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