© Crown copyright Met Office Mid-infrared observations of the water vapour continuum from CAVIAR...

© Crown copyright Met Office

Mid-infrared observations of the water vapour continuum from CAVIAR field campaignsStuart Newman and co-workers

Cosener’s House, Abingdon, December 2009


Acknowledgements

Many thanks to colleagues involved in collaboration

Imperial College NPLPaul Green, Ralph Beeby Tom Gardiner, Marc Coleman

University of Reading FAAMKeith Shine, Igor Ptashnik, Liam Tallis

Met OfficeJonathan Taylor, Fiona Hilton, Andrew Collard, Steve Wardle, Jean-Claude

Thelen, Stephan Havemann

(and many others…)


Contents

This presentation covers the following areas

• NPL laboratory calibration of ARIES

• Camborne case study: B400

• A preliminary look at Jungfraujoch data

• Water vapour continuum at 4 m


NPL lab calibrations


CAVIAR work package 3.2Low temperature blackbody calibration

• ARIES calibrated against NPL low temperature blackbody

• Range of temperatures relevant to atmospheric remote sensing – we achieved a range of -75 to +30 ºC

• ARIES blackbody target temperatures also varied as a test of target emissivity


ARIES target emissivity tests

Positive bias with “hot” target at 81ºC, “cold” target at 20ºC

Negative bias with “hot” target at 12ºC, “cold” target at 41ºC

(c.f. NPL target with 0.9975 emissivity at -74.8ºC)


Finding best fit target emissivities


ARIES target emissivity results

• “hot” target at 81ºC, “cold” target at 20ºC• “hot” target at 12ºC, “cold” target at 41ºC


Applying lab calibration to flight data

• Most impact seen for cold scenes (uplooking data)

• Aim to recalibrate ARIES flight data once nonlinearity correction to longwave MCT detector has been applied

• Alan Vance is working on ARIES lineshape


Camborne case study: flight B400


B400 flight (18 Sep 2008)

• Very good clear sky conditions

• Flight included “chase” of radiosonde for water vapour intercomparison

• 6 dropsondes released

• 8 aircraft runs over Camborne at different altitudes

• Coincides with near-nadir IASI overpass


Comparison of humidity data

• Spiral ascent of FAAM aircraft to track radiosonde balloon

• Radiosonde (corrected), aircraft hygrometer and dropsondes generally agree well

• Slightly larger discrepancies are apparent at higher altitudes


Comparison with simulations

residual differences (obs-calc) / K

Down-looking brightness temperatures


Residuals due to continuum?

Retrieved continuum strength (relative to MT_CKD) compared with results of Rowe and Walden

IASI spectrum and selected channels sensitive to the continuum

Residuals using different sources of water vapour profiles


Sources of uncertainty

New water vapour spectroscopy in HITRAN 2008 (Coudert et al.) also has a small impact

Uncertainty in water vapour profile is important


HTFRTC (PC-based) 1dVar retrievals

• Minimization of the cost function

• J(x) = (x-x0)T B-1 (x-x0) + (y-y(x))T R-1 (y-y(x))

• x is the state vector (x0 is the a priori)

• T, ln(q) and ln(O3) (43 levels), TSurf, ε (15 PC),

• y are the observations (100 Principal Components)

• y(x) is the HT-FRTC forward model

• B is the background error covariance matrix

• R is the observations+model error covariance matrix


IASI assumed errors

This is done by modelling the impact of removing 100% continuum (drastic)

Include uncertainty due to continuum in R matrix


IASI atmospheric retrieval


Flight B400 – ARIES retrieval of upper tropospheric water vapour

ARIES retrieval initiated with ECMWF forecast - succession of 467 retrievals show moistening of upper tropopshere and deepening of this moist layer to North


Radiosonde profiles show similar structure


Spectral retrieval


Intercomparison case4 May 2007 JAIVEx flight over Gulf of Mexico

• Atmospheric stability: agreement between simulations using profiles from different dropsondes

• All residuals show slowly varying structure – related to water vapour continuum?


A preliminary look at Jungfraujoch data


Jungfraujoch

MönchJungfrau Eiger


Summary of flightsWeather conditions Comments

B466 Cloud at times over Jungfraujoch Partial NPL data

B467 Initially thin cirrus which cleared Good NPL data

B468 Good clear sky conditions FLASH sonde* + MetOp overpass

B469 Cloud at times over Jungfraujoch Partial NPL data

B470 Some thin cirrus encroaching Good NPL data

B471 Excellent clear sky conditions MetOp overpass

B472 Excellent clear sky conditions ARIES failure

B473 Excellent clear sky conditions No TAFTS

B474 Partial cloud over Jungfraujoch Cancelled am flight, pm only* Institute of Applied Physics in Bern launch radiosondes from Payerneequipped with RS92, Snow White and FLASH-B (Lyman-alpha) hygrometers


ARIES data: B471 spiral descent

FAAM spiral descent measurements of temperature (Rosemount de-iced probe) and dew point (FWVS)

ARIES zenith data during spiral descent over Jungfraujoch


Continuum at 4 m


Test of dimer model at 4 m?

(data courtesy Igor Ptashnik)

4 m = (2500 cm-1)


B400 case study (Camborne)

Weak signal?


Other cases…

• Ideally need data at night to avoid solar scattering at 4 m

• A high water vapour loading is beneficial as the self continuum signal is much stronger

• FAAM aircraft flew at night during Middle East Validation Experiment (MEVEX) based in Oman in 2009 – potentially a useful case study

• But need to be careful about effects of desert dust aerosol (large particles may affect IR wavelengths)


Aerosol impact?

• (courtesy Steve Wardle, IR aerosol optical depth inferred from visible nephelometer scattering probe via Mie scattering calculations)


ARIES downwelling spectrumLow level run off coast of Oman

ARIES dataLBLRTM sim


Any other data available?

• US Atmospheric Radiation Measurement (ARM) facilities operate AERI IR interferometers at sites around the globe


ARM site: Nauru (tropical Pacific)


ARM site: Barrow (Alaska)


Trends?

• Include data from Barrow, Oklahoma and Nauru

• Fit data to simulations by varying continuum strength in LBLRTM

• For water vapour self continuum a scaling factor of at least 10 is required


• Strow, L. L., S. E. Hannon, S. De-Souza Machado, H. E. Motteler, and D. C. Tobin (2006), Validation of the Atmospheric Infrared Sounder radiative transfer algorithm, J. Geophys. Res., 111, D09S06

AIRS data (L. Strow)


ARIES downwelling spectrumLow level run off coast of Oman

Self x10


Summary and future work

• NPL lab calibrations this year have been used to estimate ARIES target emissivities and are good basis for absolute ARIES calibration for Jungfraujoch campaign

• Analysis of data from Camborne flight shows evidence (backed up by JAIVEx case and Rowe and Walden paper) that some revisions to continuum in strong water vapour band may be beneficial

• HITRAN 2008 updates (some strong water vapour transitions increased in strength by 5-10%) are being included in LBLRTM simulations for Jungfraujoch campaign

• HTFRTC 1dVar code is being used (with ARIES and IASI) to account for uncertainties in water vapour profiles; in turn an improved continuum may improve quality of retrievals of atmospheric water vapour

• Evidence from ARIES data that self continuum at 4 m is stronger than currently modelled in MT_CKD


Questions and answers


CAVIAR work package 3.2Low temperature blackbody calibration

• ff

• Following on from first set of calibrations at NPL, tests (Alan Vance) showed that ARIES exhibited weak secondary field of view displaced ~6 degrees from primary FoV


• Secondary FoV exhibited similar shape (approx. 3 degrees full angle spread) as primary FoV

• Ratio FoV1:FoV2 approx. 1.000 : 0.027, i.e. approx. 2.7 % radiance contribution – important for uplooking zenith data

• ARIES was returned to manufacturer to replace wedged input window, removing the secondary FoV problem

Radiance map of primary FoV Radiance map of secondary FoV


Finding best fit target emissivities


CAVIAR flights

9 flights

Runs 15,000 to 35,000 feet

Spiral descents over Jungfraujoch

Dropsondes from high level

MetOp underflights

FL150

FL350


Jungfraujoch


Profile comparison #1: B471

© Crown copyright Met Office Mid-infrared observations of the water vapour continuum from CAVIAR...

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