A FIRST look at the Far Infrared
Dan Feldman
Yuk Yung IR Meeting
May 9, 2007
Introduction• 15-100 μm unmeasured• >50% contribution to OLR,
tropospheric cooling rates
• Upper-tropospheric water, cirrus clouds modulate OLR, cooling rates
• Cooling rates inferred from a suite of indirect measurements
• Cirrus causes radiative heating in mid-IR, cooling in far-IR
Figures from Mlynczak et al, 2001
Remote sensing of far-infrared: a frontier in spectroscopy
• FIRST = Far Infrared Spectroscopy of the Troposphere• FTS w/ 0.6 cm-1 unapodized resolution, ±0.8 cm scan
length• Multilayer beamsplitter
– Germanium– Polypropylene– Optically inactive over broad spectral ranges
• 5-200 μm spectral range• NeDT ~ 0.2 K• 10 km IFOV, 10 multiplexed detectors• LN2 cooled• Scan time: 1.4-8.5 sec
Prototype Configuration and Test Flight
Pictures from http://stratocat.com.ar/fichas-e/2005/FSU-20050607.htm
Mid- and Far Infrared Spectra Comparison:LBLRTM
AIRS vs. FIRST T, H2O, O3 AKs
• T, O3 averaging kernels similar• FIRST has superior H2O averaging kernels due
to spectral coverage
AIRS FIRST
Test Flight 1
• Ft. Sumner, NM• Clear-sky conditions• Aqua overpass
AQUA MODIS L2 Cloud Fraction
• Ft Sumner, NM• Clear-sky conditions• Aqua overpass
Measurements and RTM comparison: Test Flight 1
Ft. Sumner, NM; Clear-sky conditions; Aqua, Cloudsat overpass
Test Flight 2AQUA MODIS L2 Cloud Fraction
CALIOP Images
• 2-channel lidar on Calipso platform
• Linear-polarized signal
• Complementary to Cloudsat
• Minimum sensitivity: τ=0.005
• Few clouds over test flight 2
Measurements and RTM comparison: Test Flight 2
Spectral Sensitivity to Clouds
• AIRS and FIRST instruments can detect clouds from window band
• Interaction between UT H2O and clouds notably observed in FIR spectra
Conclusions• Far infrared spectroscopy provides new view of upper-
tropospheric H2O and cirrus clouds– FIRST instrument is capable of such measurements
• FIRST vs AIRS– Agreement in CO2 15 μm, window band– Disagreement in O3 9.8 μm band, H2O 6.3 μm band
• FIRST vs LBLRTM– Reasonable agreement in H2O rotational bands, CO2 v2 and
window band– Disagreement in O3 9.8 μm band, H2O 6.3 μm band
• New test flight coincident with Cloudsat and Calipso in presence of cirrus would be very interesting