The Far-Infrared Frontier of
Terrestrial Remote Sensing
GPS Planetary Science Seminar
October 16, 2007
Daniel Feldman, Yuk Yung (Caltech)
Kuo-Nan Liou (UCLA)
Marty Mlynczak, Dave Johnson (LaRC)
Presentation Outline
• Motivation for studying the far-infrared
• FIRST instrument description
• Mid-IR vs Far-IR capabilities
– Clear-sky
– Cloudy-sky
• Lessons learned from collocated test flights
• Multi-instrument analysis of clouds
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Outline
Atmospheric Energy Balance
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Motivation
Earth Radiation Budget Cartoon From Liou, 2002
The far infrared frontier
• Current spectrometers don’t
measure 15-100 μm (650 – 100 cm-1)
• FIR, through H2O rotational band,
contributes substantially to OLR,
tropospheric cooling rates
• Far-IR processes inferred from
other spectral regions
• Mid-IR passive spectroscopy
• Microwave active and passive
sounding
• Vis/NIR lidar
No spectral measurements
beyond line
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Motivation
Figure from Mlynczak et al, SPIE, 2002
Clouds and the far-infrared
• In tropics, cirrus causes
increased tropospheric
radiative heating in mid-IR &
increased cooling in far-IR
• In sub-arctic winter, cirrus
leads to less mid-IR heating,
enhanced IR cooling
• Interaction between UT H2O
and cirrus clouds requires
knowledge of both
• Currently inferred from
measurements in other
spectral regions
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Motivation
Figure adapted from Stackhouse & Stephens, 1991
CLARREO Mission
• NRC Decadal Survey
recommended CLARREO for
radiance calibration, climate
monitoring
• CLARREO specified to cover
200 – 2000 cm-1 with < 2 cm-1
resolution
• NIST traceability
requirement
• Prototyped far-IR instruments
provide a science and
engineering test-bed for next
generation of satellite
instruments
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Motivation
Figure adapted from Anderson presentation, 2007
Remote sensing of far-infrared:
a frontier in spectroscopy
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FIRST = Far Infrared Spectroscopy of the Troposphere
Developed at NASA LaRC, Marty Mlynczak PI, NASA IIP project
FTS w/ 0.6 cm-1 unapodized resolution, ±0.8 cm scan length
Multilayer beamsplitter
– Germanium
– Polypropylene
– Optically inactive over broad spectral ranges in far-infrared
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5-200 μm (50 – 2000 cm-1) spectral range
NeDT ~ 0.2 K
10 km IFOV, 10 multiplexed detectors
LN2 or liquid helium cooled
Scan time: 1.4-8.5 sec
FIRST instrument
Prototype Configuration and Test Flights
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FIRST instrument
Pictures from http://stratocat.com.ar/fichas-e/2005/FSU-20050607.htm
AIRS and FIRST T, H2O, O3 clear-sky retrievals
•Standard retrievals using a linear Bayesian update for
clear-sky cases
• Mid- and Far-IR profile retrievals
• T profiles comparable
• FIRST has superior H2O profile performance in upper
troposphere
• Normalized averaging kernels quickly reveal relative
retrieval power
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Mid-IR vs. Far-IR: clear
Spectra and residuals
• Much of the initial residual
structure removed during
retrieval
• Tsurf not retrieved here
• Retrieval using FIRST vs
AIRS shows structure at low
wavenumbers and larger
residuals in H2O v3 band
• Upper troposphere H2O
retrieval power
difference revealed
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Mid-IR vs. Far-IR: clear
Cooling Rate Information
• Information from the spectrometers can impart understanding of cooling rate profiles
(Feldman et al, 2007).
• Far-IR measurements provide marginal improvement of upper-troposphere clear-sky
cooling rates
• Comparable performance for tropical, sub-arctic winter atmospheres
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Mid-IR vs. Far-IR: clear
Cloud & H2O signatures
• Ice cloud absorption, extinction, asymmetry
factors can be reasonably parameterized
with:
• Cloud-water content profiles
• Cloud effective radius profiles
• AIRS spectra utilize 8-12 µm window band
and 6.3 µm H2O band for cloud and H2O
profile retrieval
• Degree to which cirrus cloud properties can
be retrieved with H2O profiles is active area
of research
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Mid-IR vs. Far-IR: cloud
Suite of A-Train measurements
• Polar-orbiting sun-synchronous constellation of satellites with diverse instrumentation
•Aqua
• AIRS (mid-IR passive spectrometer for sounding)
• AMSU (Passive microwave sounder)
• MODIS (vis/near-IR imaging spectroradiometer)
• CloudSat (94 GHz cloud pulse radar)
• CALIPSO (2-channel vis/near-IR lidar)
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Multi-instrument analysis
Test Flight 2
AQUA MODIS L1B RGB Image
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Test flights
Ft. Sumner, NM; stratiform clouds; Aqua, Cloudsat/CALIPSO overpass
FIRST and AIRS Cloud Signatures
•Instrument collocation
•FIRST balloon-borne spectra
•AIRS
•MODIS
• Residuals are consistent with clouds ~ 5 km,
De ~ 60 μm
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Test flights
Active sounders
• CloudSat and CALIPSO near
collocation
• No signal from CloudSat
•CALIPSO signal consistent
with FIRST residual
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Multi-instrument analysis
Conclusions
• FIRST provides a comprehensive description of the far-infrared
which is relevant to CLARREO development
• FIRST clear-sky retrievals vs. AIRS
– Improved H2O retrieval relative to AIRS
• Relies on low instrument FIR NeDT
– Implied cooling rate information difference is negligible.
• FIR instrumentation can impart significant info on cloud radiative
effect
• Multi-instrument analysis is powerful for understanding FIRST test
flight spectra
• In the future:
• Orbital simulation environment testing is necessary to compare the relative
abilities of mid- and far-IR under different, realistic conditions
• FIRST radiance calibration is required to gain quantitative information out
of test flights about H2O lines and cloud cover.
Acknowledgements
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NASA ESSF Program
Jack Margolis and the YLY IR Radiation Team at Caltech
Tom Pagano, Duane Waliser, Eric Fetzer and Alex Ruzmaikan of JPL
Tony Clough and the RT Team at AER, Inc.
The CloudSat and CALIPSO product support teams