CGMS-41 NASA-WP-06
11 June 2013
Prepared by NASA
Agenda Item: WGII/4
Discussed in Working Group II
ATMOSPHERIC SOUNDING PRODUCT DEVELOPMENT AND CAL/VAL
ACTIVITIES AT NASA USING AIRS/AMSU ON AQUA, CRIMSS ON SNPP, AND
NAST-I ON NASA HIGH-ALTITUDE AIRCRAFT PLATFORMS
T. Pagano, E. Fetzer, B. Lambrigtsen, J. Teixeira, Steve Friedman
NASA Jet Propulsion Laboratory, Pasadena, CA
A. Larar, Daniel Zhou, Xu Liu
NASA Langley Research Center, Hampton, VA
Joel Susskind
NASA Goddard Spaceflight Center, Greenbelt, MD
Executive summary
Atmospheric sounding at NASA is a broad based effort to measure, characterize and
understand the Earth’s atmosphere from ground, airborne and spaceborne platforms. This
paper covers recent activities at NASA’s JPL, GSFC and LaRC centers in support of the
AIRS, CrIS, IASI and NAST-I instruments. The AIRS and AMSU instruments on the EOS
Aqua Spacecraft continue to operate well, however loss of channels has been seen. In AIRS,
the impact is minimal, and over 50 channels were recovered using redundant detectors.
AMSU loss of channel 5 severely impacted the AIRS/AMSU Level 2 Version 5 products, but
the Version 6 mitigates this and other problems and was released in February of 2013 at the
GES/DISC. The NASA teams have demonstrated Level 2 retrievals from the CrIS/ATMS
(CrIMSS) instruments on the Suomi NPP satellite and has concluded the instrument is of high
value to the scientific community. Comparisons of AIRS, CrIS and IASI have shown excellent
radiometric agreement under normal conditions but differences do exist that are being
carefully examined. The NAST-I aircraft sounder continues to provide excellent validation of
the spaceborne sounders. New retrieval methodologies are being developed and utilized to
improve computational speed, accuracy and error estimation. These new retrievals run on all
three IR sounder instruments and will form the basis for a next generation retrieval for Version
7. Scientific interest in the sounding instruments remains high with over 100 peer reviewed
publications released in 2012 using AIRS data and numerous more with the other sounder
instruments.
CGMS-41 NASA-WP-06
11 June 2013
Atmospheric Sounding Product Development and Cal/Val Activities at NASA
using AIRS/AMSU on Aqua, CrIMSS on SNPP, and NAST-I on NASA High-altitude
Aircraft Platforms
1
INTRODUCTION
The Atmospheric Infrared Sounder (AIRS) and companion instrument, the Advanced
Microwave Sounding Unit (AMSU) were launched on May 4, 2002 on the NASA Earth
Observing System Aqua spacecraft and managed by NASA JPL. They are facility
instruments designed to support measurements of atmospheric temperature, water
vapor and a wide range of atmospheric constituents in support of weather forecasting
and scientific research in climate and atmospheric chemistry. The AIRS measures the
hyperspectral infrared spectrum of the atmosphere in 2378 channels from 3.7-15.4
microns with 13.5 km spatial resolution and nearly twice global daily coverage. The
AMSU is a microwave radiometer with 15 channels ranging from 23.8 GHz to 89 GHz.
National Weather Prediction (NWP) centers worldwide assimilate the AIRS and AMSU
data with these instruments providing among the highest forecast improvement of any
sensors. The spectra are also used to retrieve cloud cleared radiances, atmospheric
temperature and water vapor profiles, and trace gas constituents with high accuracy
and product resolutions approaching 50km per retrieval. The Cross-track Infrared
Sounder (CrIS) on the Suomi National Polar-orbiting Partnership (NPP) has similar
spectral range with 1305 spectral channels and similar spatial resolution as AIRS. The
Advanced Technology Microwave Sounder (ATMS) also on Suomi NPP has 22
channels ranging from 23 GHz to 183 GHz and spatial resolution ranging from 15.8 to
75 km. In addition to the CNES Infrared Atmospheric Sounding Interferometer (IASI) on
the European METOP spacecraft, these instruments form the basis for the satellite
component of atmospheric sounding at NASA.
In addition to the satellite instruments, the airborne NAST-I is the NASA / National
Polar-orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounder
Testbed-Interferometer which LaRC maintains and deploys internationally, in support of
satellite measurement system (sensor, algorithm, direct and derived data product)
calibration / validation (cal/val) activities and NASA Earth system science advancement.
NAST-I serves as an ideal validation sensor since it measures the same level-1 quantity
as many sensors it helps to validate (i.e. infrared spectral radiance), and does so at
higher spectral and spatial resolutions.
This working paper is organized with a report from the AIRS Project at NASA JPL and
GSFC, followed by a report from the Atmospheric Sounding team at NASA LaRC.
Finally, science highlights from the peer reviewed literature that involved use of AIRS
data is highlighted at the end of this document. Further information can be found at
http://airs.jpl.nasa.gov.
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2
NASA ATMOSPHERIC INFRARED SOUNDER (AIRS) PROJECT (JPL/GSFC)
The AIRS Project carries out instrument maintenance, operations, and calibration for
AIRS and AMSU instruments, algorithm integration and testing for the AIRS/AMSU
geophysical and gridded products, and validation and demonstration that the data
products are suitable for research applications. The project provides user
documentation and support, education and public outreach, and science team
coordination (meetings, etc.). Data production, archive, and distribution are performed
by the NASA/GSFC Goddard Earth Sciences (GES) Data and Information Services
Center (DISC), NOAA NESDIS, and UW for the direct broadcast communities.
2.1
Instrument Operations
The AIRS instruments flies on the NASA Aqua spacecraft, managed and operated by
NASA GSFC. The spacecraft is in excellent health and anticipated to continue operating
until the fuel is exhausted in the 2022 timeframe.
AIRS remains in excellent health and continues to perform extremely well. Several
minor anomalies have occurred over the course of the mission, the most recent
occurring February 8, 2013, when a single-event upset (SEU) caused the instrument
fault management system to shut down the scanner. After examination of instrument
telemetry before and after the event indicated that AIRS was in good health, the
scanner was simply restarted and AIRS was returned to normal operating mode on
February 9, 2013.
AIRS engineering telemetry includes measurements of several hundred voltages and
currents related to instrument health. For most of the parameters measured, no longterm trends exist. A few parameters are changing, but the rates of change are so low
that danger levels are not expected to be reached for many years. The AIRS cooler
active drive levels are important examples, because they are critical to the health of the
instrument. For the past two years, the Cooler A and Cooler B drive levels have been
increasing at ~0.1 % per year. At present rates, neither cooler would reach its yellow
limit for over 50 years. AIRS can operate with just one cooler if necessary, but two are
being used because lower drive levels result in longer expected lifetimes.
The AIRS operations team carefully monitors the health of the 2378 IR channels, of
which 2104 channels (wavelengths 3.75 µm to 13.7 µm) are photovoltaic (PV). The 274
longest wavelength channels are photoconductive (PC). Over the nearly 11 years since
launch, about 180 PV channels have degraded by becoming noisier than they were at
launch. The noise changes are due to a buildup of radiation, either slowly over time or,
in most cases, due to radiation hits in the South Atlantic Anomaly or in the polar horns
of the Van Allen belt. (PC channels are highly resistant to radiation damage.) Each
AIRS PV channel consists of two detectors whose data are combined (through a
weighted average) on board. The weights are commandable, and when one of a
channel’s detectors suffers a radiation hit but the other does not, the channel can be
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restored to near at-launch quality by changing the weights. A revised weight table
uploaded to AIRS on January 21, 2012 restored about 90 channels to in-spec
performance.
Additional recent activities included execution of a standard calibration sequence during
the Aqua lunar roll maneuver to enable AIRS to view the moon. The resulting data will
be of value in the long-term calibration of AIRS. Given the excellent condition of the
AIRS hardware and the availability of the AIRS design and development database,
AIRS is expected to continue to perform well for the life of the spacecraft (through
2022).
2.2
Instrument Calibration
AIRS radiances have demonstrated stability and accuracy suitable for a wide range of
weather and climate studies, and they are now assimilated operationally at Numerical
Weather Prediction (NWP_ centers worldwide for weather forecasting. AIRS radiances
are routinely used to intercompare and calibrate with GOES, MODIS, CrIS, IASI, and
the High Resolution Infrared Sounder (HIRS). Recent studies by the AIRS Team show
agreement in zonal PDF’s of better than 50 mK between the AIRS, CrIS and IASI
instruments.
None of the AIRS channels used by the NWP centers have degraded. However, small
spectral and radiometric shifts over time, if left uncorrected, increase the difficulty of
monitoring climate trends using AIRS radiances and may cause long-term trend artifacts
in the geophysical products (Level 2). Tables have been developed for upload to the
AIRS instrument to replace bad or noisy detectors with their healthy redundant
detectors. In addition, a tool has been developed to de-noise (and flag appropriately) all
noisy channels and to fill the twelve small gaps that exist in the spectra using Principal
Component Reconstruction (Level 1C). This step will greatly facilitate the use of AIRS
Level 1B data for the calibration of vintage GOES, HIRS, and AVHRR thermal infrared
channels. Additionally, the tool will resample as determined by members of the AIRS
science team frequencies to a common grid to better than 0.2 ppm for all observations.
Lastly, the team continues to improve radiometric accuracy of the AIRS instrument
through more thorough and refined analysis of the pre-flight and in-flight calibration
data. The recent improvements and other QC related improvements will be
implemented in Version 7.
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2.3
AIRS/AMSU Geophysical Product Development and Validation
A description of the AIRS/AMSU
geophysical data products and
their validation status is given in
Table 1. The products consist of
calibrated radiances from the
instruments, and numerous
atmospheric and surface
geophysical variables. The scale of
resolution is approximately 45 km
at nadir for most products except
those in L1B and cloud top height
which are 13.5 km for AIRS and 45
km for AMSU. Level 3 daily, 8 day
and monthly gridded products are
also available. An example
monthly map of carbon dioxide is
shown in Figure 1.
Algorithms for all products are
provided by the AIRS science
team, and integrated at NASA JPL.
Products are validated by NASA
and the scientific community
against ground, airborne, balloon
and other in-situ observations
obtained from operational systems
and research experiments. The
AIRS project at JPL performs
configuration management,
extensive functional testing of the
changes, and packaging and
delivery to the operational
production centers (NOAA, GSFC,
and direct broadcast). A
considerable amount of data
product generation is also required
at the AIRS facility to provide the
matchup and analysis data
necessary for testing and validation.
Table 1. AIRS Data Products and Validation
Status
Accuracy
(V5)
Val Status
(V5)
L1B-AIRS
<0.2K
Stage 3
L1B-VIS
15-20%
Stage 1
L1B-AMSU
1-3 K
Stage 3
L1B-HSB
1-3 K
Stage 3
Cloud Cleared IR Radiance
L2
1.0 K
Stage 2
Sea Surface Temperature
L2
1.0 K
Stage 3
Land Surface Temperature
L2
2-3 K
Stage 2
Temperature Profile
L2
1 K / km
Stage 3
Water Vapor Profile
L2
15% / 2km
Stage 3
Total Precipitable Water
L2
5%
Stage 3
Fractional Cloud Cover
L2
20%
Stage 3
Cloud Top Height
L2
1 km
Stage 3
Cloud Top Temperature
L2
2.0 K
Stage 2
Carbon Monoxide
L2
15%
Stage 2
Post-Proc
L2
1-2 ppm
5%
Stage 1
Stage 2
Ozone Profile
L2
20%
Stage 2
Land Surface Emissivity
L2
10%
Stage 2
L1B-Flag
0.5 K
Stage 1
AIRS Product
Product
Core: Radiances
AIRS IR Radiance
AIRS VIS/NIR Radiance
AMSU Radiance
HSB Radiance
Core: Geophysical
Carbon Dioxide
Total Ozone Column
IR Dust
Research Products
Methane
OLR
Sulfur Dioxide
L2
2%
Stage 2
L2-Support
5 W/m2
Stage 3
L1B-Flag
1 DU
Stage 1
*Necessary Products are required to retrieve accurate temperature profiles (1K/km)
in all conditions
Validation Status Definitions (Com m on to all Aqua Instrum ents)
Stage 1: Validation Product accuracy has been estimated using a small number of
independent measurements obtained from selected locations and time periods and
ground-truth/field program effort.
Stage 2: Validation Product accuracy has been assessed over a w idely distributed
set of locations and time periods via several ground-truth and validation efforts.
Stage 3: Validation Product accuracy has been assessed, and the uncertainties in
the product w ell-established via independent measurements made in a systematic
and statistically robust w ay that represents global conditions.
The AIRS/AMSU Version 5 Level 2 data products have provided high value to the
scientific community since their release in July 2007. However, progressive degradation
of AMSU channel 5 performance resulted in a significant degradation of yield in 2012.
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AIRS-Only Version 5 data products were available to mitigate this loss, but with slightly
reduced performance in cloudy areas.
Numerous improvements to the
Level 2 data products have been
successfully implemented in the
AIRS/AMSU Version 6, which was
completed in October 2012, and is
now in production at the GES DISC.
These improvements include less
sensitivity to loss of AMSU
channels, removal of an apparent
50-100 mK/yr bias drift in the midtropospheric temperature product,
improved yield and sensitivity of
temperature and water vapor near
the surface, improvements to
surface emissivity, and the inclusion
of a radiative transfer algorithm
Figure 1. AIRS mid-tropospheric carbon dioxide
(RTA) to accommodate scattering
for April 2013. Over 10 years of data are now
for cloud and dust retrievals. In
archived.
addition, new cloud products,
including cloud top thermodynamic phase, ice cloud optical thickness, and ice cloud
effective diameter, are produced. The AIRS-Only Version 6 is also in production at the
GES DISC. All Version 6 Level 2 and Level 3 data are scheduled to be released to the
public on March 1, 2013, accompanied by a complete set of User Guides and a detailed
Performance Verification Test Report. The remaining work on Version 5 and mostly
Version 6 will involve a closer look at product accuracies trends, and yield through a
comprehensive characterization and validation of the products and publishing results in
the peer reviewed literature.
Support for Version 7 will be driven by the needs of the scientific and operational users
and ROSES investigators. Necessary improvements for Version 7 include transitioning
the cloud and AIRS-Only products into standard products, generating higher spatial
resolution products, and incorporating data from other instruments (e.g., MODIS) and a
new retrieval methodology providing improved error characterization. New versions,
including and beyond Version 7, will be developed in close collaboration with the NASA
NPP CrIS/ATMS science teams.
Also completed in 2012 were the AIRS Standard Level 3 gridded product (1-day, 8-day,
and monthly) and a special gridded product consisting of temperature and humidity
profiles designed for comparisons with CMIP5 models as part of the Observations for
Model Intercomparison Projects (Obs4MIPs) (see http://esg-datanode.jpl.nasa.gov/esgfweb-fe/). Obs4MIPs is a key project for NASA that illustrates the relevance of remote
sensing observations for the improvement of climate prediction, and AIRS has been at
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the forefront of this effort. The AIRS will continue to play an essential role in future
Obs4MIPs activities. One key activity will be to support the development of an AIRS
simulator to compute AIRS-like radiances from geophysical fields.
2.4
Operational Products and User Services
An essential new goal of NWP centers is the full utilization of cloud and water vapor
information from IR sounders. The AIRS science team has received renewed interest
from the Joint Center for Satellite Data Assimilation (JCSDA) and NOAA, among others,
to address this goal. This will involve sharing experience with cloudy radiances, cloud
properties, water vapor, and possible OSSE studies. The AIRS Project will continue to
work with NOAA and the NASA Short-term Prediction Research and Transition Center
(SPoRT) to develop a prototype application for use with the Advanced Weather
Interactive Processing System-2 (AWIPS-2) terminals to display AIRS data. Work in
2012 demonstrated the skill and value of AIRS lower tropospheric water vapor
compared to the current AWIPS-2 product. NASA will continue to host the atmospheric
sounding community at annual NASA Sounding Science Team Meetings as well as
focused workshops on IR sounding for weather, climate and atmospheric composition.
NASA will continue to archive project documents, provide feature stories, user services,
product multimedia, and a refereed publications database on the AIRS Project website:
http://airs.jpl.nas.gov.
3
SUOMI NPP CRIMSS SCIENCE TEAM ACTIVITIES
A NASA-funded S-NPP sounding science team is in the process of assessing the
performance of the CrIMSS sounding suite and data products derived from it to support
climate research. Following is a synopsis of findings to date, as reported in a recent
interim report to NASA.
The performance of the Cross-track Infrared Sensor (CrIS) and Advanced Technology
Microwave Sounder (ATMS) flying on Suomi-NPP, and the quality of the NOAA IDPS
products derived from these sensors were evaluated. Both sensors are performing
extremely well in orbit, clearly producing Sensor Data Records (SDRs) sufficiently
accurate for NWP applications. Further, it is clear that the CrIS instrument is very well
suited to continue, and in several ways to improve on, the high accuracy climate record
from high resolution IR spectra begun by the AIRS instrument on the NASA EOS Aqua
platform. This being said, a number of small liens on the CrIS and ATMS SDRs
produced by the NOAA/IDPS will limit their utility for climate research, and further
analysis of the SDRs (and their associated algorithms) is still needed for a full
understanding of the climate quality of these records.
The retrievals from the Cross-track Infrared Microwave Sounding Suite (CrIMSS)
Environmental Data Records (EDRs) as produced by the NOAA/IDPS is not on par with
the SDRs. However the NOAA Unique CrIS ATMS Processing System (NUCAPS)
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system which is based on the AIRS/AMSU Version 5 processing system is producing
good retrievals. A similar system at GSFC using the AIRS Version 6 production system
on CrIMSS data is producing significantly better retrievals. Finally, retrievals have also
been produced using the LaRC retrieval system. We will be generating monthly mean
products from each system from CrIMSS and compare them to the AIRS/AMSU
monthly means. The robustness of well calibrated hyperspectral infrared radiances for
retrieval of atmospheric temperature and water vapor has been demonstrated. More
work is required to evaluate the differences amongst these systems and with the trace
gas products. Improved error estimation is a high priority for the next version.
4
NAST-I AND ULTRA-SPECTRAL INFRARED REMOTE SENSING AT LARC
Atmospheric sounding efforts led by researchers within the Science Directorate (SD) at
NASA Langley Research Center (LaRC) contribute to the implementation and
advancement of ultraspectral infrared remote sensing systems for weather, climate,
chemistry, and radiation applications, and increase the societal benefits achievable from
measurements by such systems on current and future satellite missions. While LaRC
atmospheric sounding efforts are centered about the NAST-I airborne sensor regional
deployments for satellite system validation (e.g., AIRS, IASI, and CrIS), subsequent
analysis and radiative transfer and retrieval algorithm advancements have achieved
global satellite and NWP program benefits internationally.
NAST-I is the NASA / National Polar-orbiting Operational Environmental Satellite
System (NPOESS) Airborne Sounder Testbed-Interferometer which LaRC maintains
and deploys internationally, in support of satellite measurement system (sensor,
algorithm, direct and derived data product) calibration / validation (cal/val) activities and
NASA Earth system science advancement. NAST-I serves as an ideal validation sensor
since it measures the same level-1 quantity as many sensors it helps to validate (i.e.
infrared spectral radiance), and does so at higher spectral and spatial resolutions. LaRC
also implements a set of internal algorithms for fast radiative transfer modeling and
geophysical product retrievals to enable an independent assessment of derived level-2
products [Zhou et al., 2007a; 2009; Liu et al., 2007; 2009]. Methodologies have been
validated with NAST-I data, implemented on AIRS and IASI, are now supporting the
most recently-launched Suomi NPP CrIS (Cross-track Infrared Sounder) sensor. Table
2 summarizes NAST-I data products and their relevance toward field experimental
campaign validation and science objectives. Earlier field campaign comparisons have
shown IASI and AIRS spectral radiances both matching coincident NAST-I observations
to within ~ 0.1 K (band-averaged). NAST-I can also serve as a calibration reference to
compare other measurements indirectly, exploiting the fact that its accuracy stability is
less uncertain than absolute accuracy itself; employing this approach for a JAIVEx flight
has demonstrated longwave band differences between IASI and AIRS on the order of
less than 0.05 K [Larar et al., 2010]. Such comparisons demonstrate the utility of
airborne FTS sensors, such as the NAST-I, to serve as reference calibration standards
for enabling inter-satellite cross-validation.
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The most recent field experiment involving NAST-I and the first airborne cal / val
campaign implemented for the Suomi NPP satellite was conducted 6-31 May, 2013 on
the NASA ER-2 aircraft based out of Palmdale, CA; this was jointly sponsored by NASA
SMD and the NOAA NESDIS JPSS program. Aircraft campaign flights focused on both
Sensor Data Record (SDR) and Environmental Data Record (EDR) validation for the
Cross-track Infrared Sounder (CrIS), Advanced Technology Microwave Sounder
(ATMS), and Visible Infrared Imager Radiometer Suite (VIIRS) instruments aboard the
SNPP satellite. In addition to NAST-I, the ER-2 payload included the NAST-M (MIT-LL),
MASTER (NASA AMES), AVIRIS (NASA JPL), and S-HIS (UW-Madison) sensors.
Some joint flights were also conducted with the UK Met Office BAe146 aircraft that was
based nearby in Tucson, Arizona. Flight profiles included regions within Mexican
airspace, i.e. over the Gulf of California to ensure capturing clear air over water during
satellite overpass times, and over several instrumented sites including ground-based
FTIR and radiosondes launches.
Table 2. NAST-I data product campaign and science relevance.
NAST-I DATA PRODUCT
CAMPAIGN RELEVANCE
Radiance
Science
 Infrared spectrum (3.5 – 15 m) with
0.25 cm-1 spectral resolution.
 Temperature and trace species
information helpful to characterize the
boundary layer and free troposphere;
contributes to climate, air quality and
biogeochemistry studies
 Information on water vapor, ozone,
cirrus clouds, and vertical profiles of
infrared spectral radiance in the upper
troposphere and lower stratosphere
region beneficial for better understanding
the radiative forcing of this region on
climate system
 Information on local meteorology,
infrared heat budget, and trace gas
evolution
 Infrared spectral radiative
heating/cooling information during aircraft
ascents and descents.
Atmospheric Thermodynamics
 3-d characterization of atmospheric
state (temperature & moisture).
 Profiles with vertical resolution of ~1-2
km for clear air or above clouds, vertical
region dependent; horizontal resolution of
< 130 m per km flight altitude
Atmospheric Composition
 Profiles in clear air or above clouds
with vertical resolution of 2 – 10 km,
depending on altitude and atmospheric
constituent
 Tropospheric CO and O3 (PBL and
free troposphere) from nominal flight
altitude; other trace species (e.g., CO2,
CH4, N2O, and SO2) also possible during
platform ascents / descents.
Cloud Microphysical & Radiative
Properties
Validation
 Contributes toward radiance and
geophysical product validation, e.g.,
AIRS, IASI, CrIS, MODIS, etc.
 Inter-comparisons by spectral and
spatial convolution to common resolution
and coverage
 High spatial resolution enables the
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 Effective top height, temperature, &
spatial extent
 Effective particle size & optical depth
 Spectral emissivity
Surface Properties
 Temperature and spectral emissivity
4.1
effects of scene variability to be
assessed
Future sensor studies
 Direct and derived NAST-I products
contribute toward instrument
specification and retrieval studies for
future sensors, e.g., missions defined
within the National Research Council
Earth Science Decadal Survey (NRC
DS)
Retrieval of geophysical products
LaRC employs a hybrid retrieval algorithm approach to simultaneously deal with clouds
and trace species within the retrieval process [Zhou et al, 2007b]. The state of practice
approach is to avoid clouds, through hole-hunting or cloud-clearing, but the LaRC
approach is aimed at properly using cloudy ultrapectral data, and thus improving the
accuracy and yield of derived geophysical products. The algorithm retrieves quantities
to describe the thermodynamic state of atmosphere, surface and cloud properties, and
trace species amounts from advanced sounder data. It was initially developed using
NAST-I data and then implemented for both AIRS and IASI in support of validation of
these sensors and their corresponding level-2 product retrieval algorithms; and, more
recently, is being implemented for CrIS.
Figure 2 shows the evolution of NAST-I team retrievals for surface temperature and
emissivity products [Zhou et al., 2011; 2013]. The retrieval was first developed for
applying to regional airborne data from NAST-I field campaigns and has since evolved
to global-scale application on satellite data. The top images in the figure demonstrate
the ability to separate surface temperature and emissivity via a simultaneous retrieval
applied to NAST-I data from the CLAMS field campaign. The plots in the middle section
of the figure illustrate application to IASI data, utilizing measurements over the Sahara
desert during 2007, and show radiance sensitivity to using the correct surface emissivity
(i.e. via radiance residuals as one goes from unity to retrieved emissivity in the green to
red curve, respectively). Finally, the bottom images in Figure 2 show week 1 global
products for the 4-year weekly climatology of global land surface temperature and
emissivity derived from IASI measurements obtained during June 2007 – June 2011.
These types of global data products have been made available to the international
community in support of research and operational programs. LaRC has more than a
dozen international collaborators using these IASI global surface climatology data within
their programs.
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Figure 2. Retrieval of global surface temperature and emissivity: evolution from
regional airborne NAST-I development to implementation on global scale for satellite
products from IASI.
4.2
Fast forward radiative transfer model
LaRC fast radiative transfer algorithm development is focused on enabling greater
benefit to be achieved from ultraspectral remote sensing systems. The challenge being
addressed is that modern ultraspectral sensors have high information content, but
handling data volume is very problematic. Such instrument systems often have two
orders of magnitude more spectral channels and increased dimensionality than
traditional sensors. Due to computational expense, however, only a few hundred
channels are typically used in retrievals and data assimilation which yields sub-optimal
results.
The LaRC solution approach is to develop a super fast and accurate radiative transfer
(RT) model (Principal Component Radiative Transfer Model, PCRTM), in an Empirical
Orthogonal Function (EOF) domain [Liu et al., 2006, 2007; 2009], to enable using
information from all channels by reducing channel dimensionality. This enables an
operational processing alternative, and independent forward model, radiance, and
geophysical product validation that is an order of magnitude faster than current channelby-channel based RT models. This model approach has been validated using clear
scenes from NAST-I, AIRS, and IASI, and the achieved accuracy compares favorably
with international community standards (i.e., via the ITWG (International TOVS Working
Group) Radiative Transfer Working Group radiative transfer algorithm interPage 11 of 22
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comparison). This model has also been implemented at LaRC within an independent
set of physical retrieval algorithms using all channels for several sensors (i.e., NAST-I,
AIRS, IASI, and CrIS).
Figure 3. IASI CO retrieval using PCRTM. a) comparison of IASI instrument
measured and PCRTM calculated, and b) example CO retrieval global product.
Figure 3 shows an example implementation of PCRTM for a global trace gas CO
retrieval from IASI. Figure 3-a) shows a comparison between IASI measured (OBS) and
PCRTM calculated (CALC), wherein CO is excluded from the calculations in the middle
plot but included within those for the bottom plot. Notice that the feature near 2020-2250
cm-1 is removed from the OBS-CALC curve when the CO profile is explicitly included in
the radiative transfer algorithm (bottom plot) and, hence, that the CO signal in the IASI
measurements is above the noise level and therefore the ability to retrieve this trace
gas. Also of significance is that this figure shows the calculations with PCRTM are able
to match the measurements at a level below the noise.
The motivation behind PCRTM is that line-by-line accuracy is desired at computational
speeds much quicker in order to be feasible for radiative transfer calculations in
operational processing environments. This approach is fast since principal components
are used to reduce dimensionality in the spectral domain, but since the entire spectral
extent is represented the fast model is able to keep high accuracy. While
implementation for any advanced sensor is beneficial, the largest impact would be
realized for large-format imaging spectrometers applications wherein thousands of
spectral channels from thousands of spatial pixels must be handled.
4.3
Suomi NPP/JPSS CrIMSS operational algorithm
The LaRC NAST-I team is supporting efforts to mitigate risk for the CrIMSS (Cross-track
Infrared Microwave Sensor Suite) operational algorithm being used for Suomi NPP and
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JPSS operational processing producing EDRs (Environmental Data Records) [Liu et al.,
2010]. Activities have focused on porting and implementing the operational code onto
LaRC computers, testing it using proxy and synthetic data and, most recently,
implementing these algorithms on new CrIMSS flight data. Proxy data have been
generated using data from the Aqua and Metop platforms. This has helped to identify
areas of deficiencies in both forward and inverse models, testing robustness of the
operational code, checking error handling capability of the OPS code, and EDR
performance evaluations. LaRC activities are focusing on identifying issues, adding
improvements, and assisting code implementation for operational processing. Figure 4
shows sample CrIMSS EDR algorithm products for 24 February 2012.
Figure 4: Retrieved 500 mb a) temperature (K) and b) water vapor (g/kg) from the
CrIMSS algorithm using Suomi NPP data from February 24, 2012
In addition to working with the official operational algorithm being used for CrIMSS,
LaRC is also contributing toward CrIMSS algorithm and product risk mitigation through
the development and implementation of two other independent L2 retrieval algorithms
for CrIS, including both channel- and PC-based approaches for single field-of-view
(SFOV) IR retrievals for CrIS. These algorithms have been developed and are currently
being implemented on the CrIS flight data.
4.4
NWP data assimilation impact studies
One of the major objectives of current and future weather satellite measurement
programs is to help improve global NWP. Positive impact has already been shown with
current AIRS and IASI radiance measurements from several NWP models at various
forecast centers (e.g., UK Met Office, the GMAO, and the JCSDA). However, the
studies with these models using satellite ultra-spectral sounding measurements still
need huge efforts to fully use these satellite data. Since only a limited number of
channel radiances are currently used in these models, the full potential of ultra-spectral
measurements is not currently being realized. Dimensionality reduction could be
achieved using a transformation such as PCRTM or, possibly, this could be achieved in
the geophysical domain by usage of retrievals produced utilizing most of the channels
as an alternative approach in NWP assimilation. The LaRC team is participating with
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NOAA on an NWP impact study using the Global Forecast System (GFS) NWP model.
The focus of this effort is to evaluate the impact of improved surface emissivity
assimilation on model forecast skill. By using LaRC-provided IASI IR land surface
emissivity in the GSI data assimilation system, the GFS global forecast skill is improved
in terms of anomaly correlations (ACs) for geopotential height, temperature, and wind
over all vertical levels. A positive impact on GFS forecast has been achieved from 4 to 7
days.
5
AIRS SCIENCE HIGHLIGHTS
More than 600 published articles have used AIRS data since launch. This section
describes some recent studies grouped into general themes of Weather, Climate and
Atmospheric Composition (including carbon dioxide), citing some of those articles.
5.1
Weather
AIRS radiance data are assimilated
routinely at virtually all operational
weather prediction centers around
the world. These centers use a
service provided by the NOAA
National Environmental Satellite,
Data, and Information Service
Center for Satellite Applications and
Research, distributing AIRS
radiance data worldwide with a
latency of less than 3 hours.
The European Centre for MediumFigure 5. ECMWF Forecast Error
Range Weather Forecasts
Contribution (FEC) for a variety of
(ECMWF) was the first center to
observational systems, showing AIRS as the
adopt operational use of AIRS data
single most important instrument in
and has found the impact on
improving weather forecasts, second only to
operational forecasts of assimilating
a suite of four AMSU-A units. FEC is an
AIRS and IASI data to be roughly
estimate of the forecast error due to the
comparable and second only to the
absence of a particular system, so that a
collective impact of assimilating four
larger FEC implies a more positive
AMSU units [Cardinali and Healy
2012] (See Figure 5). All major NWP contribution. (Cardinali and Healy, 2012).
centers in the U.S. have also shown
notable improvements when assimilating AIRS data into their operational forecast
systems [Baker et al. 2012]. AIRS is one of the highest-ranked contributors to global
forecast skill [WMO 2012].
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AIRS offers much potential for forecast improvement, as most models currently
assimilate only clear-sky AIRS radiances. Assimilation of AIRS cloud-cleared
geophysical products, significantly improve predictions of tropical rainfall intensity
(compared to forecasts with only clear radiances) [Zhou et al. 2010]. Assimilating quality
controlled AIRS temperatures in partly cloudy conditions gives substantially better 7-day
forecasts of an extreme rainfall event over the Indus River Valley of Pakistan in 2010
versus using only clear-sky radiances [Reale et al., 2012]. Other regional studies
assimilating AIRS retrievals show significant improvements in weather forecasts over
Indian [Singh et al. 2011]; improved regional analyses and heavy-precipitation forecasts
[Zavodsky et al. 2012]; and improved tropical cyclone predictions [Li et al. (2012);
Miyoshi and Kunii 2012]. AIRS observations are also used to test model forecasts of
cloud parameters [Garand et al. 2011]
Other studies examined weather processes with AIRS data near tropical cyclones and
severe weather. One study finds that environmental relative humidity generally
increases with tropical cyclone intensity and intensification rate in nine years of
observations [Wu et al. 2012], and another examines the interactions of relative
humidity and dust near Hurricane Isabel [Pan et al. 2011]. AIRS ozone retrievals can be
improved near hurricanes, and additional information can be obtained in cloudier
regions [Wang et al. 2012], and reanalysis can be improved near severe weather by the
incorporation of additional AIRS information [Botes et al. 2012]. AIRS near-surface air
temperatures during western North American winter snowstorms are more realistic than
those in reanalysis of the National Centers for Environmental Prediction (NCEP) [Guan
et al. 2012].
Over polar regions, AIRS data and model reanalysis characterize the structure of
inversions in the Arctic atmospheric boundary layer, important in the Arctic surface
energy budget [Pavelsky et al. (2011); Devasthale et al. (2011], and AIRS-derived
relative humidity, air temperature, and surface temperature constrain the moisture flux
from an Arctic polynya [Boisvert et al. 2012].
Several recent studies have used AIRS data to examine gravity waves, with results that
include a better understanding of Andean mountain wave effects on regional and global
circulation [Alexander and Teitelbaum 2011], a validation of the distribution of modeled
gravity waves [Choi et al. 2012], the identification of previously unknown gravity wave
sources [Eckermann and Wu 2012, Hoffmann et al. 2013], and the detection and
characterization of shallow inertia-gravity waves [Gong et al. 2012]. A study integrating
data from several sources finds a suggestion that AIRS stratospheric gravity waves are
associated with ozone loss over Antarctica [Lambert et al., 2012].
5.2
Climate
The combined use of AIRS water vapor, winds from NASA’s Modern Era Retrospective
Analysis for Research and Applications (MERRA), and estimates of surface evaporation
and precipitation lead to closure of the atmospheric hydrological cycle both regionally
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and globally [Wong et al. 2011a, b]. The anomalies of global mean, and especially
tropical mean, outgoing longwave radiation values derived from AIRS and CERES are
strongly correlated with El Niño [Susskind et al. 2012].
Several recent studies use AIRS and AMSU data to obtain new information on clouds.
These include a global climatology of bulk microphysical properties of semi-transparent
cirrus clouds from AIRS data [Guignard et al. 2012]; the global distributions of clear-sky,
heterogeneous and homogeneous clouds at the AIRS scale [Kahn et al. 2011a]; the
radiance signature of deep convective clouds [Aumann et al. 2011]; and AIRS
temperature and humidity atmospheric structure in the presence of deep clouds [Zelinka
and Hartmann 2011].
A merged AIRS and Microwave Limb Sounder data set shows that tropical upper
tropospheric water vapor distributions are different depending on whether the QuasiBiennial Oscillation and the El Niño Southern Oscillation are in or out of phase, thereby
suggesting a connection between stratospheric dynamics and tropospheric variability
[Liang et al. 2011]. Small variations in AIRS temperature over the Niño 3.4 region of the
Pacific Ocean are traced to changes in solar output during the declining phase of solar
activity [Ruzmaikin and Aumann 2012].
AIRS temperature and humidity profiles are being used to derive large-scale stability
parameters [Su et al. 2011]. AIRS observations of boundary layer structure and
CloudSat observations of low-level clouds show the relationship between marine
boundary layer clouds and lower tropospheric stability [Yue et al. 2011], and AIRS nearsurface temperature perturbations are collocated with underlying ocean temperature
fronts off South America and the east coasts of Asia and North America [Shimada and
Minobe 2011]
AIRS data have been used to help develop and evaluate model parameterizations
[Russo et al. 2011, Molod 2012] and to evaluate CMIP5 models [Jiang et al. 2012a,
Tian et al. 2013]. Climate models systematically underestimate thermodynamic variance
at small scales [Kahn et al. (2011)]. AIRS and CERES observations, together with
climate model simulations, suggest that mid-tropospheric relative humidity (RH) is an
observational constraint on climate sensitivity [Fasullo and Trenberth 2012]. The
coherence between RH and albedo variations underscores the utility of RH in
diagnosing clouds, which are the main reason for albedo variability and uncertainty in
model-predicted climate sensitivity. This paper also shows that realistic models have the
highest projected warming in doubled CO2 experiments, a result receiving considerable
media attention when it first appeared.
5.3
Atmospheric Composition
AIRS CO2 during strong and weak monsoons years correlates well with the Tropical
Biennial Oscillation index. A proposed mechanism of vertical transport in the Western
Walker Cell yields greater CO2 concentrations during strong monsoon years [Wang et
al. 2011a]. AIRS free tropospheric CO2 during El Niño and La Niña shows effects of a
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changing Walker Circulation, with a signal similar to --but about twice as large as-- that
generated in the Model for Ozone and Related Chemical Tracers (MOZART) model
[Jiang et al. 2012b]. The incorporation of data from AIRS and the Greenhouse Gases
Observing Satellite (GOSAT) into a carbon cycle data assimilation system with the
Local Ensemble Transform Kalman Filter (LETKF) helps constrain surface carbon
fluxes, the greatest source of uncertainty in the carbon budget [Kang et al. 2012].
Principal component analysis and empirical mode decomposition applied to AIRS midtropospheric CO2 shows that the first mode amplitude matches the in situ interannual
growth trend, while higher-order modes display a semiannual oscillation. Retrieved CO2
concentrations closely match isentropes at middle and high latitudes, in agreement with
models [Ruzmaikin et al. 2012]
6
SUMMARY AND CONCLUSIONS
Atmospheric sounding continues to be a high priority at NASA. The NASA Atmospheric
Infrared Sounding (AIRS) instrument continues to operate successfully with full NASA
support. With loss of over 100 channels since launch due to radiation exposure, the
majority of the 2378 channels remain intact and provide sufficient information for
continuity of the geophysical data products. Improvements in the Version 6 Product
Generation System for AIRS have increased yield and accuracy considerably and
removed a large bias trend in the temperature. The NAST-I instrument is in full
operations and is currently being utilized to validate the CrIS instrument on Suomi NPP.
The team continues to validate the data products and improve retrieval methodologies
in preparation for a complete production system using the CrIS and ATMS instruments.
The science interest in these instruments is still very high as indicated by the numerous
science findings made over the last two years.
7
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