The Far-Infrared Frontier of Terrestrial Remote Sensing

GPS Planetary Science SeminarOctober 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

OutlineOutline

Atmospheric Energy Balance

Earth Radiation Budget Cartoon From Liou, 2002 MotivationMotivation

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

Figure from Mlynczak et al, SPIE, 2002 MotivationMotivation

No spectral measurementsbeyond line

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

Figure adapted from Stackhouse & Stephens, 1991 MotivationMotivation

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

Figure adapted from Anderson presentation, 2007 MotivationMotivation

Remote sensing of far-infrared: a frontier in spectroscopy

• 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

• 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 instrumentFIRST instrument

Prototype Configuration and Test Flights

Pictures from http://stratocat.com.ar/fichas-e/2005/FSU-20050607.htm FIRST instrumentFIRST instrument

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

Mid-IR vs. Far-IR: clearMid-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

Mid-IR vs. Far-IR: clearMid-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

Mid-IR vs. Far-IR: clearMid-IR vs. Far-IR: clear

• 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

Mid-IR vs. Far-IR: cloudMid-IR vs. Far-IR: cloud

Cloud & H2O signatures

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)

Multi-instrument analysisMulti-instrument analysis

Ft. Sumner, NM; stratiform clouds; Aqua, Cloudsat/CALIPSO overpass

Test Flight 2

AQUA MODIS L1B RGB Image

Test flightsTest flights

FIRST and AIRS Cloud Signatures•Instrument collocation

•FIRST balloon-borne spectra•AIRS•MODIS

• Residuals are consistent with clouds ~ 5 km, De ~ 60 μm

Test flightsTest flights

Active sounders

Multi-instrument analysisMulti-instrument analysis

• CloudSat and CALIPSO near collocation• No signal from CloudSat •CALIPSO signal consistent with FIRST residual

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• 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