higher. The new
TOMS version-8 algorithm
improves the sonde–satellite total column agreement by only 1%–2%.
TABLE 1. SHADOZ stations; further technical details given in Table A1 in Thompson et al. (2003a).
many data points taken under mostly tropical conditions, we have learned something about the ozonesonde
measurement (Thompson
et al. 2003a). Statistics from the
first large set of tropical ozone
soundings show the following:
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QUASI-BIENNIAL OSCILLATION.
Prior to SHADOZ, sounding data were neither frequent enough nor close
enough to the equator
(where the QBO effect is
most pronounced) to fully
document the ozone response to the QBO. In the
stratosphere, near the ozone
partial pressure maximum
(25–27 km), the QBO variation is evident in many
SHADOZ profiles (Logan
et al. 2003). The QBO, a
downward-propagating oscillation between easterly
and westerly winds, has a
period of about 28 months
FIG. 4. Five-day back trajectories, initialized at pressure levels shown with color
(Reed 1964). In the first half
code, created by the GSFC kinematic trajectory model (Schoeberl and
of 1998 the winds above
Newman 1995), initialized with NCEP winds at the pressure levels indicated;
+ marks each 24-h transit segment.
30 hPa (~24 km) are easterly; throughout the year
Although the SHADOZ PTU soundings have not there is a shift to westerly winds that causes the ozone
been used as widely as ozone observations, the archive to increase at that pressure, over and above the noroffers a unique set of radiosondes (sample profiles are mal seasonal increase. Examples of profile changes for
in Fig. 2). Temperature measurements from both a re- 1998 appear in Fig. 5. The reader is referred to Logan
sistive-type (VIZ/Sippican) and capacitive-type et al. (2003) for a complete discussion of the QBO and
(Vaisala) thermistor are used in SHADOZ. The two ozone using SHADOZ sondes and concurrent sateltypes of sensors do not provide identical values lite data.
(Schmidlin 1988; Nash and
Schmidlin 1987); differences in the stratosphere
can exceed 1∞–1.5∞.
Capacitive thermistor data
are corrected in the sonde
software at each station.
Resistive thermistor measurements are not corrected
at the station but may be
when used by analysis centers, for example, NCEP.
Ozone variability and meteorology. With unprecedented
coverage, the principal accomplishment of SHADOZ
has been to provide striking
examples of the links among
tropical meteorology, ozone,
and pollution (Thompson
et al. 2003b). We illustrate
this with several examples.
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FIG. 5. Profiles from Nairobi, Kenya, illustrate stratospheric ozone increases
under the influence of the QBO in 1998 (a) Feb relative to (b) Oct. In 1999
there was not a noticeable difference in Feb–Oct phases. Profiles in 2001 (not
shown) were similar to those for 1998.
ZONAL WAVE-ONE PATTERN IN
OZONE. As mentioned above,
satellites like TOMS detect a
standing wave-one pattern
in the total ozone column at
the equator (Fishman and
Larsen 1987; Shiotani 1992;
Shiotani and Hasebe 1994;
Kim et al. 1996; Newchurch
et al. 2001). Researchers
have attempted to resolve
the issue: is the wave-one
pattern due to more ozone
over the Atlantic in the troposphere, the lower stratosphere, or both? The ozone
measurement from profiling satellites is least accurate
in the lower stratosphere so
it has been difficult to anFIG. 6. The structure of the zonal wave one in tropospheric ozone based on
swer the question with suf0.25-km mean mixing ratios from all SHADOZ stations (except Irene), 1998–
ficient precision in the ab2002. Between eastern South America and the western Indian Ocean, ozone
sence of in situ ozone data
and sources of photoreactive ozone precursors are more concentrated durin the Tropics. Given the
ing biomass burning and lightning episodes. Further enrichment of ozone over
the Atlantic is a consequence of a tendency for ozone to subside in that relongitudinal and temporal
gion, compared to more active convection over the central Pacific. Thus, more
coverage of the SHADOZ
tropospheric ozone appears from ~50∞∞ W to 100∞∞ E than at other longitudes,
data, it is possible to examgiving a wavelike appearance to the integrated ozone column (Figs. 13 and 14
ine the wave pattern for the
in Thompson et al. 2003a).
first time. The sonde record
confirms the amplitude of
the wave—10–25 DU, depending on time of year— 1996; Moxim and Levy 2000; Martin et al. 2002; Peters
and shows that it is predominantly, if not completely, et al. 2002). Furthermore, subsidence is prevalent over
in the troposphere. The structure is summarized in the south tropical Atlantic where midtropospheric
Fig. 6, where a cross-sectional view of the ozone mix- ozone accumulates (Krishnamurti et al. 1996). It aping ratio (annually averaged) in the troposphere and pears that northern tropical ozone adds to the south
lower stratosphere is illustrated. Details of the wave Atlantic ozone burden at times of the year when
vary over the course of a year but the basic appear- southwesterly flows are followed by interhemispheric
transport (Thompson et al. 2000; Edwards et al. 2003;
ance remains similar to Fig. 6.
How does the wave come about? It apparently re- Jenkins et al. 2003). The high-ozone feature over the
sults from photochemical activity, dynamical patterns, Atlantic is enhanced by a persistent recirculation of
and the general circulation. Photochemical reactions ozone pollution in an anticyclonic high pressure sysinvolve free-radical processes that form and destroy tem over southern Africa (Garstang et al. 1996; Tyson
ozone at rates determined by local conditions: UV et al. 1997). Figure 7 shows the resulting gyre with arradiation, temperature, source species, and loss pro- rows indicating where ozone pollution heads west
cesses, including removal at the surface. At any point toward the Atlantic. Ozone pollution also heads east
in the atmosphere the ozone concentration is in an from Africa toward La Réunion (arrows in Fig. 7).
equilibrium fixed by the net rate of formation or loss Over the eastern Indian and Pacific Oceans
and net transport. Above the boundary layer, ozone (Watukosek, Java, and Fiji–Samoa–Tahiti), freein the Tropics is most concentrated over eastern South tropospheric ozone is at a minimum because convecAmerica, the Atlantic, Africa, and the Indian Ocean tion brings clean, lower-tropospheric ozone to the
(40∞W–90∞E). In this region, photochemical sources middle and upper troposphere (Fig. 6).
Similar patterns to those seen in SHADOZ sondes
(biogenic, from biomass burning, lightning formation
of ozone precursors) are abundant (Pickering et al. prevailed over the tropical Atlantic Ocean during the
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Convective activity at La
Réunion, most pronounced
in February–March, is indicated by the appearance of
relatively lower levels of
ozone in the troposphere
above 5 km. Convective influence is more pronounced
at Natal (Fig. 8b) and Fiji
(Fig. 8c) than at La Réunion
(Fig. 8a). Signified by a local
minimum in the middle–
upper troposphere (within
the 35 ppbv contour), the
impact of convection is
strongest at Natal during
the February–April period.
Fiji (Fig. 8c), in the South
Pacific convergence zone,
shows signatures of convective mixing throughout the
year (Pickering et al. 2001).
FIG. 7. Schematic representation of flows and processes contributing to the
At all three stations, during
persistent zonal ozone maximum observed over the south tropical Atlantic.
the period from August to
Besides biomass fires, lightning-derived nitric oxide, biogenic sources of nitric
oxide, and nonmethane hydrocarbons provide the precursor gases that lead
the end of the year, there is
to photochemical ozone formation. The dynamic patterns all operate at some
a localized ozone maxipoint in an annual cycle.
mum between 5 and 10 km.
Analysis of the ozone peaks
Southern African Fire Atmospheric Research Initia- contributing to these features shows origins over retive (SAFARI)/Transport and Atmospheric Chemis- gions of biomass burning, mostly from Africa.
try near the Equator—Atlantic (TRACE-A) field camPerhaps the most surprising result of SHADOZ is
paigns (Fishman et al. 1996) in September–October the extent of week-to-week variability in ozone
1992. During that mission, aircraft and sonde sam- throughout the troposphere. Because most SHADOZ
pling showed that the south Atlantic ozone maximum stations are remote from sources, the ozone vertical
is due to elevated ozone mixing ratios (> 40 ppbv) that structure versus time (curtain graphs, Fig. 9) essenare most concentrated in the free troposphere (2– tially reflects short-term meteorological change. In
8 km) where the photochemical lifetime of ozone is many cases, advection of pollution alternates with
1 month. From Ascension Island ozonesondes and convection. Pollution leads to high ozone mixing raback trajectories, it was inferred that tropical Atlan- tios in the midtroposphere at Nairobi (green and yeltic ozone was composed roughly equally of a “back- low , Fig. 9a) and convection brings up low-ozone air
ground ozone” amount and imported pollution, each and cleans it out. The convective signature is corrobo25 DU in column amount. Two-thirds of the pollu- rated by variations in relative humidity (Fig. 9b),
tion originated over Africa, the remainder was from where the higher relative humidity, green-red above
South America (Thompson et al. 1996).
8–10 km, shows vertical redistribution of ozone from
the surface.
Variability, pollution, convection, and long-range transport.
There are two practical consequences of the variSeasonal variability in tropospheric ozone at three ability of ozone at SHADOZ sites. First, simple averSHADOZ sites, based on monthly averaged data from aged profiles, often used for analysis of trends, may
5 yr, is illustrated in Fig. 8. The chemical tropopause not be statistically robust. In Figs. 10a and 10b, sea(100 ppbv) is lower at La Réunion (Fig. 8a) than at sonally averaged [March–April–May (MAM) and
the other two stations shown. In all three cases, the September–October–November (SON)] mean ozone
tropopause is at its greatest altitude between February profiles at Nairobi, based on 4 yr of data, are shown
and April, and is lowest in August through October. along with the individual profiles that comprise the
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mean. The distribution of mixing ratios at a typical
altitude (Fig. 10c) is so broad that the mean represents
a relatively small fraction of the observations. The statistics are fairly similar during the two periods, although when tropospheric ozone column amounts
are analyzed, the highest and lowest 20% occur, respectively, in the second half and first half of the year.
Trends might be manifested in a modified distribution, particularly at the extremes. In an intriguing
classification of ozone profiles from aircraft ascents and
descents (surface–11 km) over one African location,
Diab et al. (2003) showed ozone distributions correlating with a half-dozen prototypical synoptic situations.
A second implication of the variability observed in
SHADOZ data concerns the nature of ozone fluctuations that satellites and models are attempting to capture. For example, simulation of convective episodes
apparently detected in SHADOZ ozone may challenge convective parameterizations in models.
Similarly, convection, pollution, and stratospheric–
tropospheric exchange episodes observed in
SHADOZ are not resolved in satellite ozone data that
are often available as monthly averages (Martin et al.
2002). Daily (or better, continuous) satellite data are
required to track regional and intercontinental ozone
transport (see, e.g., Thompson et al. 2001). One approach to obtain more accurate ozone retrievals could
be to combine SHADOZ data with an assimilation
model that includes photochemistry and appropriately resolved sources and transport.
SHADOZ PARTICIPATION IN CAMPAIGNS.
To give greater tropical coverage, SHADOZ station
data have been augmented with ozone launches from
field campaigns in which the interaction of ozone
chemistry and meteorological processes has been
studied. The campaign data are also available at the
SHADOZ Web site.
Aerosols99 and INDOEX campaigns. An Atlantic cruise
on the National Oceanic and Atmospheric Administration (NOAA) R/V Ronald H. Brown in January and
February 1999 (Fig. 3 gives the cruise track) showed
that tropical tropospheric ozone profiles north of the
ITCZ may be very different from south of the ITCZ.
The shipboard measurements showed convective influence north of the ITCZ, and an ozone column 4–
5 DU lower than south of the ITCZ where ozone accumulated in descending air. In Fig. 11, the
convection is signified by better mixing and more
uniform ozone and humidity profiles (solid lines); the
higher ozone with drier air (dashed lines) represents
conditions south of the ITCZ. Satellite data showed
AMERICAN METEOROLOGICAL SOCIETY
FIG. 8. Tropospheric ozone variability, on an annually
averaged basis at three SHADOZ stations, representative of the (a) subtropical Indian (La Réunion), (b) Atlantic (Natal, Brazil), and (c) Pacific (Samoa) Oceans.
Based on data from 1998 to 2002.
that the hemispheric ozone contrast was characteristic of the entire tropical Atlantic. Because biomass
burning added pollution north of the ITCZ, the expectation had been that the greater ozone column
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FIG. 9. Week-to-week tropospheric variability at Nairobi, indicated in altitude vs time graphs of the (a)
ozone mixing ratio and (b) relative humidity for 1998–2001. Means of 0.25 km are used. Nairobi uses a
capacitive-type temperature sensor (temperature profiles also available at SHADOZ Web site) for which
corrections are incorporated into the reduction software.
would be to the north. Instead, greater column ozone
is south of the ITCZ, a phenomenon referred to as the
“tropical Atlantic ozone paradox” (Thompson et al.
2000; Edwards et al. 2003; Jenkins et al. 2003).
The Aerosols99 cruise coincided with the beginning of the Indian Ocean Experiment (INDOEX)
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OCTOBER 2004
campaign (Zachariasse et al. 2001) during which
soundings were taken for SHADOZ on Male Island
(Kaashidhoo Observatory, 5∞N, 73∞E) in the
Maldives. In Chatfield et al. (2004), SHADOZ soundings and daily tropospheric ozone satellite maps were
used with trajectories and corroborative data [e.g.,
outgoing longwave radiation, fire, and lightning
counts from the Tropical Rainfall Measuring Mission
(TRMM) satellite, smoke aerosol from TOMS] to
trace individual high-ozone layers to lightning and
African and South American biomass fires. Ozone in
the Kaashidhoo observatory sondes (35 DU) was
similar to the TOMS measurement over the Indian
Ocean downwind of southern Asia in late February
1999. Some of this ozone was lofted above 9 km, then
transported over Africa to the region of Atlantic
subsidence (Fig. 7). Signatures of Indian Ocean ozone
were detected in the 3 March 1999 Nairobi sounding and at Ascension Island on 11 March 1999
(Fig. 12). Not only are individual pollution layers at
Ascension Island traceable to transport from the Indian Ocean, a gradual increase in background ozone
at Ascension Island during February–March 1999 is
attributed to pollution from south Asia (Chatfield
et al. 2004).
SAFARI-2000. A campaign at Lusaka, Zambia, during
SAFARI-2000 (Swap et al. 2003) confirmed the vertical stability of pollution layers during the southern
African dry season, a phenomenon first characterized
during the 1992 SAFARI(92)/TRACE-A (Garstang
et al. 1996). Recirculation of pollution layers over
south-central Africa produced very high tropospheric
ozone throughout the free troposphere, in contrast to
relatively clean profiles often found at fixed SHADOZ
sites. Biomass burning over Angola, Namibia, Zimbabwe, and Mozambique, as well as South Africa and
Zambia, itself contributed to a layer of pollution ozone
> 100 ppbv over Lusaka (Thompson et al. 2002).
Figure 13 shows a curtain graph of the ozone mixing
ratio, averaged in 1-km layers, for nine soundings in
the 2000 SAFARI (2000) campaign. A localized disturbance, following the fifth sounding, cleaned out the
boundary layer and reduced ozone aloft, but a thinner stable layer at 2–3 km remained intact. In addition to the pollution aloft, boundary layer ozone during the first four SAFARI-2000 launches exceeded
90 ppbv, the highest level yet measured over southern Africa. Up to one-half of this pollution appeared
to be from local vehicles and urban burning practices
(Thompson et al. 2002).
FIG. 10. Nairobi seasonal mean profile (a) MAM, and
(b) SON to 20 km, based on 4-yr data, with individual profiles contributing to mean. (c) Histogram
of ozone mixing ratios from Nairobi soundings,
binned at 5 ppbv intervals, at 450 hPa (between 5
and 6 km).
AMERICAN METEOROLOGICAL SOCIETY
OCTOBER 2004
| 1559
FIG. 11. Mean profiles of ozone and relative humidity,
taken north (solid) and south (dashed), respectively, of
the ITCZ on the Aerosols99 cruise of the R/V Ronald
H. Brown during a Jan–Feb 1999 Atlantic transect from
Virginia to Cape Town, South Africa (Thompson et al.
2000). Sounding data reside in SHADOZ archive (online
at http://croc.gsfc.nasa.gov/shadoz).
JOSIE. SHADOZ is a vital part of several international
efforts sponsored by the United Nations and several
International Council of Scientific Unions (ICSU)
bodies to assure integrated (satellite, airborne, balloon, ground-based) observing strategies to detect
global change. The World Meteorological Organization (WMO) is developing a standard procedure for
ozonesonde operations to be used at Global Atmospheric Watch (GAW) stations. It turns out that
although all SHADOZ stations use the ECC sonde,
small variations in instrument type (manufacturer)
and preparation procedures may affect the ozone
measurement. Thus, SHADOZ data and techniques
are being used to evaluate details of sonde operations
and data handling. In September 2000, SHADOZ
investigators participated in the 2000 Jülich
Ozonesonde Intercomparison Experiment (JOSIE)
under WMO sponsorship (WMO 1998b). All of the
methods used in SHADOZ were tested against a calibrated spectroscopic instrument in chamber tests
conducted in September 2000 at the Forschungszentrum in Jülich, Germany (information online at
1560 |
OCTOBER 2004
FIG. 12. A sequence of soundings taken during INDOEX
(1999) in which ozone pollution originating over south
Asia is detected successively at the Maldives
(Kaashidhoo Observatory), Nairobi (East Africa), and
Ascension Island in the Atlantic. Black arrow points to
progression of pollution from Indian Ocean over Africa
then adding to the background ozone over Ascension
in early March 1999.
www.fz-juelich.de/icg/icg2/forschung/Josie).
Preliminary analysis of results shows that instrument
properties (manufacturer and concentration of the
chemical solution) affect the ozone measurement and
may account for small variations seen among
SHADOZ stations when sonde total ozone is compared to a ground-based or satellite total ozone
column (Thompson et al. 2003a). A WMO-sponsored
field comparison of sondes was made in April 2004
in the Balloon Experiment on Standards for Ozone
Sondes (BESOS; online at http://croc.gsfc.nasa.
gov/besos). A large balloon was flown with a UV
spectrophotometer and several types of sonde
instruments.
WMO’s GAW seeks to increase the number of
ozonesonde stations in developing countries. Several
SHADOZ stations provide a model for GAW’s “twinning” concept. Station operations, infrastructure, and
gases are provided by the country in which the station is located. A partner country initiates sonde
launches with a local agency, either a meteorological
department or a space institute. Training, expend-
ful launch of NASA’s Aura
spacecraft, SHADOZ is well
poised for validation of the
four ozone sensors on board.
ACKNOWLEDGMENTS.
SHADOZ is supported by NASA’s
Atmospheric Chemistry Modeling and Analysis Program
(ACMAP) and the TOMS project.
It would not exist without the
generous and enthusiastic participation of the following:
NASA’s Wallops Flight Facility,
NOAA, National Space Development Agency of Japan (NASDA),
Lembaga Penerbangan dan
Antariksa Nasional (LAPAN, the
National Institute of Aeronautics
and Space of Indonesia), Instituto
Nacional de Pesquisas Espaciais
(INPE, the National Space Agency
of Brazil), the South African
Weather Service, the Swiss MeFIG. 13. Succession of nine soundings, 1-km mean mixing ratio, taken from
teorological Agency, the Kenyan
6 to 11 Sep 2000, over Lusaka, Zambia (15.5∞∞ S, 28∞∞ E). Launches were made
Meteorological Department, the
during SAFARI-2000 campaign (Thompson et al. 2002); the data reside
Royal Dutch Meteorological Inin the SHADOZ archive. A stable ozone pollution layer from 2 to 5 km
stitute (KNMI), the University of
coincides in most of these soundings with a double temperature inversion.
the South Pacific (Suva, Fiji),
Below the stable layer is the mixed layer. Five kilometers is a typical height
CNRS, and the University of
of a capping layer of tropical fair-weather cumulus.
Réunion (France). We are grateful to the WMO Global Atmoables, and data processing are provided by the out- spheric Watch project for the opportunity to participate in
side sponsor. In this manner the National Aeronau- the JOSIE intercomparison activity. Thanks to the BAMS
tics and Space Administration (NASA) and the Bra- Atmospheric Chemistry Editor, two anonymous reviewers,
zilian Space Agency (INPE) started the first tropical and B. N. Duncan (UMBC/GEST and GSFC) for their
soundings in 1978 when the Nimbus-7 TOMS satel- comments.
lite instrument was launched. Twinning between the
Swiss Meteorological Agency and the Kenyan Meteorological Department led to the start of Nairobi sonde
launches in 1996. Other examples in SHADOZ are the
Japanese and Indonesian Space Agency operation on
Java and the meteorological services of the Netherlands and Surinam at Paramaribo, Surinam.
SUMMARY. More than 2000 ozone and PTU profiles
archived between 1998 and 2003 are now at the
SHADOZ Web site (online at http://croc.gsfc.nasa.gov/
shadoz). SHADOZ data have been used to improve
the profile climatology for models and satellite algorithms, elucidate the structure of the zonal wave one
in tropospheric ozone, and delineate characteristics
of the ozone response to the QBO. With the successAMERICAN METEOROLOGICAL SOCIETY
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