EAQUATE An International Experiment For Hyperspectral Atmospheric Sounding Validation
BY J. P. TAYLOR, W. L SMITH, V. CUOMO, A. M. LARAR, D. K. ZHOU, C. SERIO, T. MAESTRI, R. RIZZI, S. NEWMAN, P. ANTONELLI, S. MANGO, P. DI GIROLAMO, F. ESPOSITO, G. GRIECO, D. SUMMA, R. RESTIERI, G. MASIELLO, F. ROMANO, G. PAPPALARDO, G. PAVESE, L. MONA, A. AMODEO, AND G. PISANI
The validation of advanced infrared sounding satellites requires a diverse set of coordinated synergistic observations utilizing ground based and^airbome instrumentation.
The European Aqua Thermodynamic Experi- hyperspectral sounding instruments, modeled after ment (EQUATE) was held in September 2004 the experimental Geosynchronous Imaging Fourier in both Italy and the United Kingdom to vali- Transform Spectrometer (GIFTS), are being planned date data from the Atmospheric Infrared Sounder for implementation during the next decade. (AIRS) instrument on the Earth Observing System The focus of this initial experiment was placed (EOS) Aqua satellite. It also aimed to demonstrate on the validation of the AIRS instrument on the how combinations of ground-based and airborne EOS Aqua satellite. During EAQUATE, the Proteus systems are useful for validating hyperspectral sat- aircraft, carrying five separate remote sensing ellite sounding observations from satellites due for instruments [the NPOESS Airborne Sounder launch through this decade. The era of hyperspectral Testbed-Interferometer (NAST-I), NAST-Microwave atmospheric sounding began with the launch of the (NAST-M), Scanning High-Resolution Infrared AIRS experiment of the EOS Aqua satellite during Sounder (S-HIS), Far-Infrared Sensor for Cirrus May 2002. Following AIRS, the Infrared Atmospheric (FIRSC), and Microscale Measurement of Atmospheric Sounding Interferometer (IASI) was launched Pollution Sensor (micro-MAPS)], was stationed in aboard the Meteorological Operational (METOP) Naples, Italy, from 4 to 11 September and in Cranfield polar-orbiting satellite in October 2006, and the United Kingdom, from 11 to 19 September 2004. Cross-Track Infrared Sounder (CrIS) instrument is During the Italian portion of the campaign, Proteus to be orbited aboard the National Polar-Orbiting underflew Aqua in coordination with ground-based Environmental Satellite System (NPOESS) Prepatory remote sensing measurements, including several Project (NPP) operational series of polar orbiters in Raman lidar water vapor and temperature profilers late 2009. Operational geostationary satellite imaging and radiosondes, provided by •
|CA|I M|TE0|0|JDGIC# L SCJCIETY Unauthenticated | Downloaded 10/09/21 05:34 AM UTC the Istituto di Metodologie per l'Analisi Ambientale (IMAA), the Dipartimento di Ingegneria e Fisica delFAmbiente (DIFA), the University of Basilicata in Potenza, and the Univer- sity of Naples (UNINA), in Napoli, Italy. During the U.K. portion of the campaign, the Proteus underflights of Aqua were coordinated with the U.K. Facility for Airborne Atmospheric Measurements (FAAM) BAel46-301 aircraft, which flew a large payload of in situ measurement instru- FIG. I. The Proteus aircraft showing instrumentation fitted during ments, including dropsondes EAQUATE. and remote-sensing instru- ments [e.g., the Airborne Research Interferometer Oceanic and Atmospheric Administration (NOAA)- Evaluation System (ARIES) interferometer spec- National Aeronautics and Space Administration trometer], useful for validating the Aqua satellite (NASA) Integrated Program Office (IPO) for the observations. A brief description of the instrumen- NPOESS carried five separate radiometers. The tation used in the EAQUATE campaign is given in Proteus generally maintained a flight altitude in "Instrumentation," followed by a description of some the range of 15-17 km, depending upon flight of the results from "The Italian phase" and "The U.K. duration, when underflying the Aqua satellite and phase." The "Conclusions" discuss the merits of cali- overflying the IMAA/DIFA ground sites and the bration and validation of satellite instruments and the FAAM BAel46-301 aircraft measurements. required synergy of different observing systems. The results presented in this paper often rely on FAAM BAel46-30l (Met Office and NERC). The complex retrieval theory. A good review of a range FAAM BAel46-301 (Fig. 2) is jointly funded by the of the retrieval techniques used here is presented in Met Office and the Natural Environment Research Zhou et al. (2006) and Amato et al. (2002). Council (NERC). Capable of operating between 15 m and 10.5 km, the aircraft can carry a scientific INSTRUMENTATION. Full details of the key payload of 4,000 kg. For the EAQUATE campaign instrumentation used in the EAQUATE campaign the scientific payload consisted of a combination of is given in Table 1. spectrometers for measuring the radiation from the visible to the microwave region of the electromagnetic Proteus (IPO). The Proteus aircraft (Fig. 1), sponsored spectrum plus an array of instrumentation to char- by the Department of Defense (DoD)-National acterize the troposphere both in terms of thermody-
AFFILIATIONS: TAYLOR AND NEWMAN—Met Office, Exeter, Devon, Interuniversitario per le Scienze fisiche della Materia, United Kingdom; SMITH—Hampton University, Hampton, Virginia, University of Naples, Napoli, Italy and University of Wisconsin—Madison, Madison, Wisconsin; CORRESPONDING AUTHOR: Dr. Jonathan P. Taylor, CUOMO, ROMANO, PAPPALARDO, PAVESE, MONA, AND AMODEO—Istituto Met Office, FitzRoy Road, Exeter, Devon, EXI 3PB, United di Metodologie per l'Analisi Ambientale, CNR, Tito Scalo, Italy; Kingdom LARAR AND ZHOU—NASA Langley Research Center, Hampton, E-mail: [email protected] Virginia; SERIO, DI GIROLAMO, ESPOSITO, GRIECO, SUMMA, RESTIERI, The abstract for this article can be found in this issue, following the AND MASIELLO—Dipartimento di Ingegneria e Fisica dell'Ambiente, table of contents. University of Basilicata, Potenza, Italy; MAESTRI AND RIZZI—Physics DOI: 10.1175/BAMS-89-2-203 Department, Alma Mater Studiorum, University of Bologna, Bologna, Italy; ANTONELLI—Mediterranean Agency for Remote In final form 6 July 2007 Sensing, Benevento, Italy; MANGO—NPOESS Integrated Program ©2008 American Meteorological Society Office, Silver Spring, Maryland; PISANI—Consorzio Nazionale
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC namical variables like temperature and water vapor, cold space and hot calibration targets are provided by and also particulates in the form of aerosols and a 360° rotation of the scan mirror every 2.67 s. The cloud particles. The main focus of the work during AIRS radiance measurement accuracy and its ability EAQUATE was cloud-free atmospheres, but one to achieve the 1-K/l-km sounding accuracy objective flight in cirrus clouds was conducted under an Aqua are to be validated using the EAQUATE surface and overpass. During the EAQUATE flights dropsondes airborne datasets. were launched to give vertical profiles of temperature and water vapor. During satellite overpasses up to THE ITALIAN PHASE. The Italian phase of 14 sondes were launched in quick succession, giving the EAQUATE measurement campaign took place high levels of spatial detail. In addition to the ther- between 6 and 10 September 2004. modynamic measurements, the BAel46-301 made Figure 3 shows the times for all measurements; continuous measurements of carbon monoxide and note that two flights of the Proteus aircraft were ozone concentrations and also measured aerosol and coincident with Aqua overpasses. cloud particle size, shape, and concentration. The Proteus aircraft flew legs at high altitudes along the line of the Aqua overpasses at 0101 UTC Ground based (IMAAIDIFAIUNINA). A wide range of 8 September and 0056 UTC 10 September (Fig 4). instrumentation was used during EAQUATE mea- The numbers of overpasses of the ground sites by the surement campaign: ground based, airborne, and Proteus aircraft are given in the figures. satelliteborne. During the Italian phase of the experiment, the NAST-I, AIRS retrieval validation. The NAST-I instru- ground-based instrumentation was located at three ment on Proteus covers the entire spectral range different sites located in southern Italy. These systems of AIRS at greater resolution, allowing the direct were operated continuously throughout the Proteus comparison of radiances and level-2 products, like airborne measurement period, which included the temperature and water vapor. During the night of Aqua overpass times. 9-10 September 2004 the Proteus aircraft flew a flight track over southern Italy that was coincident with Aqua AIRS (NASA). The AIRS instrument flying aboard the AIRS overpass at 0056 UTC 10 September. The the EOS Aqua polar-orbiting satellite is the first space- aircraft track passed over the Potenza site, allowing borne spectrometer designed to meet the 1-K/l-km for a direct comparison of the satellite, aircraft, sounding accuracy objective of future operational satel- and ground-based profile information (Fig. 4). The lite sounders by measuring the infrared spectrum quasi NAST team of scientists has developed an inversion continuously from 3.7 to 15.4 ^m, with high spectral scheme. Detailed NAST-I physically based empirical resolution (v/Sv = 1200/1). The sensitivity requirements, orthogonal function (EOF) regression and simulta- expressed as noise-equivalent differential temperature neous matrix inversion (i.e., nonlinear multivariable (NEDT), referred to a 250-K target temperature and physical iteration) can be found in Zhou et al. (2002). ranges from 0.1 K in the 4.2-^m lower-tropospheric sounding wavelengths to 0.5 K in the 15-^m upper-tropospheric and stratospheric sounding spectral region. The AIRS instrument provides spectral coverage in the 3.74-4.61-, 6.20-8.22-, and 8.8-15.4-^m infrared wave- bands at a nominal spectral resolution of v/dv = 1200, with 2378 IR spectral samples and four visible/near-infrared (VIS/NIR) channels between 0.41 and 0.94 /mi. Cross-track spatial coverage with -16-km linear resolution, depending on scan angle, and views of FIG. 2. The FAAM BAel46-30l aircraft, showing instrumentation.
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC TABLE 1. Key instruments used during the EAQUATE campaign. Sensor Sensor description Products Location Reference NPOESS Airborne Spectral range of 3.6-16.1 Profiles of temperature Northrop-Grumman Smith et al. (2005); Sounding Testbed- jL/m, spectral resolution (0.25 and water vapor, a vertical Proteus Aircraft Zhou et al. (2002) Interfero meter cm-'); FOV: size of 130 m km"1 resolution of 1-2 km, (NAST-I) of aircraft altitude (e.g., from the surface temperature and Proteus altitude of 16 km, 2.0-km surface emissivity, direct spatial resolution is achieved) radiance validation of satellite instruments; scans across track
NPOESS Airborne 29 spectral channels in the Temperature and water Northrop-Grumman Blackwell et al. Sounding Testbed- 50-425-GHz range (i.e., eight vapor sounding information Proteus Aircraft (2001) Microwave (NAST-M) channels in the 50-56-GHz through nonprecipitating oxygen band, seven channels on clouds and maps out the the 118-GHz oxygen line, seven spatial distribution of precipi- channels on the 183-GHz water tating cells in the atmosphere vapor line, and seven channels on the 425 oxygen line); ground resolution of 130 m km-1 of flight altitude
Scanning High Spectral range 3.0- Profiles of temperature Northrop-Grumman Tobin et al. (2006) resolution 17.0-jum resolution 0.5 cm"1 and water vapor and direct Proteus Aircraft Interferometer spatial resolution 100 m km-1 of radiance validation of NAST-I Sounder (S-HIS) flight altitude views upward and and satellites. downward
Far Infrared Band-I upwelling radiation in the Observe the microphysical Vaneket al. (2001) Spectrometer for 10-47 cm-1 (300-1400 GHz) properties of cirrus clouds Cirrus (FIRSC) band-2 upwelling radiation in the 80-135 cm"1 (75-135 jum) spectral resolution of 0.1 cm-1
Microscale Nadir-viewing gas filter Radiometric measurement of Northrop-Grumman Online at www. Measurement of radiometer 4.67-jLvm band carbon monoxide Proteus Aircraft resonance.on.ca/ Atmospheric Pollution of carbon monoxide; spatial mm aps.html Sensor (MicroMAPS) resolution of 52 m km-1 of flight altitude
ARIES Thermal Infrared Bomem FTS 3.3- to 16-jL/m Radiative transfer model On board FAAM Fiedler and Interferometer region with 0.5 cm-1 spectral validation, temperature BAel46-30l Newman (2005); resolution with both upward and profiling and profiling of Atmospheric Research Taylor et al. (2003) downward pointing various species (e.g., water Aircraft, operating vapor, ozone), cloud and between 15 m and surface emissivity studies 10.5 km
MARSS Microwave Operates at AMSU-B channels Satellite intercomparisons, On board FAAM McGrath and Radiometer 16-20 (89-183 GHz) and scans radiative transfer model BAel46-30l Hewison (2001) both upward and downward validation, temperature and Atmospheric Research (+/-40° along track) humidity profiling, column Aircraft, operating liquid water retrievals, between 15 m and precipitation, ice cloud and 10.5 km surface emissivity studies
Deimos Microwave Four channels (each of two Satellite intercomparisons, On board FAAM McGrath and Radiometer orthogonal polarizations at radiative transfer model BAel46-30l Hewison (2001) 23 and 50 GHz); upward or validation, surface emissivity Atmospheric Research downward viewing studies Aircraft, operating between 15 m and 10.5 km
The Tropospheric 12.5—l25-/jm region with Study of upper-tropospheric On board FAAM Canas et al. (1997) Airborne Fourier 0.1 cm-1 spectral resolution; and lower-stratospheric BAel46-30l Transform upward and downward views water vapor and cirrus Atmospheric Research Spectrometer (TAFTS) clouds Aircraft, operating (owned by Imperial between 15 m and College, United 10.5 km Kingdom)
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC Atmospheric State Temperature, water vapor, Wide range of measurements On board FAAM Online at www. Parameters ozone, carbon monoxide, liquid made at frequencies between BAel46-30l faam.ac.uk water, ice water, particle size and 64 and 1 Hz Atmospheric Research shape, aerosol size scattering and Aircraft, operating chemistry, etc; dropsondes. between 15 and 10.5 m
Raman lidar*3 Transmitter: Nd:YAG 355 nm, Vertical profiles, both in 1: Potenza laser Di Girolamo et 532 nm, prf 20 Hz daytime and nighttime, of and Basilicata Bosenberg (2001); aerosol backscatter and 48°38'45.23"N, Matthias (2004); extinction coefficients, water I5°48'29.32"E, Bockmann (2004); vapor mixing ratio, atmo- 770 m ASL; 2: UNINA Pappalardo et al. spheric temperature Napoli 40°50'I8"N, (2004) I4°I0'59"E, 118 m ASL; 3: The Istituto di Metodologie per l'Analisi Ambientale (IMAA) in Tito Scalo (40°36'4.32"N, I5°43'25.32"E, 760 m ASL)
Raman lidar Nd: YAG laser with third har- Water vapor mixing ratio IMAA in Tito Cornacchia et al. monic generator (355 nm) prf profiles from about 60 m Scalo (40°36'4.32"N, (2004) 100 Hz above lidar station up to the I5°43'25.32"N, 760 UTLS m ASL) Infrared Interferometer ABB MR-100 spectral range Water vapor, temperature IMAA in Tito 500-5000 cm-1 resolution of profiles, and information on Scalo (40°36'4.32"N, 0.5 cm"1 cloud properties I5°43'25.32"E, 760 m ASL) REFIR/BB (Radiation FTS measuring over the range Study of the water vapor IMAA in Tito Explorer in Far 100-1050 cm-1 with a resolution rotational bands Scalo (40°36'4.32"N, Infrared/Bread Board) of 0.5 cm"1 I5°43'25.32"E, 760 m • ASL) Radiosonde Vaisala-type RS90 and RS92 Profiles of pressure, temper- IMAA in Tito sondes ature, and relative humidity Scalo (40°36'4.32"N, I5°43'25.32"E, 760 m ASL)
Microwave radiometer 12 channels Profiles of temperature and IMAA in Tito water vapor from surface to Scalo (40°36'4.32"N, 10 km and low-level cloud I5°43'25.32"E, 760 m liquid water content ASL)
Ceilometer Vaisala cloud-base detector Cloud base up to 7.5 km, IMAA in Tito 15-km vertical resolution; Scalo (40°36'4.32"N, five profiles a minute I5°43'25.32"E, 760 m ASL)
FIG. 3. Chart showing times of measurements for all instru- ments involved in the Italian phase of EAQUATE.
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC FIG. 4. The four Proteus flight tracks: (a) 6, (b) 7, (c) 8, and (d) 9 Sep 2004. The black circles indicate the positions of Napoli, Tito Scalo, and Potenza, showing that the Proteus overflew each of these sites on every flight.
NAST-I measurements and its inversion algorithm 2004; Di Girolamo et al. 2004, 2006). Under cloud- have been validated through numerous dedicated free conditions, the retrieved effective surface skin field experiments (e.g., Smith et al. 2005). Processing temperature is equivalent to the skin temperature. both AIRS and NAST-I data through this inversion Figure 5 shows the skin temperature retrieved from scheme allows the intercomparison of thermody- AIRS and NAST-I, with a full spatial resolution of namic parameters. -16 and ~2 km, respectively. Profiles of temperature AIRS original single field-of-view (FOV) radiances and moisture retrieved from AIRS and NAST-I are are put through the NAST team inversion scheme to validated with dedicated radiosonde and Raman lidar produce AIRS retrievals to compare with the NAST-I observations during the EAQUATE. In the figure, the retrievals degraded to AIRS full spatial resolution lines with arrows indicate the Aqua and Proteus flight (i.e., AIRS single FOV). The same inversion algorithm directions with associated time. The open cycles rep- is applied to both AIRS and NAST-I data to minimize resent AIRS single FOV within NAST-I ground track the impact of inversion algorithm differences on the swath width. All NAST-I retrievals from the NAST-I retrieval products. However, it should be noted that single FOV falling into the AIRS FOV are averaged the forward radiative transfer models used differ for comparison with the AIRS retrieval. in that the Stand-Alone AIRS Radiative Transfer The mean profile of the section shown in Fig. 5 Algorithm (SARTA) (Strow et al. 2003) is used (indicated with the open cycles) is plotted in Fig. 6 for the AIRS retrieval while the optimal spectral with a Vaisala-type radiosonde and two Raman lidar sampling (OSS) fast molecular radiative transfer observations from Potenza. One Raman lidar station model (Moncet et al. 2003; Liu et al. 2003) is used for (labeled lidar 1) is located at Potenza (40°39'N, the NAST-I retrieval. These retrievals can then be 15°48'E; 730 m above sea level), while another Raman compared directly with the measurements of dedi- lidar station (labeled lidar 2) and a radiosonde cated radiosonde and Raman lidar (e.g., Cuomo et al. station are located at Tito Scalo, Potenza (40°36'N,
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC 15°44'E; 760 m above sea level). A dedicated radiosonde was launched while Proteus was passing over Potenza. The Raman lidar data were extensively processed (i.e., a 10-min integration time and variable vertical averaging) in order to reach a higher altitude for retrieval validation as Aqua and Proteus were passing over. The Raman lidar data plotted in Fig. 6 are the mean profiles of Aqua (0056 UTC) and Proteus (0037 UTC) overpasses. The retrievals from both AIRS and NAST-I compare favor- ably to Raman lidar and radiosonde observations. However, a few noticeable features are included, such as follows: 1) there are integra- tion time differences between AIRS and NAST-I; in other words, the Aqua satellite passed over quickly while the Proteus's flight from south to north took more than 1 h, 2) the averaged profile-reduced vertical resolution of a single FOV FIG. 5. AIRS- and NAST-I retrieved effective surface skin temperature retrieval has a lower vertical reso- (K) during the night of 9-10 Sep 2004. lution (approximately 2-3 km), while radiosonde and Raman lidar observations have a much higher vertical resolution (15-500 m, de- pending on altitude), 3) a dry bias in the humidity measurement from a Vaisala-type radiosonde at altitudes above ~8 km has been noticed here as well as in other validations, and 4) the fine vertical structures (i.e., resolution) of the retrieved profiles are partially due to the instrumen- tation characteristics, such as the spectral resolution and instrument noise, which could cause a difference in the profile. Overall, the retriev- als from the two different sounders compare favorably with each other. The AIRS-NAST-retrieved tempera-
FIG. 6. Retrievals of (top) temperature and (bottom) relative humidity using AIRS and NAST-I data processed through the NAST team inversion scheme. Radiosonde and Raman lidar profiles from Potenza (Ptz) are plotted as references.
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC FIG. 7. Temperature retrieval over tree-covered land FIG. 8. Profiles of water vapor mixing ratio. Key to lines near Tito Scalo on 10 Sep. First guess shown as black and symbols as per Fig. 7. solid line with radiosonde shown as blue curve. The results of the physical retrieval conducted on NAST-I data using the 2- and 3-band scheme are shown in red conifer trees, was determined by fitting a set of 16 circles and green crosses (respectively). clear-sky spectra recorded by NAST-I (Masiello et al. 2006). A physical retrieval scheme using the a IASI ture and moisture profile differences, plotted in the code (Amato et al. 2002; Carissimo et al. 2005) has right-hand panels of Fig. 6, are within the uncertainty been used to retrieve temperature (Fig. 7) and water levels for these parameters. This implies that both vapor profiles (Fig. 8). The physical retrieval scheme instruments are well calibrated and that the radiative can be run using either two or three spectral bands. transfer models are equivalently accurate. The two-spectral band option uses bands between 670-830 and 1100-1200 cm"1. The three-band op- NAST-I retrieval over land. One of the NAST-I tion uses an additional band of 1450-1600 cm-1. The footprints at 0032 UTC 10 September was over a three-band option includes a portion of the water small tree-covered hill close to the IMAA/Consiglio vapor vibrational band at 6.7 fim. The inclusion of Nazionale delle Ricerche (CNR) Institute. The this band had a negligible impact on the tempera- emissivity of the surface, which was covered by ture retrieval and yielded a modest impact upon the retrieved water vapor in the lower atmosphere. In principle, especially for water vapor, one would have expected a larger im- pact by the inclusion of band three. The fact here is that the NAST-I band 3 has a relatively larger noise in comparison to that of bands 1 and 2; therefore, the related radiances are in part filtered out because of the large variance weight they had been assigned in the observational covari- ance matrix. Although illustrative, the retrieval example shown in FIG. 9. Schematic of aircraft operations during the U.K. phase of Figs. 7 and 8 gives evidence EAQUATE. of the vertical resolution
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC improvement that can be reached with hyperspectral THE U.K. PHASE. During the U.K. phase of sounding in the infrared. The fine structure in the water EAQUATE, the Proteus aircraft worked in close vapor retrieval in between 600 and 800 mbar is nicely collaboration with the BAel46-301, studying reproduced, which leads us to conclude that the NAST-I the atmosphere during two AIRS overpass days. hyperspectral observation is capable of retrieving water Flights were flown on 14 and 18 September 2004. vapor with a vertical resolution of 1-2 km in the lower During this phase of EAQUATE the aim was to troposphere. It is also worth noting that the inversion in validate AIRS in the remote marine environment Figs. 7 and 8 has been obtained with a research-oriented under cloud-free conditions by stacking the AIRS inversion scheme, in which the retrieval can be con- satellite, Proteus aircraft, and BAel46-301 aircraft strained by the degrees of freedom (Serio et al. 2007). vertically over each other during the overpass, as For the analysis at hand, the final degrees of freedom indicated in Fig. 9. The Proteus aircraft was operat-
of the H20 retrieval was 7.2. It can be shown that below ing the NAST-I and S-HIS interferometers, while 6.5 degrees of freedom the vertical resolution for water the BAel46-301 made measurements of the in situ vapor rapidly decrease to levels for which structures of a state of the troposphere and the surface; dropsondes length scale of the order of 1-2 km are no more resolved. were also launched to more finely characterize the The performance of the retrieved temperature is not as atmospheric column. good in the boundary layer where the impacts of the The conditions on 14 September 2004 are clearly spectrally variable and nonunit emissivity is a contribut- shown in the Moderate Resolution Imaging Spectro- ing factor. In the layers below 800 mb the temperature radiometer (MODIS) image in Fig. 10. After transiting differences are still around 1 K, while the mixing ratio to the operating area, the BAel46-301 conducted a differences are 0.1 g kg1 at the surface (using the two- profile descent to an altitude of 15 m above the sea band scheme). surface in the area of interest and then flew a straight
FIG. 10. The (top left) sub-satellite track, (bottom right) BAel46-30l track, and (bottom left) BAel46-30l alti- tude time series overlaid on the Aqua MODIS image for 1320 UTC 14 Sep 2004. The portion of the sub-satellite track studied during this flight is shown as the red line.
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC and level run at 30 m along the line of the subsatellite proper spatial and temporal coincidence needed to track to measure the sea surface temperature. During best validate the radiometric performance of satellite- this low-level run of the BAel46-301 the Proteus based measurements like Aqua. The high spectral and aircraft flew at an altitude of around 14 km along spatial resolution aircraft sensor radiance data can the same track in the same direction. The ground be spectrally and spatially convolved, respectively, to speed of the BAel46-301 and Proteus were perfectly simulate what should be measured by the spatially and matched and both aircraft remained stacked above temporally coincident satellite observations during each other for this run. Having characterized the sea overpass events. The much higher spatial resolution surface, the BAel46-301 profiled to 8.5 km and flew of the aircraft sensor data can play an important role backward and forward along the subsatellite track, in validating satellite-derived data products under the launching dropsondes en route. Having burned a bit conditions of variable surface and atmospheric radi- more fuel, the BAel46 was then able to climb to an ance (e.g., resulting from clouds) within the satellite altitude of 10.6 km, continuing along the subsatellite sensor footprint. track. During all of these high-level runs the BAel46 The 11-^m window band (MB31) from the MODIS made measurements with the ARIES interferometer aboard Aqua has been used to determine the most and its other radiometers. Finally, the BAel46-301 spatially uniform scene of coincidence between AIRS conducted a long, descending profile back to the sur- IFOVs and the NAST-I and S-HIS instruments on face to get a final look at the tropospheric structure of the Proteus aircraft for the flight on 14 September temperature, water vapor, ozone, carbon monoxide, 2004. and clouds. Figure 11 shows a comparison of the resulting The evaluation of satellite data can be conducted in averaged nadir spectra at their original as-measured terms of retrieval performance, but also, importantly, spectral resolutions for NAST-I, S-HIS, and AIRS. in terms of direct radiance validation. The Proteus Such a first-order comparison covering the entire aircraft flies at an altitude that puts it above most of spectral extent effectively demonstrates the desired the atmosphere, and hence it is ideally suited for the overlapping nature of these spectra, along with some important job of direct radiance validation using the expected differences. Most notably, spectral structure NAST-I and S-HIS instruments. NAST-I and S-HIS is better resolved with the higher spectral resolution have been compared on many occasions before and NAST-I; the AIRS grating has gaps in coverage over their calibration is well characterized. During the the spectral extent; and spacecraft versus aircraft EAQUATE campaign the BAel46 and Proteus on oc- altitude differences cause upwelling radiance observ- casion both operated at 8.5 km, allowing for a direct able by AIRS to deviate from that seen by NAST-I
comparison of NAST-I, S-HIS, and ARIES, these data and S-HIS in select spectral regions (e.g., 03 and
are not shown here for brevity. C02 bands). Figure 12 shows this same comparison of infrared spectral radiance in more detail. This NAST-I, S-HIS, and AIRS radiance intercomparison. Only intercomparison serves to exemplify the high degree observations from aircraft platforms can provide the of radiometric (and spectral/spatial) fidelity of these airborne Fourier transform spec- trometer (FTS) systems (with NAST-I and S-HIS differing, on average, by 0.08 K) and their utility for direct radiance valida- tion of satellite sensors (with the mean difference between NAST-I and AIRS having a magnitude < 0.15 K). While such results do vary slightly in magnitude when examining different underflights resulting from, in large part, the degree of spatial/temporal collo- cation combined with the amount of geophysical field nonunifor- FIG. 11. Averaged nadir spectra at their original as-measured spectral resolutions for NAST-I (red), S-HIS (green), and AIRS (cyan) for the mity, a similar relationship has intercomparison region on 14 Sep 2004. typically been observed in other
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC -1 FIG. 12. Spectral radiance intercomparison for a 100 cm interval in the infrared window region: (a) for NAST-I reduced to S-HIS spectral resolution, S-HIS as measured, AIRS as measured, and NAST-I, with the AIRS spec- tral response functions applied; and for (b) difference plots with S-HIS and AIRS subtracted from the spectrally equivalent versions of the NAST-I measurement shown in (a).
cases analyzed. One consistent conclusion reached the Proteus aircraft flew a series of runs up and down among the different cases examined regards the im- a track measuring the upwelling radiances with its portance of such airborne measurements for satellite range of spectrometers, the BAel46 flew along the sensor calibration/validation; that is, while radiance same track launching a series of nine dropsondes to simulations using available ground truth (radio- characterize the temperature and water vapor struc- sondes, NWP analysis fields, etc.) are an important ture of the troposphere. This dataset is being used contributor to validation, better agreement is almost to evaluate the performance of the NAST-I retrieval always achieved upon infusing directly measured ra- scheme, which is in turn being used to validate the diance data from these very well-calibrated airborne AIRS instrument retrieval. In addition to this valida- FTS sensors; this is particularly important for non- tion study the dataset allows a study of the horizontal uniform and/or rapidly varying scenes, those of most variability of the thermodynamic structures in the meteorological interest for research and operational atmosphere, which are important to understand when usage of such satellite sensor systems. considering the calibration/validation of satellite instruments. Figure 13 shows the positions of NAST-I Spatial variability of water vapor and the importance nadir view retrievals of temperature and water vapor of in situ measurements. The flight of 18 September between 1221:00 and 1308:53 UTC (a total of 208 2004 saw the BAel46-301 and Proteus aircraft flying retrievals), where the data have been filtered to together under an AIRS overpass that occurred at remove points with aircraft roll in excess of 10°. 1254 UTC. The Proteus flew at a pressure of 150 hPa, The times of the dropsondes were 1252, 1255, while the BAel46 operated at around 350 hPa. While 1258, 1301, 1303, 1312, 1315, 1319, and 1350 UTC;
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC the relative humidity difference does not vary from one sounding to another above about 3% (absolute RH). At lower levels around 800 hPa the humidity differences grow to nearer 6% for distances in excess of 40 km. In the boundary layer the story is totally different; the weather on this day consisted of a high pressure system with dry air aloft and a moist bound- ary layer between the surface and around 875 hPa. The data from the retrievals in the boundary layer show that the variability in relative humidity grows with horizontal distance. It is hoped that IR sounders like AIRS and future sounders like IASI and CrIS will be able to allow retrieval of relative humidity The position of the -I nadir retrievals FIG. 13. NAST to within 10% over a 1-km-thick layer. This analysis (black line) and the launch and splashdown positions of the nine dropsondes (red). Launch points are those suggests that to evaluate a retrieval to this level of to the west. accuracy will require in situ truth measurements from within -10 km of the retrieval. Clearly, the thus, are all within 30 min of the NAST-I retrievals, horizontal scales of importance will differ from day with many of them being significantly closer in to day with the prevailing meteorological conditions time. To give an idea of scale, the horizontal distance and also with the horizontal averaging of the remote from the launch to splashdown position of the most sensing instrument. However, this analysis is indica- northern dropsonde shown in Fig. 13 is 10.97 km. tive of the importance of gaining high-quality and Understanding spatial variability in water vapor and spatially correlated in situ data, as were available temperature fields is critical to the interpretation of during the EAQUATE campaign. remote-sensing data and any data used in a valida- Figure 15 shows the nine dropsonde profiles of tion exercise; here, NAST-I retrievals will be used for temperature (red lines), the AIRS retrievals from the this purpose. For each pressure level of the retrievals six pixels in the geographic region of Fig. 13 (blue the absolute value of the relative humidity difference lines), and the 208 retrievals of temperature from between adjacent retrieval pixels (1.75 km) has been NAST-I (black crosses). The AIRS retrievals are computed and the average has been taken. There were version-3 level-2 product 3x3 FOV averages with a 208 retrievals in all, so for one pixel difference 207 spatial scale of 45 km. Figure 16 shows the profiles humidity differences are available for each pressure of relative humidity in the same way. Several features level. This exercise was carried out again looking at the differences for all possible 2-pixel differences (equivalent to spatial scales of around 3.5 km) through to all possible 50-pixel differences (equivalent to spatial scales of around 105 km). Figure 14 shows, as a contour plot, the aver- age difference in relative humidity, evaluated from NAST-I retrievals, as a function of pressure and horizontal scale. For high levels in the atmosphere (above 700 hPa) the vari- ability of water vapor is low, and hence for all scales FIG. 14. Contour plot of the average difference in relative humidity from evaluated up to 105 km NAST-I retrievals as a function of pressure and horizontal spatial scale.
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC the dropsondes clearly show signifi- cant variation. The RMS difference computed from all AIRS-minus- sonde combinations (54 in total) is shown as a dashed-dotted line, and this shows that the RMS difference is within 1 K, except around 500 hPa. The differences between the NAST-I and sonde profiles are presented at a higher vertical resolution. The solid line is the difference between the average difference of all NAST-I retrievals and the average of all the sondes, while the dotted line is the average difference of the NAST-I retrieval and its closest sonde. The . 15. Profiles of temperature from the nine dropsondes (red), the FIG agreement between NAST-I and six AIRS pixels in the region (blue), and the 208 NAST-I retrievals the sonde is generally within 1 K; (black crosses). the exception is around 800 hPa, where the difference is around 1.8 K; are apparent: the NAST-I retrievals of temperature interestingly, this occurs in a region of maximum differ from the dropsondes the most around the 750- change in relative humidity. 800-hPa area, which is where the relative humidity is The relative humidity difference (Fig. 18) is pre- changing most rapidly. Furthermore, the NAST-I and sented with the same line styles as the temperature AIRS retrievals of relative humidity are generally too difference figure. The AIRS retrievals are within moist in the boundary layer. 10% of the dropsonde, except for the single point Figure 17 shows the difference in temperature at 850 hPa, which corresponds to the top of a sharp retrievals while Fig. 18 shows the difference in relative inversion and is indicative of the inability to resolve humidity. The differences are defined as the retrieval such sharp features. The RMS difference is also minus the sonde. Prior to computing the difference within 10%, with the exception of the point at the the profile of each dropsonde was resampled to the boundary transition. The NAST-I retrievals of relative respective pressure levels of the two retrievals (AIRS humidity are in better agreement with the dropsonde and NAST-I). This was done by taking the average data, but once again perform worst at the boundary of the sonde data over the respective model layers. The NAST-I retrieval product is defined at 32 levels in the troposphere below the altitude of the BAel46 (which launched the drop- sondes), whereas the AIRS retrieval is only defined at seven levels in this region. The levels at which the AIRS profile is defined are coarse, and therefore the comparison of "aver- age" relative humidity across a sharp inversion that straddles two layers is always going to be difficult. Considering first the temperature differences, the difference between the average of the six AIRS retrievals and the average of the nine drop- sondes (dashed line) shows that the FIG. 16. Profiles of relative humidity from the nine dropsondes (red), the six AIRS pixels in the region (blue), and the 208 NAST-I retriev- AIRS retrievals are generally within als (black crosses). +/-1 K, except around 500 hPa where
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC with the mean difference between NAST-I and AIRS being <0.15 K. During the U.K. phase of the campaign the airborne interferometers on the BAel46-301 and Proteus were directly compared during a wingtip-to- wingtip intercomparison run, which gives confidence in the radiometric performance of the airborne interferometers. Working from this position of confidence in the direct radiometric performance of the various instruments, the data gathered during EAQUATE have been used to evaluate the range of level-2 products derived from AIRS. This has been done through direct comparison with similar level-2 prod- ucts from the airborne interferometers (NAST-I and FIG. 17. Profile of temperature difference (retrieval minus sonde). The solid line is the difference between S-HIS) and through comparison with lidar and drop- the average of all NAST-I retrievals and the average sonde profiles. Comparison of retrievals during the of all nine dropsondes. The dotted line is the average Italian and U.K. phases of EAQUATE have shown that difference between sonde and the NAST-I retrieval the 1-K and 10% accuracy required for temperature where each retrieval has been differenced from its and humidity is mostly being met by AIRS. closest sonde in distance. The dashed line is the differ- In conducting this type of analysis there are many ence between the average of all six AIRS retrievals and issues associated with the representativeness of the the average of all sondes. The dashed dotted line is the RMS difference between AIRS and the sondes. various data types that should be considered. Direct radiance validation is fairly straightforward provided that the instrument artifacts, like instrument line layer transition. Although only a single case study, shape, are accounted for correctly. Comparison of these results clearly show the ability of IR sounders retrievals is more complex because of the fact that a like AIRS and NAST-I to retrieve profile information retrieved product is reported at pressure levels (which of temperature and relative humidity at the 1 K and are a function of the forward radiative transfer model 10% RH level. used by the researcher), which by definition represent
CONCLUSIONS. The EAQUATE campaign brought together an extensive range of airborne and ground-based observing systems to provide valida- tion of AIRS level 1 and 2 products. The campaign focused on the provision of high-quality radiance spectra from the NAST-I and S-HIS interferometers on the Proteus aircraft, backed up with detailed measurements of the tropospheric temperature and water vapor structure. During the Italian phase of the campaign a network of ground-based lidar systems provided the detail of the tropospheric water vapor structure at high temporal resolution. During the U.K. phase of the campaign detailed measurements of the atmosphere over the marine environment were provided by the extensively equipped FAAM BAel46- FIG. 18. Profile of relative humidity difference (retrieval 301 aircraft; a critical element of this was the use of minus sonde). The solid line is the difference between multiple dropsondes. the average of all NAST-I retrievals and the average The radiance performance of satellite instruments of all nine dropsondes. The dotted line is the average difference between sonde and NAST-I retrieval where like AIRS can be evaluated through direct compari- each retrieval has been differenced from its closest son with higher spectral resolution instruments like sonde in distance. The dashed line is the difference NAST-I and S-HIS, which are resampled spectrally between the average of all six AIRS retrievals and the and spatially to the AIRS specification. Analysis here average of all sondes. The dashed dotted line is the shows excellent agreement between the instruments, RMS difference between AIRS and the sondes.
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC information over a finite pressure interval arising intercomparison in the framework of the EARLINET from the weighting function of the spectral channels. project. 2. Aerosol backscatter algorithms. Appl. In contrast, dropsonde or radiosonde data are point Opt., 43, 977-989. measurements. Great care needs to be used therefore Bosenberg, J., and Coauthors, 2001: EARLINET: in describing differences between disparate types of A European Aerosol Research Lidar Network. measurement, and the reader needs to be aware of Advances in Laser Remote Sensing, A. Dabas, C. Loth, these issues when drawing conclusions. and J. Pelon, Eds., Ecole Polytechnique, 155-158. The EAQUATE campaign has shown the critical Canas, A. A., J. E. Murray, and J. E. Harries, 1997: importance of coordinated measurements by a range The tropospheric airborne fourier transform of observing systems. Over the marine environ- spectrometer. SPIE, 3220, 91-102. ment coordinated operations between high-altitude Carissimo, A., I. De Feis, and C. Serio, 2005: The and tropospheric aircraft are critical in satellite physical retrieval methodology for IASI: The
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Unauthenticated | Downloaded 10/09/21 05:34 AM UTC Moncet, J. L., and Coauthors, 2003: Algorithm Strow, L. L., S. E. Hannon, M. Weiler, and K. Overoye, Theoretical Basis Document (ATBD) for the Cross 2003: Prelaunch and in-flight radiometric calibration Track Infrared Sounder (CrIS) environmental data of the Atmospheric Infrared Sounder (AIRS). IEEE records (EDR), Vl.2.3. AER Document P882-TR-E- Trans. Geosci. Remote Sens., 41, 274-286. 1.2.3-ATBD-03-01, 157 pp. [Available online at eic. Taylor, J. P., S. M. Newman, T. J. Hewison, and ipo.noaa.gov/IPOarchive/SCI/atbd/cris_atbd_03_ A. McGrath, 2003: Water vapour line and continuum 09_01.pdf.] absorption in the thermal infrared—Reconciling Pappalardo, G., and Coauthors, 2004: Aerosol lidar in- models and observations. Quart. J. Roy. Meteor. Soc., tercomparison in the framework of the EARLINET 129, 2949-2969. project. 3. Raman lidar algorithm for aerosol Tobin, D. C., H. E. Revercomb, C. C. Moeller, and T. S. extinction, backscatter, and lidar ratio. Appl Opt., Pagano, 2006: Use of Atmospheric Infrared Sounder 43, 5370-5385. high-spectral resolution spectra to assess the calibra- Serio, C., G. Grieco, and G. Masiello, 2007: Capability tion of Moderate resolution Imaging Spectroradi- of high spectral resolution observations in the in- ometer on EOS Aqua. /. Geophys. Res., Ill, D09S05, frared to detect water vapor structures. Technical doi:10.1029/2005JD006095. Digest Fourier Transform Spectroscopy (FTS) Vanek, M. D., 2001: Far-infrared sensor for cirrus Hyperspectral Imaging and Sounding of the (FIRSC): An aircraft-based Fourier-transform spec- Environment (HISE), Optical Society of America, trometer to measure cloud radiance. Appl. Opt., 40, CD-ROM, HThB2. 2169-2176. Smith, W. L., D. K. Zhou, A. M. Larar, S. A. Mango, Zhou, D. K., and Coauthors, 2002: Thermodynamic H. B. Howell, R. O. Knuteson, H. E. Revercomb, product retrieval methodology for NAST-I and and W. L. Smith Jr., 2005: The NPOESS Airborne validation. Appl Opt., 41, 6957-6967. Sounding Testbed Interferometer—Remotely sensed , and Coauthors, 2006: AIRS retrieval validation surface and atmospheric conditions during CLAMS. during the EAQUATE. Proc. SPIE, 6362, 636 224. J.Atmos. Sci., 62,1117-1133.
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