determination of atmospheric temperature, water vapor, and heating rates from mid- and far- infrared...
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Determination of atmospheric temperature, water vapor, and heating rates from mid- and
far- infrared hyperspectral measurements
AGU Fall Meeting, Wednesday, December 12, 2007GC34A-02
D.R. Feldman (Caltech); K.N. Liou (UCLA); Y.L. Yung (Caltech);
D. G. Johnson (LaRC); M. L. Mlynczak (LaRC)
Presentation Outline
• Motivation for studying the far-infrared• FIRST instrument description• Sensitivity tests of mid-IR vs far-IR capabilities
– Clear-sky– Cloudy-sky
• Multi-instrument data comparison• Climate model considerations• Conclusions
OutlineOutline 2
The Far-Infrared Frontier
3
• Current EOS A-Train measure 3.4 to 15 μm, don’t measure 15-100 μm
• IRIS-D measured to 25 μm in 1970
• Far-IR, through H2O rotational band, affects OLR, tropospheric cooling rates
• Far-IR processes inferred from other spectral regions
• Mid-IR, Microwave, Vis/NIR
• Interaction between UT H2O and cirrus clouds requires knowledge of both
• Currently inferred from measurements in other spectral regions
Figures derived from Mlynczak et al, SPIE, 2002 MotivationMotivation
No spectral measurementsto the right of line
FIRST: Far Infrared Spectroscopy of the Troposphere
• FTS w/ 0.6 cm-1 unapodized resolution, ±0.8 cm scan length
• Multilayer beamsplitter– Germanium on polypropylene– Good performance over broad
spectral ranges in the far-infrared• 5-200 μm (50 – 2000 cm-1)
spectral range• NeDT goal ~0.2 K (10-60 μm),
~0.5 K (60-100 μm)• 10 km IFOV, 10 multiplexed
detectors• Cooling
• Spectrometer LN2 cooled• Detectors liquid He cooled
• Scan time: 1.4-8.5 sec• Balloon-borne & ground-based
observations
FIRST instrumentFIRST instrument 4
FIRSTAIRS AIRS
Retrieval Sensitivity TestFlow Chart
5
Model Atmosphere A priori Atmospheric State)
Random Perturbations
Synthetic Measurement
RTM + Noise
A priori spectrum
RTM
Retrieval algorithm
A prioriuncertainty
Analyze retrieved state, spectra, and associated statistics
Sensitivity testsSensitivity tests
T(z)H2O(z)O3(z)CWC(z)CER(z)
Clear-Sky Retrieval Test
6 Sensitivity testsSensitivity tests
• AIRS and FIRST T(z) retrievals comparable.• FIRST better than AIRS in H2O(z) retrievals 200-300 mbar.• Residual signal in far IR seen 100-200 cm-1 → low NeDT critical
Clear-Sky Heating Rates
• Spectra provide information about fluxes/heating rates• Error propagation (Taylor et al, 1994; Feldman et al, In Review) can be used • Heating rate error for scenes with clouds generally higher due to lack of
vertical cloud information7 Heating RatesHeating Rates
Tropical Conditions Sub-Artic Winter Conditions
Extrapolating Far-IR with Clouds• Retrieval of T(z), H2O(z), CWC(z),
CER(z) difficult with AIRS spectra• Use AIRS channels to
extrapolate far-IR channels?– Depends on cloud conditions, T(Z),
H2O(z)
– Low BT channels from 6.3 μm band ≈ low BT channels in far-IR
– High BT channels scale from mid- to far-IR
– For tropics, channels with BT 250-270 K (emitting ~ 5-8 km) are complicated
8 CloudsClouds
Test Flight on September 18, 2006:Ft, Sumner NM
AQUA MODIS L1B RGB Image
9
AIRS FootprintsFIRST Balloon
CloudSat/CALIPSO Track Test flightTest flight
CloudSat/CALIPSO signals
10 Test flightTest flight
• CloudSat and CALIPSO near collocation• No signal from CloudSat • CALIPSO signal consistent with FIRST residual
• Instrument collocation• FIRST balloon-borne spectra• AIRS• MODIS
• Residuals are consistent with clouds ~ 5 km, De ~ 60 μm
11
FIRST and AIRS Cloud Signatures
CloudDetected !
Test flightTest flight
Climate Model Considerations
12
• Climate models produce fields that specify mid- & far-IR spectra.
• Multi-moment statistical comparisons of measured spectra and modeled spectra avoid subtle biases from data processing.– Spectral and atmospheric state spaces should be considered jointly.
• Far-IR climate model analysis requires more far-IR data– Far-IR extrapolation should retain physical basis and be verified with
measurements.
– Agreement with CERES is only partial verification and presents a non-unique checksum
• Future work to assess how spectra impart information towards climate model processes.
Model evaluationModel evaluation
Conclusions• AIRS measures mid-IR, but far-IR is not covered A-Train spectrometers.• FIRST provides thorough description of far-IR but limited spectra are
available.• FIRST clear-sky T retrievals comparable, improved UT H2O retrieval
relative to AIRS– Implied cooling rate information difference is small .
• Extrapolating far-IR channels good for Tb ~ 220 K, ok for Tb ~ 300 K, difficult for Tb ~250-270 K.
• Multi-instrument analysis with A-Train facilitates comprehensive understanding of FIRST test flight spectra.
• AIRS mid-IR spectra can validate climate models, but far-IR should not be neglected.
13 ConclusionsConclusions
Acknowledgements
• NASA Earth Systems Science Fellowship, grant number NNG05GP90H.
• Yuk Yung Radiation Group: Jack Margolis, Vijay Natraj, King-Fai Li, & Kuai Le
• George Aumann and Duane Waliser from JPL• Xianglei Huang from U. Michigan and Yi Huang from Princeton• AIRS, CloudSat, and CALIPSO Data Processing Teams
14 Thank you for your timeThank you for your time
Cloud Radiative Effect (CRE)
15
• CRE = TOA clear broadband flux – TOA broadband flux
• CERES provides collocated measurements of CRE from broadband radiometers– Most CERES products contain
multiple data-streams
• AIRS L3 CRE lower than CERES CRE– Other A-Train sets
(CloudSat/CALIPSO) can arbitrate difference
CloudsClouds
Towards CLARREO
16
• 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
• Further orbital simulations required to test how mid-IR state space uncertainties appear as far-IR spectral residuals
• More integrated A-train analyses w.r.t. Far-IR warranted
• Larger Far-IR dataset analysis needed to demonstrate utility of long wavelength measurements for climate monitoring
• Don’t forget about 50-200 cm-1 (200-50 μm).
Future directionsFuture directions