The Atmospheric Infrared Sounder (AIRS) introduces new Level-1C (L1C) products. AIRS Version 6.6 L1C processing addresses data quality and sampling issues as well as spectral drift, making a clean, easy to use product. This will be the first version of AIRS L1C permanently hosted at the Goddard Earth Science DAAC, making it easy for users to access. A later Version 7 L1C will incorporate v7 Level-1B (L1B) calibration improvements and use a modern netCDF4 format. We focus on the spectral changes in the AIRS instrument and the new L1C feature that corrects for it.
The Atmospheric Infrared Sounder (AIRS) radiometric calibration coefficients convert the counts measured from the instruments A/D converters (Level 1A) to SI traceable radiance units (Level 1B). The calibration equations are based on how the instrument operates and follow a simple second order relationship between counts and radiance. Terms are included to account for nonlinearity of the detectors, emissivity and temperature knowledge of the on-board calibrator (OBC) blackbody and radiometric offset due to coupling of the polarization of the scan mirror with the spectrometer. In this paper, we re-derive the radiometric calibration equation with a little more rigor and account for the view angle of each of the 4 space views. We then derive new polarization coefficients from the 4 space views over the mission and use them re-derive the coefficients for blackbody emissivity and nonlinearity. We then compare new coefficients (Version 7k) with the latest operational version of the AIRS radiometric calibration coefficients (Version 5). The AIRS Version 5 coefficients were sufficiently adequate that an update has never been made since AIRS launch in 2002. However, it can be seen, when we compare to the Cross-track Infrared Sounder (CrIS), that better agreement is made in Version 7. The impact of the new coefficients is highest at cold scene temperatures and very warm temperatures.
By the end of 2013 NASA and EUMETSAT will have accumulated more than 11 years of AIRS, 6 years of IASI and one year of CrIS data. All three instruments were nominally specified to support the NWC for short term weather forecasting with a five year lifetime, but continue to exceed the accuracy requirement needed for weather forecasting alone. This allows use of their data for a much broader range of applications, including the calibration of broad-band instruments in space and climate research. We illustrate calibration aspects with examples from AIRS, IASI and CrIS using spatially uniform clear conditions, simultaneous nadir overpasses and random nadir samples. The differences between AIRS, IASI and CrIS for the purpose of weather forecasting are small and we expect that the excellent forecast impact demonstrated by the combination of AIRS and IASI will be continued by the combination of CrIS and IASI. Clear data are useful for calibration, but contain no climate signal. The analysis of random nadir samples from AIRS and CrIS identifies larger biases for observation of extreme conditions, represented by 1% and 99%tile data than for non-extreme observations. This is relevant for climate analysis. Resolution of these differences require further work, since they can complicate the continuation of trends established by AIRS with CrIS data, at least for extrema. The unequaled stability of the AIRS data allows us to evaluate trends using random nadir sampled data. We see an increasing frequency in severe storms over land, a decreasing frequency over ocean. The 11 years of AIRS data are too short to tell if these trends are significant from a climate change viewpoint, or if they are parts of multi-decadal oscillations.
KEYWORDS: Infrared radiation, Sensors, Calibration, Spectroscopy, Mirrors, Infrared spectroscopy, Signal to noise ratio, Space operations, Black bodies, Staring arrays
The Atmospheric Infrared Sounder (AIRS) is a hyperspectral infrared instrument on the EOS Aqua Spacecraft, launched
on May 4, 2002. AIRS has 2378 infrared channels ranging from 3.7 μm to 15.4 μm and a 13.5 km footprint at nadir.
The AIRS is a “facility” instrument developed by NASA as an experimental demonstration of advanced technology for
remote sensing and the benefits of high resolution infrared spectra to science investigations. AIRS, in conjunction with
the Advanced Microwave Sounding Unit (AMSU), produces temperature profiles with 1K/km accuracy on a global
scale, as well as water vapor profiles and trace gas amounts for CO2, CO, SO2, O3 and CH4. AIRS data are used for
weather forecasting, climate process studies and validating climate models. The AIRS instrument has far exceeded its
required design life of 5 years, with over 10 years of operations as of September 2012. While the instrument has
performed exceptionally well, with little signs of wear, the AIRS Project continues to monitor and maintain the health of
AIRS, characterize its behavior and improve performance where possible. Radiometric stability has been monitored and
trending shows better than 16 mK/year stability. Spectral calibration stability is better than 1 ppm/year, and a new gain
table was recently uploaded to recover 100 significantly degraded or dead channels by switching to their redundant
counterpart. At this time we expect the AIRS to continue to perform well for the next decade.
It has been widely accepted that an infrared sounder in low polar orbit is capable of producing climate quality data, if the spectral brightness temperatures have instrumental trends of less than 10 mK/yr. Achieving measurement stability at this level is not only very demanding of the design of the instrument, it is also pushes the state of art of measuring on orbit what stability is actually achieved. We discuss this using Atmospheric Infrared Sounder (AIRS) L1B data collected between 2002 and 2011. We compare the L1B brightness temperature observed in cloud filtered night tropical ocean spectra (obs) to the brightness temperature calculated based on the known surface emissivity, temperature and water vapor profiles from the ECMWF ReAnalysis (ERA) and the growth rates of CO2 , N2O and Ozone. The trend in (obscalc) is a powerful tool for the evaluation of the stability of the 2378 AIRS channels. We divided the channels into seven classes: All channels which sound in the stratosphere (at pressure levels below 150 hPa), 14 um CO2 sounding, 4 um CO2 P-branch sounding, 4um CO2 R-branch sounding, water vapor sounding, shortwave surface sounding and longwave surface sounding. The peak in the weighting function at 1050 hPa separates sounding and surface channels. The boundary between shortwave and longwave is 5 μm. Except for the stratosphere sounding channels, the remaining six groups have (obs-calc) trends of less than 20 mK/yr. The longwave surface channels have trends of 2 mK/yr, significantly less than the 8 mK/yr trend seem in the shortwave window channels. Based on the design of the instrument, trends within a group of channels should be the same. While the longwave and shortwave trends are less than the canonical 10 mK/yr, the larger trend in the shortwave channels could be an artifact of using the pre-launch determined calibration coefficients. This is currently under evaluation. The trend in (obs-calc) for the non-surface sounding channels, in particular for stratosphere sounding and upper tropospheric water channels, is dominated by artifacts created in calc, most likely due to changes in the ERA Ozone and water vapor. Based on this argument the best estimate of the trend for the channels within a channel group is given by the surface sensitive channels within the group. Based on this consideration we estimated the trend of all AIRS longwave channels as 2 mK/yr, while the shortwave channels have a trend of 8 mK/yr.
The ultraspectral infrared radiances obtained from satellite observations provide atmospheric, surface, and/or cloud
information. The intent of the measurement of the thermodynamic state is the initialization of weather and climate
models. Great effort has been given to retrieving and validating these atmospheric, surface, and/or cloud properties. Error
Consistency Analysis Scheme (ECAS), through fast radiative transfer model (RTM) forward and inverse calculations,
has been developed to estimate the error budget in terms of absolute and standard deviation of differences in both
spectral radiance and retrieved geophysical parameter domains. The retrieval error is assessed through ECAS without
assistance of other independent measurements such as radiosonde data. ECAS re-evaluates instrument random noise, and
establishes the link between radiometric accuracy and retrieved geophysical parameter accuracy. ECAS can be applied to
measurements of any ultraspectral instrument and any retrieval scheme with associated RTM. In this paper, ECAS is
described and demonstration is made with the measurements of the METOP-A satellite Infrared Atmospheric Sounding
Interferometer (IASI).
Infrared sounders, such as the Atmospheric Infrared Sounder (AIRS), the Infrared Atmospheric Sounding Interferometer
(IASI), and the Cross-track Infrared sounder (CrIS), have a cloud-impenetrable disadvantage in observing the
atmosphere and surface under opaque cloudy conditions. However, recent studies indicate that hyperspectral, infrared
sounders have the ability to detect cloud effective-optical and microphysical properties and to penetrate optically thin
clouds in observing the atmosphere and surface to a certain degree. We have developed a retrieval scheme dealing with
atmospheric conditions with cloud presence. This scheme can be used to analyze the retrieval accuracy of atmospheric
and surface parameters under clear and cloudy conditions. In this paper, we present the surface emissivity results
derived from IASI global measurements under both clear and cloudy conditions. The accuracy of surface emissivity
derived under cloudy conditions is statistically estimated in comparison with those derived under clear sky conditions.
The retrieval error caused by the clouds is shown as a function of cloud optical depth, which helps us to understand how
well infrared sounders can observe the atmosphere and surface through clouds.
The radiometric and spectral system performance of space-borne infrared radiometers is generally specified
and analyzed under strictly cloud-free, spatially uniform and warm conditions, with the assumption that the
observed performance applies to the full dynamic range under clear and cloudy conditions and that random
noise cancels for the evaluation of the radiometric accuracy. Such clear conditions are found in only one
percent of the data. Ninety nine percent of the data include clouds, which produce spatially highly nonuniform
scenes with 11μm window brightness temperatures as low as 200K. We use AIRS and IASI
radiance spectra to compare system performance under clear and a wide range of cloudy conditions.
Although the two instruments are in polar orbits, with the ascending nodes separated by four hours, daily
averages already reveal surprisingly similar measurements. The AIRS and IASI radiometric performance
based on the mean of large numbers of observation is comparable and agrees within 200 mK over a wide
range of temperatures. There are also some unexpected differences at the 200 -500 mK level, which are of
significance for climate applications. The results were verified with data from July 2007 through January
2010, but many can already be gleaned from the analysis of a single day of data.
It is widely accepted that the knowledge of the frequencies of the spectral response functions (SRF) of the channels of
hyperspectral sounders at the 10 parts per million (ppm) of frequency level is adequate for the retrieval of temperature
and moisture profiles and data assimilation for weather forecasting. However, SI traceability and knowledge at the 1 ppm
level and better are required to separate artifacts in the knowledge of the SRF due to orbital and seasonal instrument
effects from diurnal and seasonal effects due to climate change. We use examples from AIRS to discuss a spectral
calibration that uses the SI traceable upwelling radiance spectra to achieve an absolute accuracy of 0.5 ppm.
The radiometric intercomparison of instruments is a key element in developing climate-quality data records. In this study
we compare data from the first two years of the Infrared Atmospheric Sounding Interferometer (IASI) with the matching
data from the Atmospheric Infrared Sounder (AIRS). We compare observed spectra in cloud-free areas of the tropical
oceans at night to spectra calculated using data from the European Centre for Medium-Range Weather Forecasts
(ECMWF). We use five frequencies-three window channels, one mid-tropospheric sounding channel, and one lower
stratospheric sounding channel. The use of ECMWF data as a transfer standard permits comparisons of many more
points distributed more widely over the globe than is possible with the traditional simultaneous nadir overpass (SNO)
technique. The analysis shows that AIRS and IASI daily mean brightness temperatures track each other within 100 mK,
in spite of the fact that the instruments are in different orbits. AIRS was launched into polar orbit on the EOS Aqua
spacecraft on May 4, 2002. It is a grating spectrometer with 2378 channels in the range 3.7 to 15.4 microns. IASI was
launched into polar orbit in October 2006 on the METOP-A spacecraft. IASI is a Fourier transform spectrometer
covering 3.7 to 15.5 microns in three bands with a total of 8461 channels.
Atmospheric thermodynamic parameters and surface properties are basic meteorological variables for weather
forecasting. A physical geophysical parameter retrieval scheme dealing with cloudy and cloud-free radiances observed
with satellite ultraspectral infrared sounders has been developed and applied to data from the Infrared Atmospheric
Sounding Interferometer (IASI) and the Atmospheric InfraRed Sounder (AIRS). The retrieved parameters presented
herein are from radiance data gathered during the Joint Airborne IASI Validation Experiment (JAIVEx). JAIVEx
provided intensive aircraft observations obtained from airborne Fourier Transform Spectrometer (FTS) systems, in-situ
measurements, and dedicated dropsonde and radiosonde measurements for the validation of the IASI products. Here,
IASI atmospheric profile retrievals are compared with those obtained from dedicated dropsondes, radiosondes, and the
airborne FTS system. The IASI examples presented here demonstrate the ability to retrieve fine-scale horizontal features
with high vertical resolution from satellite ultraspectral sounder radiance spectra.
The Atmospheric Infrared Sounder (AIRS), launched on the EOS Aqua spacecraft on May 4, 2002, has been in
routine operations since late August 2002. In this paper we analyze the first four years of AIRS Level 1B data
(calibrated radiances) from September 1, 2002 through August 31, 2006 for stability and accuracy of the radiometric
and spectral calibration. Both are key to linking the AIRS record to previous and future instruments. The analysis
shows that the AIRS absolute radiometric accuracy is better than 200 mK with a stability of better than 10 mK/year.
Both upper limits are due to the difficulty of finding ground truth data of sufficient quality. The instantaneous
knowledge of the channel frequencies is better than 1 ppmf. Compared to the nominal frequency set adopted in
September 2002, AIRS channel frequencies are slowly shifting to higher frequencies at the rate of about 1
ppmf/year. The term "ppmf" means "parts per million in frequency". For all but the most critical climate
applications, using the nominal frequency set is sufficient for the radiative transfer code used in atmospheric
parameter retrievals.
The Atmospheric Infrared Sounder (AIRS) flying on NASA's EOS-AQUA platform has channels sensitive to both sulfur
dioxide (SO2) and nitric acid (HNO3). We have developed a simple regression retrieval for both of these gases that
illustrates the potential for AIRS, and other hyperspectral sounders, to retrieve these two gases. We have cross-validated
the SO2 retrievals to those from the OMI instrument flying on the EOS AURA platform. Similarly, we have cross-validated
the HNO3 retrievals with limb retrievals of HNO3 by from MLS instrument, also flying on the AURA platform.
The AIRS instrument was launched in May 2002 into a polar sun-synchronous orbit onboard the EOS Aqua Spacecraft. Since then we have released three versions of the AIRS data product to the scientific community. AIRS, in conjunction with the Advanced Microwave Sounding Unit (AMSU), produces temperature profiles with 1K/km accuracy on a global scale, as well as water vapor profiles and trace gas amounts. The first version of software, Version 2.0 was available to scientists shortly after launch with Version 3.0 released to the public in June 2003. Like all AIRS product releases, all products are accessible to the public in order to have the best user feedback on issues that appear in the data. Fortunately the products have had exceptional accuracy and stability. This paper presents the improvement between AIRS Version 4.0 and Version 5.0 products and shows examples of the new products available in Version 5.0.
The Atmospheric Infrared Sounder (AIRS) sounding suite, launched in 2002, is the most advanced atmospheric
sounding system to date, with measurement accuracies far surpassing those of current operational weather satellites.
From its sun-synchronous polar orbit, the AIRS system provides more than 300,000 all-weather soundings covering
more than 90% of the globe every 24 hours. Usage of AIRS data products, available to all through the archive system
operated by NASA, is spreading throughout the atmospheric and climate research community. An ongoing validation
effort has confirmed that the system is very accurate and stable and is close to meeting the goal of providing global
temperature soundings with an accuracy of 1 K per 1-km layer and water vapor soundings with an accuracy of 20%
throughout the troposphere, surpassing the accuracy of radiosondes. This unprecedented data set is currently used for
operational weather prediction in a number of countries, yielding significant positive impact on forecast accuracy and
range. It is also enabling more detailed investigations of current issues in atmospheric and climate research. In addition
to the basic soundings related to the hydrologic cycle, AIRS also measures a number of trace gases, the latest such
product being the global distribution of carbon dioxide. We discuss some examples of recent research with AIRS data.
The Atmospheric Infrared Sounder (AIRS) has been operating since Sept. 2002 and is being used operationally by several weather centers. Routine retrieval processing is done by NASA. Daytime AIRS measurements in the 4.3um region show large (upto 12 K) brightness temperature shifts compared to nighttime observations. The daytime shifts result from the preferential absorption of solar radiance in the upper atmosphere by CO2 and other molecules. This energy is transferred to many of the 4 um CO2 bands, driving them into a state of Non-Local Thermodynamic Equilibrium (NLTE). We present comparisons of observations against the results of a Fast Model we developed for the AIRS instrument, that includes this effect. This algorithm is fast enough to be used for retrievals, and will be especially useful for the 4.3um R branch head channels that are used for temperature sounding.
We present an intercomparison of retrieved dust parameters obtained from
analyzing AIRS and MODIS satellite data. Recent papers have highlighted
using AIRS data to retrieve dust top (layer) height, loading and particle size.
Different methods have been used, such as assuming a fixed particle size
and dust top height before fitting radiance
data from selected AIRS channels, or using lookup tables to retrieve dust
loading, height and particle size. In this paper we use the combination
of dust retrievals from MODIS visible and AIRS thermal infrared channels
to provide information on dust top height by forcing the error term (or
intercept of the linear regression of dust optical depths retrieved from
MODIS and AIRS) to zero. When available, GLAS measurements will be used to
validate dust top height. Collocated ship based M-AERI observations, obtained in March 2004 during the AEROSE campaign will also be analyzed to verify this
approach.
Precise measurements of CH4 in a column of near surface air, and in partial columns above this, would be very valuable in identifying sources/sinks of atmospheric CH4, and its transport. For this purpose we have proposed a grating mapping spectrometer (GMS) for deployment as an Instrument of Opportunity (IOO) on the NPOESS that acquires data in the 2990 to 3050 cm-1 spectral region. It will provide measurements of CH4 absorption of sunlight in the weaker CH4 features in the region, and of thermal emission in the stronger CH4 features in the region. It is the combination of the two that provides the vertical information. The IOO will acquire spectra on a crosstrack swath centered on nadir, and with 1/2 width of 55 degrees on each side of nadir (about 2800 km full width swath on the ground for a nominal 828 km satellite altitude). This with footprints that are about 3.1 km on a side at nadir. The small footprint facilitates cloud screening, and identification of CH4 source hotspots. A capability to project the slit to nadir along the direction from satellite to sun will be utilized for over the ocean viewing in order to facilitate measurements in solar glitter. It will obtain spectra with resolution n < 0.58 cm-1 and sample spacing < 0.17 cm-1. Based on the spectral characteristics and currently achievable very low-noise we do a linear error analysis (Rodgers, [1]) for the simultaneous retrieval of multi-column CH4, humidity, and surface parameters and 13CH4 total column. We show that useful multi-column CH4 retrievals can be obtained, with good near surface sensitivity in sunlit conditions. We also show the 13CH4 column can be retrieved with precision better than 3%. Retrieval of 13CH4 column in the earth's atmosphere is analogous in difficulty to retrieval of the major CH4 isotope column in the Martian atmosphere by a similar GMS deployed on a Mars orbiter. We show that H2O vertical information can be retrieved from these measurements and discuss the potential for ethane column retrieval.
Measurements of the column CH4, CO and CO2 are high priorities of the NPOESS Pre-Planned Product Improvement (P3I) data sets. Risk reduction for existing NPOESS instruments, including mitigation of daytime CO2 SWIR non-LTE effects, is also a high priority. We have proposed an NPOESS Instrument Of Opportunity (IOO) to address these priorities. It consists of two grating mapping spectrometers (GMSs). One that would acquire measurements with high spectral resolution Δv < 0.13 cm-1 of CH4, CO and H2O absorption lines in reflected sunlight in the VSWIR region 4281 to 4301 cm-1, and another for measurements with Δv < 0.30 cm-1 in the SWIR region 2355 to 2430 cm-1. The IOO will acquire spectra on a crosstrack swath from nadir to 55 degrees (about 1400 km on the ground) on footprints that are about 1.55 and 3.1 km on a side at nadir for the two GMS, respectively. The small footprint facilitates cloud screening, and identification of pollution hotspots. We use linear error analysis (LEA, based on the Rodgers [1] paper) to estimate the proposed IOO's performance. The LEA indicates that the IOO should be able to provide CH4 and CO column retrieval over sunlit land (and from ocean glitter when it is viewed) that satisfies or exceeds NPOESS P3I Environmental Data Records (EDRs) requirements in all aspects except refresh where the IOO would provide every two days vs the once per day requirement. Further, it shows the VSWIR IOO data when used in combination with the NPOESS Cross Track Infrared Sounder (CrIS) [2] data should provide: (a) CO profile data with sensitivity to CO in near surface air that is enhanced compared to that in the current TERRA-MOPITT, ACQUA-AIRS and AURA-TES data sets because these are limited to thermal infrared measurements that lack sensitivity to CO in near surface air layer where there is little contrast between the air temperature and the ground surface temperature, (b) CH4 profile with sensitivity in the near surface air layer that is crucial for identifying CH4 sources/sinks (c) and significant improvement in the CrIS retrieved humidity in the near surface layer of air. We show the SWIR IOO data can be used for CO2 column retrieval with near surface air layer sensitivity in the daytime. And also that in combination with CrIS SWIR data facilitates CO2 SWIR non-LTE mitigation that is required for advanced sounding quality temperature profile (TP) retrieval from CO2 SWIR data in daytime conditions. This provides risk reduction in case of degradation in the CrIS LWIR region data.
Understanding upper troposphere humidity is important in the context of radiative forcing and climate. We present a detailed statistic comparison of upper troposphere water vapor retrieval profiles derived from the Atmospheric Infrared Sounder (AIRS) and in-situ measurements. The in-situ measurements are based on a recently compiled database of "best estimate" atmospheric state profiles, obtained from a careful selection of RS-90 radiosondes at Department of Energy Atmospheric Radiation Measurement (ARM) sites, during AIRS overpasses. The aim of this research is to improve the skill and accuracy of the retrieval algorithms in order to understand and quantify the biases between AIRS and RS-90 radiosondes.
The Earth Science and Meteorological communities are taking great interest in a new instrument released by NASA. The Atmospheric Infrared Sounder (AIRS), launched on the EOS Aqua Spacecraft on May 4, 2002, is a high spectral resolution infrared imaging spectrometer with over 2300 distinct infrared wavelengths ranging from 3.7 μm to 15.4 μm. AIRS is unique in that it provides the highest infrared spectral resolution to date while also providing coverage of over 95% of the Earth's surface every day at 15 km spatial resolution. The AIRS project is currently managed by NASA's Jet Propulsion Laboratory in Pasadena, California1. The AIRS is providing a wealth of scientific data to the Earth Science community including upper atmospheric water vapor and atmospheric composition on key greenhouse gases. It is also improving weather forecasting and the studies of processes affecting climate and weather.
The Atmospheric Infrared Sounder (AIRS) has been operating since Sept. 2002 and AIRS radiance data is being used operationally by several weather centers. A significant fraction of AIRS observations are contaminated by dust blowoff from arid areas. The almost continous spectral coverage of AIRS in both the 10-12 and 3.7 micron atmospheric windows allows excellent detection of the presence of dust. Dust signals can often survive the cloud-clearing process used in the retrieval system for AIRS, thereby contaminating the low-altitude temperature and water vapor retrieval products. We present techniques being developed to both operationally detect and mitigate the effects of dust on AIRS retrieval products.
Atmospheric Infrared Sounder (AIRS) measurements of the sea surface temperature at 2616cm-1 show an unexpected cold bias compared to the NCEP Global Real Time SST. This cold bias, which is distributed in large scale patterns which are stable on the time scale of several months, has a satellite zenith angle (sza) dependent and a sza independent component. The sza independent component is strongly temperature dependent, and is most likely related to a larger than expected gradient between the bulk and the skin temperature, possibly due to not understood regional or seasonal weather patterns. The sza dependent component of about 0.4K is most likely due to some form of aerosol which is not included in the radiative transfer. AIRS was launched 4 May 2002 on the EOS Aqua into polar orbit.
The Atmospheric Infrared Sounder (AIRS), launched in May 2002, is the first of a new generation of high-spectral resolution temperature and humidity sounders for numerical weather prediction and climate change studies. The accuracy of the AIRS radiances, and the validity of the clear sky AIRS Forward Model, have already been demonstrated. With daily global coverage by the instrument, the almost continuous wavelength coverage in the 10-12 micron and 3.7 micron atmospheric windows enables AIRS to excel at detecting cirrus clouds. This paper presents global retrievals of cloud top pressure, ice particle size and amount, using the AIRS radiances. Cloud optical depths lower than 0.1 (at 10 μm) have been detected. We also present comparisons between retrievals done using spherical particles to those done using nonspherical particles.
The Atmospheric Infrared Sounder (AIRS), launched in May 2002, is the first of a new generation of high-spectral resolution temperature and humidity sounders for numerical weather prediction and climate change studies. The accuracy of the AIRS radiances, and the validity of the clear sky AIRS Forward Model, have already been demonstrated. This paper presents global daytime measurements by the instrument, showing the effects of NLTE in the 4.3 μm CO2 band. Simulations using upper atmosphere NLTE temperatures are compared to actual AIRS measurements, for various solar angles.
The Atmospheric Infrared Sounder (AIRS), Advanced Microwave Sounding Unit (AMSU), and Humidity Sounder from Brazil (HSB) are three instruments onboard the Earth Observing System (EOS) Aqua Spacecraft. Together, they form the Aqua Infrared and Microwave Sounding Suite (AIMSS). This paper discusses the science objectives and the status of the instruments and their data products one year after launch. All instruments went through a successful activation and calibration and have produced exceptional, calibrated, Level 1B data products. The Level 1B Product Generation Executables (PGEs) have been given to NOAA and the GSFC DAAC for production and distribution of data products. After nine months of operations, the HSB instrument experienced an electrical failure of the scanner. Despite the loss of HSB, early validation results have shown the AIRS and AMSU are producing very good temperature profiles.
JPL is currently managing the instrument operations, calibration and data system for the Atmospheric Infrared Sounder (AIRS) on the EOS Aqua spacecraft. Aqua was launched on May 4, 2002 from Vandenberg Air Force Base. AIRS has 2378 infrared channels with high spectral resolution (1200) covering the 3.7 to 15.4 micron wavelength range. AIRS data are used to produce temperature and humidity profiles useful in predicting weather and monitoring climate. We discuss lessons learned on AIRS in the development and operations as well as plans for next generation systems including SIRAS, a wide field hyperspectral infrared imaging spectrometer which offers AIRS spectral performance at 24x the spatial resolution.
The Atmospheric Infrared Sounder (AIRS) was launched in early May 2002. This new high-spectral resolution sounder is the first of a new generation of temperature and humidity sounders for numerical weather prediction and climate change studies. In addition, AIRS should be able to detect several minor gases, including ozone, carbon monoxide, methane and carbon dioxide. This paper presents a preliminary comparison between observed AIRS spectra and spectra computed from the ECMWF (European Center for Medium Range Forecasting) model fields. A key component of this comparison is the selection of clear fields of view, which we limited to night views over ocean, allowing the use of the relatively well known sea surface emissivity.
The Atmospheric Infrared Sounder (AIRS) was launched in early May 2002. The temperature and humidity fields retrieved from this new high-spectral resolution sounder will be used for numerical weather prediction and climate change studies. This paper presents a comparisons between observed AIRS spectra and spectra computed from profiles using the worldwide radiosonde network, as well as spectra computed from the ECMWF (European Center for Medium Range Weather Forecasts) model fields.
A combined active-passive remote sensing system has been developed to study atmospheric radiation and cirrus cloud radiative properties at the NOAA Mauna Loa Observatory on the island of Hawaii. The active portion of this system is an eye-safe, dual-polarization lidar, while the passive portion is a Fourier transform spectro-radiometer operating in emission mode. The combined system allows unattended, remote measurements of infrared atmospheric emission and clouds with depolarization discrimination of ice and liquid.
Spectral characterization of the Atmospheric Infrared Sounder (AIRS) instrument during ground Thermal/Vacuum tests posed a number of difficult challenges due to the high spectral resolution and accurate knowledge requirements. A Fourier transform spectrometer was used in external step-scan mode to characterize the spectral response functions (SRFs) of the 2378 infrared detectors in the focal plane array which is part of the AIRS grating spectrometer. This paper summarizes the test development and characterization results. Special post-test data analysis was needed separately to determine the effects of interference in the order-separating entrance filters, which have a different temperature dependence from that of the otherwise unperturbed SRFs. This separation, which was successfully accomplished, provides calibration of the AIRS SRF shape over the full expected range of instrument temperatures.
The Atmospheric Infrared Sounder represents a quantum leap in spaceborne sounding instruments with 2,378 infrared spectral channels ranging in wavelength from 3.5to 15.5 microns. AIRS was built by NASA subcontractor Lockheed Martin Sanders (LM Sanders) in Lexington, Massachusetts and is scheduled for launch on the NASA EOS-Aqua spacecraft in December 2000. Characterization of this high spectral resolution infrared spectrometer involved extensive laboratory testing in a thermal vacuum environment at cold optical temperatures. This paper summarizes the results of that testing and gives a detailed report on the spectral, radiometric, and spatial performance of the AIRS. Based on the excellent prelaunch calibration and results of data simulation, AIRS data should significantly improve global weather forecasts and provide an important new tool for climate research.
AIRS, on the EOS-Aqua, produces global high precision spectra form 3.7-15.4 micron with spectral resolving power (mu) /(Delta) (Mu) equals1200 twice each day form 708 km orbital altitude. AIRS is the first hyperspectral IR spectrometer designed to support NOAA/NCEPs the operational requirements for medium range weather forecasting during its nominal 7 year lifetime. AIRS, together with the AMSU and HSB microwave radiometers, will achieve global retrieval accuracy of better than 1K rms in the lower troposphere under clear and partly cloudy condition. Based on the excellent radiometric and spectral performance demonstrated during the pre-launch testing, the assimilation of AIRS data into the forecast model is expected to result in a major forecast improvements. Launch of AIRS on the EOS AQUA is scheduled for May 2001.
New high-spectral resolution satellite sounders will use channels located between CO2 lines for temperature retrievals. Transmittances for these channels are dominated by spectral line wings that are strongly influenced by line-mixing and duration-of-collision effects. Previous studies demonstrated the importance of Q-branch line mixing for atmospheric sounding in the 15 micrometer region. This work presents an improved model of P/R-branch line mixing and duration-of- collision effects on CO2 transmittances in the 4.3 micrometer and 15 micrometer regions, based on laboratory and spectroscopy data. Most line-by-line codes model non- Lorentzian behavior by using the Cousin chi-function. This empirical function incorporates both P/R line-mixing and duration-of-collision effects by using many parameters. It is common to use the Cousin model parameters obtained from the 4 micrometer band in the 15 micrometer region, overestimating the amount of line-mixing. Comparisons to radiance data taken with high resolution interferometers that fly on NASA's ER-2 partially validates our model. The biggest improvements are at 4.3 micrometer where the differences are reduced by more than 2K, compared to using the Cousin model.
A new monochromatic radiative transfer algorithm based on compressed lookup tables of pre-computed atmospheric molecular absorption coefficients has been developed. These compressed look-up tables are called the kCompressed Database. Our motivation is to compute monochromatic absorption coefficients for any realistic Earth atmospheric situation (pressure, temperature, gas amount) at the same accuracy as a line-by- line code, but faster. In addition, the procedure for producing atmospheric transmittances is extremely simple, and easy to code. Although the kCompressed Database was originally developed to compute layer-to-space transmittances that are needed to produce fast transmittance models for high spectral resolution infrared temperature and humidity sounders, we have now developed a complete (non-scattering) atmospheric radiative transfer code around the kCompressed Database, called kCARTA (for kCompressed Atmospheric Radiative Transfer Algorithm). In addition, Jacobians with respect to gas amount and temperature can be rapidly performed, providing the user insight to the regions to which the measured radiance is most sensitive.
One goal of the atmospheric infrared sounder (AIRS), scheduled to fly on the EOS-PM1 satellite in 2000, is the global measurement of the atmospheric abundance of carbon monoxide (CO). ALthough it is primarily a temperature and humidity sounder for EOS, AIRS can resolve individual CO lines in a portion of the 1-0 vibration-rotation band of CO between 2170 and 2200 cm-1, but with significant noise. Taking advantage of the almost regular spacing of these lines, we have developed an algorithm to retrieve the column density of CO from AIRS spectra using standard signal processing techniques for noise reduction. Detailed simulations indicate the capability to retrieve total column densities of CO to an accuracy of approximately 10 percent. Validation of our CO retrieval algorithm has been accomplished using a combination of in situ CO profiles acquired by an instrumented Cessna and nearly coincident infrared spectra obtained by the University of Wisconsin Madison's High-resolution Interferometer Sounder (HIS) flying onboard a NASA ER-2 during the Second Convection and Moisture Experiment in August, 1995. Excellent agreement was obtained between the retrieved CO abundance, approximately 90 ppbv, and the in situ profile above the boundary layer, approximately 80-100 ppbv. Additional HIS spectra obtained near Long Island, NY show enhanced CO levels, 1400-4300 ppb if confined to portions of the boundary layer, in the smoke plume downwind from forest fires near Westhampton, NY on August 25, 1995.
We have developed a new information-content based look-up table technique for the fast computation of near- monochromatic atmospheric transmittances in the infrared that is well suited for nadir viewing satellite and airplane observations. It allow a user to quickly compute near- monochromatic radiances using a very simple algorithm that is easily ported to many machine architectures. Radiative transfer based on look-up tables of monochromatic absorption coefficients could speed calculations, but they are impractical due to their large size and the need to interpolate long wavenumber vectors in temperature and pressure. We use a singular value decomposition to transform monochromatic look-up tables of absorption coefficients into a compressed representation that is almost 100 times smaller. Moreover, temperature and pressure interpolations can be performed in this compressed representation, resulting in significant savings in computation times and computer I/O. We start with the line-by-line computation of a set of tables of absorption coefficients for each relevant gas. Each 25 wavenumber table has 10,000 wavenumber points and 1,100 temperature/pressure layers. For water vapor we add an extra dimension to these tables that spans 5 water vapor profiles to provide variability in the self-broadening of water vapor spectral lines. On average we need 37 basis vectors for water, 12 for carbon dioxide, and 6 for each of the other required gases in order to reproduce the absorption coefficient tables to an accuracy equivalent to a nadir-viewing monochromatic brightness temperature error of 0.1K.
The next generation of atmospheric temperature and humidity sounders will have thousands of radiometrically accurate spectral channels throughout the infrared. The retrieval of atmospheric parameters from these radiances will stress both the accuracy and efficiency of forward model radiative transfer algorithms. We are developing a forward model for the Atmospheric Infrared Sounder (AIRS) which will fly on the EOS PM platform. The work presented here is based on algorithms developed over a number of years by McMillin, Fleming, and others for low resolution infrared sounders (HIRS) and microwave sounders. We have developed tow 'high resolution' AIRS forward model algorithms for water vapor, one based on atmospheric layers with fixed pressures and variable water amounts, and other based on layers of fixed absorber amount but with variable pressures. These algorithms are compared for speed, accuracy, ease of development, and other factors that must be considered in developing a complex operational retrieval system.
DIAL lidar for water-vapor and temperature remote sensing in the eye-safe two micron region has been receiving much interest. Such systems rely on accurate spectral line characterization. Typically spectral line properties are taken from the HITRAN database. A series of transmittance measurements are made to complete and improve the HITRAN database in this important spectral region. A 3-meter base-path White cell attached to a BOMEM DA3.02 is used for the transmittance measurements. The White cell is set to a path length of 216 meters for all experiments. Measurements of pure water-vapor, nitrogen broadened water-vapor, pure carbon dioxide, and nitrogen broadened carbon dioxide are collected at room temperature. Data analysis is performed on water-vapor lines that are relatively temperature insensitive and carbon dioxide lines that are relatively temperature sensitive over the range of typical atmospheric temperatures. The measured spectrum is converted to the absorption coefficient and is nonlinear least squares fitted to determine the spectral line parameters. Some lines show good agreement (within a few percent) with the HITRAN database, other lines disagree by more than +/- 10%. Thus care must be exercised in applying the HITRAN database to DIAL lidar applications. New parameters are obtained that are not available on the current 1992 HITRAN database, such as the self width and pressure shift.
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