intercomparison of omi no 2 and hcho air mass factor calculations: recommendations and best...
TRANSCRIPT
Intercomparison of OMI NO2 and HCHO air mass factor calculations: recommendations and best practices
A. Lorente, S. Döerner, A. Hilboll, H. Yu and K. F. Boersma
19th OMI Science Team Meeting, KNMI, De Bilt. 2nd September 2015
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What is needed?• Although satellite retrievals have improved over the last decades,
there is still a need to better understand the uncertainties with every retrieval step
QA of ECV retrieval algorithm
step by step
QA of complete
ECV record
Provide quality assured long term climate data record
QA of independent
reference data
Harmonise, improve and quality assure independent validation data
Community best practice retrieval algorithm will be applied on NO2 and HCHO data records
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Traceability chains available at: www.qa4ecv.eu
What does QA4ECV do?
Retrieval uncertainty dominated by AMF uncertainties
• Traceability chains of ECV productions• Best practices to facilitate harmonization, evaluation and
demonstration of traceability
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What is this work about?QA of ECV retrieval
algorithm step by
step
Community best practice retrieval algorithm will be applied on NO2 and HCHO data records
Retrieval uncertainty dominated by AMF uncertainties
www.qa4ecv.eu/ecv
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What is this presentation about?
• We evaluate different approaches to calculate AMFs by different scientific groups– Altitude dependent AMFs LUT– Tropospheric total and clear-sky AMFs for specific OMI orbit
• Common settings• Preferred settings
• The objectives are:– Establish the error of forward models for UV/Vis retrievals.– Test and improve box AMF LUT for HCHO and NO2 for QA4ECV and
TROPOMI.– Overall measure of uncertainty on AMF calculation
• Recommendations and best practices for AMF calculation in the DOAS NO2 and HCHO algorithm
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AMF LUT• Altitude dependent AMFs
• at 440nm, with NO2 absorption• at 338nm with HCHO absorption
• Commons settings • Several values of surface albedo and surface height • Wide range of viewing and solar geometry• Clear sky: no aerosols• Rayleigh scattering, O3 absorption.
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New NO2 AMF LUT• New NO2 LUT that will be used in QA4ECV retrieval algorithm, DOMINO v3 and
TROPOMI.• Significant increase in reference points: chosen according to sensitivity
studies of RTMs and box AMFs.From 13 x 9 x 10 x 25 x 15 x 24…… to… 16 x 11 x 10 x 26 x 14 x 174
Pressure levels 174
Surface albedo 26
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NO2 AMF LUTCorrect vertical shape
throughout the atmosphereMean relative differences between
DAK-SCIATRAN-LIDORT models within 2%
Statistical noise due to Monte Carlo simulations Higher rel. differences for extreme geometries
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HCHO AMF LUT
• Good agreement between box AMFs, although relative differences are somewhat higher than for NO2
• Comparison of TOA reflectances showed higher differences at 340 nm – Rayleigh scattering dependency with wavelength
Correct vertical shapethroughout the atmosphere
Mean relative differences betweenmodels within 6%
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NO2 tropospheric AMFs• Common settings with choice for ancillary data from DOMINO V2:
• surface albedo (Kleipool et al. 2008)• cloud information from OMI cloud retrieval• temperature profile and NO2 a priori profile (TM4)• terrain height (Global 3km Digital Elevation Model data (DEM_3KM))
• Cloud correction via IPA• Mcl = cloudy AMF; Mcr = clear sky AMF; w = cloud rad. fraction
• Temperature correction
• Four specific pixels:• Pixel I and pixel II: polluted regions
• Southern Beijing, South Korea• Pixel III and IV: clean, remote locations
• Pacific Ocean
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Agreement in clean pixels is better (0.2%) than for polluted pixels (~1.4%)
TOTAL TROPOSPHERIC AMFs
NO2 tropospheric AMFs
• Agreement is the same with and without temperature-Applying temperature correction is more realistic
Pixel I: southern Beijing area
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Tropospheric NO2 AMFs: full orbit• First results: IASB-BIRA vs. WUR tropospheric AMFs using
common settings
Slope: 1.0087Correlation: 0.9998Points: 55343
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Tropospheric NO2 AMFs: Clear sky vs. total AMFs
Total AMF Clear sky AMF DifferenceTotal – clear sky AMF
Cloud fraction < 0.2
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Tropospheric NO2 AMFs: Clear sky vs. total AMFs
Higher differences for higher cloud fraction
As a function of cloud fraction:
Total AMFs are higher : albedo effect
Clear sky AMFs are higher: screening effect
As a function of cloud pressure:
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Tropospheric NO2 AMFs: Preferred settings
• Each group calculates AMFs with their preferred settings• Which are these preferred settings?
RTMsSurface albedo
Kleipool et al. 2008
MODIS climatology BRDF
Cloud correction
Terrain height
Temperature profile
DEM 3km
Cloud information
FRESCO +
O2-O2 retrieval
DAK
LIDORTSCIATRAN
McArtimIPA
Cloud screening
A priori profile
TM4
IMAGES CTM
ECMWF ERA-interim
GEOS-Chem
ECMWF ERA-interim
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Tropospheric NO2 AMFs: Preferred settings
• Each group calculates AMFs with their preferred settings• Project partners : WUR (KNMI), MPIC, IUP-UB, IASB-BIRA• Other international groups have been invited
Round robin exercise to encourage their active participation
Asses similarities and differences between AMF calculation approaches
Already received positive response!!
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Summary and conclusions• Uncertainty assessment in every step of the retrieval
algorithm is needed• After this comparison exercise, we are able to asses best
practices on NO2 and HCHO box AMFs LUT• Good agreement between box AMFs calculated by different groups
(within 2% for NO2, 6% for HCHO)
• Improvement of LUT for current and future missions
• NO2 tropospheric AMFs:• Recommendations
• Ancillary data: reasonable mature products• Include temperature correction
• Other choices for cloud correction (via IPA or cloud screening), aerosol treatment are important (still under investigation, more statistics with more orbits are needed)
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Back up – Reflectances • TOA Reflectance as a function of:
a/ Wavelength b/ cosine of solar zenith angle
Very good agreement between RTMs!
• Differences are generally within 1% for relevant scenarios• Exception occurs for large SZA
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Back up – Reflectances • TOA Reflectance as a function of:
a/ Wavelength b/ cosine of solar zenith angle
Very good agreement between RTMs!
• Differences are generally within 1% for relevant scenarios• Exception occurs for large SZA
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Back - up slidesREFLECTANCES• Processes that might be causing differences have been investigated:
• GENERAL CONCLUSIONInfluence of all processes Layering
SphericityRayleigh scattering
Intrinsic (light path) differences between RTMs are the most likely cause the higher differences
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Back – up slide. Box AMFs at 950 hPa• Differences dependency with different parameters
90% of the retrievalsare done here!
Relative differences within 1.5%
• Minor dependence of box AMFsdifferences with geometry parameters