NAST-I tropospheric CO retrieval validation during INTEX-NA and EAQUATE
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QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY
Q. J. R. Meteorol. Soc. 133: (S3) 233–241 (2007)
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/qj.130
NAST-I tropospheric CO retrieval validation during
INTEX-NA and EAQUATE†
Daniel K. Zhou,a * Allen M. Larar,a Xu Liu,a William L. Smith,b,c Jonathan P. Taylor,d
Stuart M. Newman,d Glen W. Sachsea and Stephen A. Mangoe
a NASA Langley Research Center, Hampton VA, USA
b Hampton University, Hampton VA, USA
c University of Wisconsin-Madison, Madison WI, USA
d Met Office, Exeter, UK
e NPOESS Integrated Program Office, Silver Spring MD, USA
ABSTRACT: Troposphere carbon monoxide (CO), as well as other trace species retrieved with advanced ultraspectral
remote sensors of Earth observing satellites, is critical in air quality observation, modelling, and forecasting. The retrieval
algorithm and the accuracy of the parameters retrieved from passive satellite remote sounders must be validated. The
Intercontinental Chemical Transport Experiment - North America (INTEXNA) and the European Aqua Thermodynamic
Experiment (EAQUATE) provide important validation of satellite observations with ongoing satellite measurement
programmes such as Terra, Aura, and Aqua. One of the experimental objectives is to validate chemical species observed from
ultraspectral sounders with aircraft in situ measurements, such as the NPOESS Airborne Sounder Testbed-Interferometer
(NAST-I). Detailed intercomparisons between aircraft in situ measured and NAST-I retrieved CO profiles were performed
to assess the retrieval capability of a passive infrared spectral remote sounder. Validation results illustrate that the CO
vertical structure can be obtained by the NAST-I. The thermal radiances are most sensitive to CO emissions from the free
troposphere. However, the profile retrieval accuracy depends on the CO uncertainty in the terrestrial boundary layer. It is
shown here that the CO distribution in the terrestrial boundary layer over the sea cannot be obtained with reliable accuracy
where there is little contrast between the surface air and surface skin temperature. Copyright 2007 Royal Meteorological
Society
KEY WORDS ultraspectral sounder; NPOESS; CO; retrieval
Received 3 November 2006; Revised 26 April 2007; Accepted 14 May 2007
1. Introduction satellite and aircraft observations has been established to
provide the necessary global and regional data needed
Tropospheric chemical reactions involving carbon mon-
to understand the complex chemistry and transport pro-
oxide (CO) extend their influence to the global climate
cesses involved in regional air pollution chemistry and
through the accumulation of greenhouse gases. Trace
its influence on the global environment.
amounts of CO are found in the atmosphere, and Levy
The Measurement of Pollution in the Troposphere
(1971) has recognized the significance in atmospheric
(MOPITT) instrument was launched on 18 December
chemistry of CO, which, due to its relatively long life-
1999 aboard the Terra satellite (Drummond, 1992; Pan
time, can be transported a great distance from its origi-
nal source. Air particles downwind of combustion (e.g. et al., 1998; Deeter et al., 2003) for space-based mea-
biomass, fossil fuels) often show elevated CO and ozone surement of CO and methane. The Atmospheric InfraRed
(O3 ) resulting from photochemical production (Fishman Sounder (AIRS) on the Aqua satellite, launched on 4 May
et al., 1990). Crutzen et al. (1979) have recognized the 2002, monitors atmospheric thermodynamic structure as
importance of improving our knowledge of tropospheric well as trace species like O3 and CO (e.g. Aumann et al.,
chemical gases relating to environmental health and the 2003). The Tropospheric Emission Spectrometer (TES)
magnitude of global climate change. The critical role of instrument, aboard the Aura satellite, was launched on
15 July 2004 and is detecting tropospheric trace species
(e.g. Worden et al., 2004). One of the objectives of these
* Correspondence to: Daniel K. Zhou, Mail stop 401A, NASA Langley missions is to monitor global CO distribution (Beer et al.,
Research Center, Hampton, VA 23681, USA.
E-mail: daniel.k.zhou@nasa.gov 2001). A great deal of effort has been given to vali-
† The contributions of Jonathan P. Taylor and Stuart M. Newman of dating satellite observations and their retrievals as well
the Met Office, Exeter were prepared as part of their official duties as as model analyses (e.g. Emmons et al., 2004). Scientists
employees of the UK Government. It is published with the permission
of the Controller of HER Majesty’s Stationery Office and the Queen’s have been using aircraft in situ measurements as well
Printer for Scotland. as chemical model predictions (e.g. Deeter et al., 2004;
Copyright 2007 Royal Meteorological Society
234 D. K. ZHOU ET AL.
Kulawik et al., 2006) to understand the retrieval accuracy 2. Experiment, algorithm, and validation
with satellite measurements.
Since the inversion of the radiative transfer equation NAST-I instrumentation, measurements, calibration, and
is an ill-posed problem, a priori information may be radiance validation are documented elsewhere (e.g.
used to stabilize the retrieval process. The a priori infor- Cousins and Smith, 1997; Gazarik et al., 1998; Smith
mation used and the vertical resolution of the retrieval et al., 1999). NAST-I is a Fourier Transform Spectrom-
determined by the averaging kernels may be taken into eter of the Michelson Interferometer design. It possesses
account when validating the retrieval accuracy. Dedi- high spectral resolution (0.25 cm−1 ) and high spatial res-
cated field campaigns, such as Transport And Chemical olution (0.13 km linear ground resolution per km of air-
Evolution over the Pacific (TRACE-P), the Interconti- craft flight altitude at nadir). The NAST-I spatially scans
nental Chemical Transport Experiment - North America cross-track to the aircraft motion at +/−48.2 degrees,
(INTEX-NA) and the European Aqua Thermodynamic thereby providing a 2.3 km ground track swath width
Experiment (EAQUATE) were conducted with intensive per km of aircraft flight altitude (e.g. a 46 km swath
aircraft in situ measurements as well as other measure- from a flight altitude of 20 km). The radiometric noise is
ments from different airborne sensors in order to accu- nominally 0.3 K, spectrum to spectrum, depending upon
rately capture the features of atmospheric species with a the spectral region and scene temperature. The spectrally
high spatial and vertical resolution (e.g. Crawford et al., random noise, spectral point to spectral point, is gener-
2004). To fully understand the vertical resolution of the ally less than 0.3 K within a given radiance spectrum.
retrievals from satellite measurements, the information It is the spectrally random component of the radiance
content of the satellite infrared radiometric measurements measurement noise which limits the ability to decon-
must be known. It is difficult to evaluate the accuracy volute the radiance spectrum with the precision needed
of satellite retrievals without knowledge of the chemical to retrieve small-scale vertical features of atmospheric
abundance, and its vertical distribution, used as a priori temperature and the absorbing constituents (e.g. water
information. vapour). NAST-I is designed to support the development
To investigate tropospheric CO vertical profile retrieval and performance validation of high-spectral-resolution
accuracy from a satellite ultraspectral sounder, the (i.e. vertical) temperature and moisture sounders being
National Polar-Orbiting Operational Environmental Satel- flown on Earth-orbiting satellites. While a large amount
lite System (NPOESS) Airborne Sounder Testbed-Inter- of data has been collected since July 1998 under a vari-
ferometer (NAST-I) field campaign data can be used ety of meteorological conditions, NAST-I observations
to provide radiometric measurements including tropo- that are coincident with in situ CO measurements are
spheric trace species. NAST-I is an ultraspectral res- very limited. However, such coincident measurements did
olution sounder (e.g. Cousins and Smith, 1997) simi- occur during the INTEX-NA and EAQUATE campaigns.
lar to current and future satellite sounders, such as the During these campaigns, NAST-I flew on the Proteus
AIRS, the Infrared Atmospheric Sounding Interferome- aircraft while the NASA DC-8 and the UK Facility for
ter (IASI), and the Cross-track Infrared Sounder (CrIS). Airborne Atmospheric Measurements (FAAM) BAE 146
NAST-I provides high spatial resolution spectral radiance aircraft provided in situ measurements during INTEX-
measurements, and profile retrievals obtained from them. NA and EAQUATE, respectively. Only one INTEX-NA
The NAST-I measurements, in conjunction with coinci- flight (22 July 2004) off the US east coast and two
dent in situ measurements which have a very high spatial EAQUATE flights (14 and 18 September 2004) over the
and vertical resolution, are vital for validating the infor- UK Celtic Sea provided data valuable for this study. CO
mation content and retrieval accuracy achievable with profiles from these three flights were observed in situ,
satellite instruments (i.e. in this case, the CO vertical which enabled the validation of retrievals obtained from
profile retrieval from satellite radiometric observations). NAST-I ultraspectral radiance measurements.
NAST-I radiometric radiance samples collected during NAST-I profile retrieval algorithms have been devel-
field campaigns, together with the in situ measurements oped in which clouds are detected and their geometri-
regarded as ground ‘truth’, are used to validate the accu- cal and microphysical properties are accounted for in
racy of a CO profile retrieval algorithm and to investigate the profile retrieval process (Zhou et al., 2007a). The
retrieval sensitivity to CO variations. This study provides NAST-I CO profile retrieval scheme is briefly described
a means of understanding the accuracy achievable from here. Under cloud-free conditions, the NAST-I CO
radiance emission measurements from satellite, as well inversion scheme is combined with a three-step proce-
as airborne, ultraspectral resolution instruments. In this dure: (1) EOF (empirical orthogonal function) regres-
paper, the focus is on CO retrieval validation in order to sion retrieval, (2) simultaneous matrix inversion, and
demonstrate the capability of capturing the tropospheric (3) CO profile enhancement inversion. Since the retrieval
CO vertical distribution from spectral radiance emission problem is ill-posed, additional information is needed
observations. The scope of this work is limited to validat- to constrain the solution. The radiosonde temperature
ing the NAST-I CO retrievals, but the results are believed and moisture profiles with regional and seasonal vari-
to be useful in understanding the sensitivity of satellite ations are used as a training dataset. Statistical sam-
infrared radiometric observations to the vertical and hor- ples of CO profiles are used with radiosonde tempera-
izontal distribution of CO. ture and water vapour profiles to obtain coefficients for
Copyright 2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: (S3) 233–241 (2007)
DOI: 10.1002/qjNAST-I TROPOSPHERIC CO RETRIEVAL VALIDATION 235
regression retrieval. Unfortunately, CO profiles are not
sampled simultaneously with the radiosonde temperature
and moisture profiles used for the statistical regression
retrieval. As a result, CO abundance profiles must be
assembled for each radiosonde profile using the relation-
ship between thermodynamic parameters (i.e. temperature
and moisture) and CO abundance profiles based on mea-
surements from aircraft made during several previous
atmospheric chemistry field campaigns. The EOF regres-
sion relations are based on the radiosonde and CO profile
training dataset. Although a model analysis might provide
a more accurate ‘first guess’ for the retrieval process, the
emphasis here is placed on how the information content
of the radiance measurements are able to improve the
CO vertical profile specified as the first guess for the
retrieval. It is shown that the CO profile retrieval accu-
racy depends heavily on the accuracy of the first guess as
expected from the CO profile radiance weighting func-
tions and the averaging kernels resulting from the inverse
solution of the radiative transfer equation. Figure 1. Selected significant averaging kernels calculated for retrieving
The last two steps of this retrieval approach employ the US standard atmosphere with NAST-I instrument specification.
physical inversion, which is a one-dimensional varia- Perturbed layers are indicated in the legend (after Zhou et al., 2005).
tional (1D-Var) solution, also known as the regulariza-
tion algorithm or the minimum information method (e.g. 2.1. INTEX-NA case of 22 July 2004
Twomey, 1963; Tikhonov, 1963; Rodgers, 1976; Hansen, INTEX-NA is an integrated atmospheric field experi-
1998). Detailed descriptions of thermodynamic parame- ment performed over North America. The objective of
ters and CO profile retrievals under clear conditions are the experiment is to understand the transport and trans-
found in Zhou et al. (2002, 2005). The averaging kernels formation of gases and aerosols on transcontinental and
are the vertical resolution functions of the retrieval and intercontinental scales and their impact on air quality and
they represent how a true atmospheric state is transformed climate. NAST-I, flown on the Proteus aircraft, partici-
to the atmospheric state specified by the retrieval pro- pated in the INTEX-NA campaign. The particular focus
cess. Detailed formulation and definition of the averaging of this study is to quantify and characterize NAST-I
kernel can be found elsewhere (e.g. Pan et al., 1998; CO profile retrieval sensitivity and accuracy. During one
Rodgers, 1990). The averaging kernels for NAST-I CO, particular flight on 22 July 2004, a NASA DC-8 air-
presented by Zhou et al. (2005), were based on NAST-I craft was ascending and descending to collect trace gas
radiances simulated from the US standard atmosphere profiles, including CO, with an instrument described by
with a surface/atmosphere thermal contrast of 3.0 K. Sachse et al. (1987), while the Proteus aircraft, carrying
Selected significant layers plotted in Figure 1 indicate the NAST-I, cruised over the DC-8.
that NAST-I radiances are most sensitive to the CO varia- As shown in Figure 2(a), flight tracks of Proteus and
tions within the 2 to 10 km altitude region. The averaging DC-8 aircraft were close and covered a large geophysical
kernels, as well as the profile weighting functions (Zhou area over the water. NAST-I retrieved temperature, mois-
et al., 2005), assist us to understand how well the CO ture, and CO profiles are plotted in Figures 2(b), (c) and
(d), respectively. Based on the validation of tempera-
vertical structure can be captured using radiance observed
ture and moisture profiles obtained during numerous field
with the NAST-I.
campaigns, NAST-I retrieved thermodynamic parameters
NAST-I CO retrieval error and the error contribution
have been shown to be relatively accurate. The NAST-I
sources (e.g. the retrieval accuracies for temperature, temperature and moisture profiles retrieved here com-
moisture, surface properties, as well as the forward pare favorably with nearby radiosondes (e.g. Smith et al.,
radiative transfer model accuracy) have been addressed 1999; Smith et al., 2005; Zhou et al., 2007b). As both
previously by Zhou et al. (2005). NAST-I CO retrieval aircraft covered a large geophysical region, a large CO
standard deviation of error (STDE) was estimated as a variation from both NAST-I retrieved profiles (with view-
function of the altitude. These are approximately 80, ing angles less than 23° ) and in situ observations are
60, 30, and 20 parts per billion by volume (ppbv) at shown in Figure 2(e) indicating CO variation in this
1.0, 1.5, 4.5, and 7.0 km, respectively. The estimated area. Detailed intercomparison is given in Figure 3. It
STDE may change depending on the nature of CO is noted that a relatively large CO variation (and/or gra-
distributions. The retrieval error should be considered dient) was also observed by the MOPITT satellite instru-
as the intercomparisons between NAST-I retrievals and ment for the same geographical region. The MOPPIT
in situ observations are presented hereafter. observations show similar CO column enhancement off
Copyright 2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: (S3) 233–241 (2007)
DOI: 10.1002/qj236 D. K. ZHOU ET AL.
Figure 2. Case of 22 July 2004: (a) Proteus and DC-8 aircraft flight tracks, (b) NAST-I retrieved temperature (K) vertical cross-section,
(c) NAST-I retrieved relative humidity (%) vertical cross-section, (d) NAST-I retrieved CO (ppbv) vertical cross-section, and (e) NAST-I CO
profiles (coloured curves) and in situ CO observations (black circles). In (b)–(d), regions where cloud prevents accurate tropospheric retrievals
are shown as white stripes.
the US eastern seaboard where the aircraft data were observed by both in situ measurements and NAST-I
obtained. It is believed that this variation was due to the retrievals (Figures 2(d) and (e)) may also be due to
transport of CO produced by Alaskan wildfires (Pfister long-range transport of Alaskan wildfire produced CO.
et al., 2005). Tropospheric CO enhancement (4–5 km) The terrestrial boundary layer (TBL) CO enhancement
Copyright 2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: (S3) 233–241 (2007)
DOI: 10.1002/qjNAST-I TROPOSPHERIC CO RETRIEVAL VALIDATION 237
Figure 3. (a) Comparison between NAST-I retrievals and in situ measurements at co-located points, with background of all in situ data (see
legend). (b) The difference between NAST-I retrievals and in situ measurements at the collocated points (see text).
observed by the in situ sensor may be due to urban with the UK FAAM BAE 146 aircraft, made two ded-
emissions. icated AIRS validation flights on 14 and 18 Septem-
In order to compare NAST-I retrievals with in situ ber 2004. Numerous in situ sensors and remote-sounding
measurements, it is necessary to find relatively close ‘col- instruments were deployed aboard the BAE 146 aircraft
located’ spots (i.e. within a cube of 2 × 2 × 2 km) for (Taylor et al., 2008) including a fast-response vacuum-
intercomparison. The latitude, longitude, and altitude of UV resonance fluorescence CO instrument (Gerbig et al.,
in situ data points were used to search for their ‘collo- 1999). Dropsondes were also released from BAE 146
cated’ NAST-I data. Despite a small time difference (i.e. aircraft during the experiments. Detailed radiometric
within a few hours) and a sample resolution difference, and thermodynamic parameter retrieval validation stud-
there are 44 ‘collocated’ data points. Figure 3 is a plot ies (Larar et al., 2006; Zhou et al., 2007b) indicate that
of in situ observations and NAST-I retrievals; the verti- high-quality datasets were collected. Both flights were
cal mean difference between NAST-I CO retrievals and conducted in the same geophysical location and at almost
in situ CO measurements is ∼5 ppbv and their STDE is the same local time, thus providing an excellent case for
∼22 ppbv. The mean difference illustrates the bias caused verifying retrieval sensitivity of day-to-day atmospheric
by differences in the vertical resolution and absolute variations of CO.
accuracy between the retrievals and in situ observations. Figure 4 plots the flight tracks of Proteus and BAE
In contrast, the STDE illustrates the residual random 146 aircraft and NAST-I retrievals for both 14 and
error caused by these differences. Here, NAST-I aver- 18 September 2004. NAST-I temperature and moisture
aging kernels and a priori information are not applied to retrievals are validated with the dedicated dropsondes
in situ data to make the in situ data vertical resolution the released from BAE 146 aircraft. Detailed validation
same (i.e. the retrieval equivalent). Therefore, the differ- indicating that the agreement is within approximately
ent vertical resolution associated with these two datasets 1 K for temperature STD and 15% for relative humidity
affects the outcome of the comparison. Thus, these statis- STD is found in Zhou et al. (2007b). The day-to-day
tics reveal the difference between the retrievals using a
variation of the atmosphere is clearly shown. A relatively
passive ultraspectral remote sensor and in situ observa-
large amount of CO in the free troposphere (FT) is
tions. As a consequence, these results reveal how well the
shown in NAST-I retrievals for 14 September compared
retrieval can capture tropospheric CO vertical features.
with those of 18 September 2004. Since the geophysical
Although NAST-I CO in the TBL is heavily dependent on
locations covered by both aircraft were almost the same,
the a priori condition, the retrievals compare favourably
and the in situ observed CO are not scattered as shown
to the in situ measurements, as shown in Figure 3.
in the INTEX-NA case of 22 July 2004, the NAST-I
cross-section averaged CO profile is used and plotted
2.2. EAQUATE cases of 14 and 18 September 2004
against the in situ profile. As shown in Figure 5, CO
During the second phase of the EAQUATE campaign profiles indicating a day-to-day difference as validated
over the United Kingdom, the Proteus aircraft, together from the comparison between NAST-I retrievals and the
Copyright 2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: (S3) 233–241 (2007)
DOI: 10.1002/qj238 D. K. ZHOU ET AL.
Figure 4. Data from 14 September 2004: (a) Proteus and BAE 146 aircraft flight tracks, (b) NAST-I temperature vertical cross-sections,
(c) NAST-I relative humidity vertical cross-sections, and (d) NAST-I CO vertical cross-sections. (e)–(h) are as (a)–(d), but for 18 September
2004.
in situ observations of the 14 and 18 September 2004 CO column content in the FT. However, the NAST-I
are plotted. A relatively small amount of CO abundance CO profile within the TBL is hardly changed from the
within the TBL indicates that strong urban emissions first guess to final physical retrieval, which is more
were not nearby, and the TBL CO day-to-day variation pronounced than in situ observations. This is explained
is barely noticed. CO variations observed in the FT are by the NAST-I CO weighting functions (i.e. Jacobian
attributed to its long-range transport from other regions. matrices) and averaging kernels shown in Figure 1.
Unlike the case of 22 July 2004 shown in Figure 3, the As a consequence, a vertical CO distribution must be
first guess is relatively poor (especially in the TBL). compensated for a total CO column amount, which is
There is nearly no difference in the first guess between exactly what happened as shown in Figure 5(b). NAST-I
the two cases of 14 and 18 September 2004 (shown CO in the FT is less pronounced than that shown by the
in Figure 5(a)). This situation provides an exceptional in situ measurements.
opportunity to test the retrieval algorithm with real Another way to assess the retrieval accuracy is to com-
measurements to understand vertical retrieval sensitivity pare the spectral radiance measurements with simulated
from NAST-I type remote sounders. Figure 5(b) plots radiances using the true and retrieved atmospheric pro-
the NAST-I CO physical retrieval profiles. The CO files. Here, using the case of 18 September 2004, NAST-I
profile separation in the FT, as shown by the in situ spectral radiances are simulated using a retrieved CO
observations between 14 and 18 September, illustrates profile as well as in situ measurements. Other parame-
the importance of the physical inversion in the retrieval ters, such as temperature, moisture, ozone, and surface
process. The ultraspectral thermal emissions, measured properties used in the radiance simulation, are from the
by remote sounders such as NAST-I, are sensitive to same retrieval. Figure 6(a) shows two spectra, one which
Copyright 2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: (S3) 233–241 (2007)
DOI: 10.1002/qjNAST-I TROPOSPHERIC CO RETRIEVAL VALIDATION 239
density. A CO profile retrieved from spectral radiance
measurements by an instrument with a given radiance
measurement capability is influenced by the first-guess
profile as well as the accuracy of the other retrieved ther-
modynamics parameters.
3. Conclusions and future work
Field campaigns with aircraft in situ observations provide
precise distributions of chemical species needed to vali-
date retrieval algorithms and results from satellite/aircraft
remote sounders. This CO evaluation study, having coin-
cident radiance and in situ measurement datasets, enables
an understanding of the accuracy of our current CO
retrieval algorithm and validates the results based on the-
oretical simulations (Zhou et al., 2005). In addition to
measurement quality (e.g. instrument noise, spectral res-
olution, cloud contamination, thermal property accuracy),
the CO first-guess profile (or a priori condition) plays a
major role in the CO profile accuracy. The TBL CO accu-
racy, mainly determined by the first guess (or a priori
information), affects the free tropospheric CO retrieval.
However, CO variations in the FT can still be captured
while CO amount variations in the TBL are not retrieved
very well. Thus, it is a challenge to obtain accurate CO
profiles (especially in the TBL) from remote sounders
such as NAST-I. The situation may change when the
difference of air and surface skin temperatures is more
pronounced over the land during the local daytime. The
larger the contrast between surface air and surface skin
temperatures, the greater the radiance sensitivity is to the
TBL gas concentrations. At the same time, the land sur-
face properties, mainly the surface emissivity, need to be
accounted for in order to obtain accurate CO retrievals
from a thermal IR ultraspectral sounder. While this work
Figure 5. CO profile intercomparisons: (a) NAST-I regression versus evaluates the NAST-I CO retrieval algorithm and its
in situ, and (b) NAST-I final physical retrieval versus in situ. Data from retrieval products, additional profile validation analyses
14 (18) September are plotted in grey (black); in situ data are plotted
for different geophysical location and seasonal conditions
as dots, and NAST-I section means as solid curves.
are desired.
is based on the retrieved CO profile and the other on Acknowledgements
the in situ measured CO profile shown in Figure 5(b),
and the difference is plotted in Figure 6(b). In order to The authors greatly appreciate the contributions of the
investigate error contributions from TBL and FT CO NASA Langley Research Center, the Space Science
abundance, the radiance differences contributed by the and Engineering Center of the University of Wiscon-
CO differences in the FT and the TBL are also plotted in sin – Madison, and the UK Met Office. The NAST-
Figure 6(b). The differences are associated with the extra I program is supported by the NPOESS IPO, NASA
amount of retrieved CO in the TBL and the lesser amount Headquarters, and NASA Langley Research Center. The
of retrieved CO in the FT in comparison with the in situ authors acknowledge support from NASA Headquarters
measured CO profile shown in Figure 5(b). With very Earth Science Division Associate Director for Research
little CO retrieval sensitivity in the TBL, the TBL error Dr. Jack A. Kaye and IPO Algorithm Division Chief Dr.
introduced by the first guess is not significantly reduced Karen St.Germain. The FAAM is jointly funded by the
through physical inversion. This TBL error affects the Natural Environment Research Council and the UK Met
FT CO retrieval accuracy; in other words, the FT error Office. The personnel who contributed to INTEX-NA and
is dependent on the TBL error. The extra or lack of CO EAQUATE field campaigns are too numerous to men-
in the TBL compensates for the lack of or extra CO in tion by name; nonetheless their personal contributions
the FT, in order to achieve an accurate total CO column are greatly appreciated.
Copyright 2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: (S3) 233–241 (2007)
DOI: 10.1002/qj240 D. K. ZHOU ET AL.
Figure 6. (a) NAST-I spectra simulated with the retrieved and in situ measured CO profile of 18 September 2004. (b) Spectral difference between
retrieved and in situ measured CO profile (dashed black), difference due to free tropospheric CO (light grey), and difference due to TBL CO
(dark grey).
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