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A REVIEW OF BIAS PATTERNS IN THE MIRAS BRIGHTNESS TEMPERATURES OVER THE OCEAN

A REVIEW OF BIAS PATTERNS IN THE MIRAS BRIGHTNESS TEMPERATURES OVER THE OCEAN. Joe Tenerelli SMOS Quality Working Group #10 4-5 Feb 2013 ESRIN. OVERVIEW.

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A REVIEW OF BIAS PATTERNS IN THE MIRAS BRIGHTNESS TEMPERATURES OVER THE OCEAN

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  1. A REVIEW OF BIAS PATTERNS IN THE MIRAS BRIGHTNESS TEMPERATURES OVER THE OCEAN Joe Tenerelli SMOS QualityWorking Group #10 4-5 Feb 2013 ESRIN

  2. OVERVIEW Whilesignificantprogess has been made by the calibration team in improving the accuracy of the calibratedvisibilities and the image reconstruction procedure, troublesomebiasesremain in the brightnesstemperaturesobtainedfrom the reprocessed and operational data. Moreover, the biasesexhibitclearseasonal and spatial trends, and significantdiscrepanciesremainbetweenascending and descending passes. For descending passes, thereis a strikingresemblancebetween the evolution of the antenna patch temperature (Tp7) and the AF FoVbias as a function of latitude and time. But anycorrespondenceislessevident in ascending passes. It has been speculatedthat the direct sunmayplay a role in the bias, but anysuchroleisdifficult to demonstrate.

  3. ANALYSIS APPROACHES • Look at the standard AF FoVbiasevolutionboth as a function of latitude and time and averaged over the South Pacific. Review qualitative relationshipwith Tp6 and Tp7 (two of the measuredphysical instrument temperatures). • Look at the possible sun impact on the image and antennatemperaturebiases. Compare the NIR drift to the AF FoV drift, and try to assess the impact of the direct sun correction by comparingbiasevolutionswith and without the correction. Use JRECON solutions to obtainbiaseswithoutsun. • Compare seaonalbiasevolution in successive years.

  4. BIAS COMPUTATIONS Data sources for the bias computations: All of the bias computations presentedhere use the level 1B and 1A data from the last reprocessingcampaign of ESA up through the end of 2011 and from the operational DPGS system for January 2012 up to the present. These data all employ the 1-Slope model for the antennaloss L1 as developed by the Level 1 calibration team. The one exception to thisisthat, on a few slides, wealso show biases for the so-called ‘Calibrated L1’ approachintroduced by Ignasi Corbella. Biases are computedboth for the reconstructed images (AF and EAF FoV) and for the NIR antennatemperatures. Hovmoller plots show averages over the open-ocean portion of all passes (Atlantic, Pacific, IndianOcean). However, for computationalreasons, the NIR antennatemperaturebiases are computed over a smallsubset of Pacific Ocean passes.

  5. BIAS COMPUTATIONS Data sources for the bias computations: Biases are computedboth for the reconstructed images (AF and EAF FoV) and for the NIR antennatemperatures. Hovmoller plots show averages over the open-ocean portion of all passes (Atlantic, Pacific, IndianOcean). For computationalreasons, the biascurves(for both the images and the NIR antennatemperatures) are, for the most part, computed over a smallsubset of daily Pacific Ocean passes. An evensmallersubset (about one pass per month per pass direction) has been selected for use by the Level 1 team for algorithmtestingpurposes.

  6. BIAS COMPUTATIONS AF FoV and EAF FoVdomainaveragedbiases: Domain-averagedbiases are calculated over both the alias-free (AF) and extended alias-free (EAF) portions of the field of view (FOV). Gridpointswithin 0.044 directorcosineunits of the domainboundariesare excludedfrom the averaging.

  7. BIAS COMPUTATIONS NIR antennatemperaturebiases: Biasbetweenmeasured and modeledantennatemperatures are computedalongeachhalf-orbit. Scenebrightnessismodeled over entire front halfspace, whichincludescelestialsky radiation directly incident at the antennas and the contribution of the direct sun to Txx and Tyy. The sunbrightnessisassumed to beunpolarized. Contribution iscomputed for each NIR separatelyusing the correspondingantenna patterns. Cross-polarizationterms are excluded.

  8. BIAS COMPUTATIONS Latitudinal averaging and the bias trend curves: The biascurvespresenthere have been computed by averaging per-epochbiases (for bothNIR antennatemperatures and the images) between 40oS and 5oN. This range of latitudes isused to avoid impact of land and ice on the antennatemperatures.

  9. A LITTLE REMINDER OF THE MAGNITUDE OF THE DRIFT PROBLEM

  10. THREE-MONTH MEAN MAPS (USING ONLY ALIAS-FREE FIELD OF VIEW) SHOW CLEARLY THE GLOBAL BIAS EVOLUTION To producethesemapswe have used the first Stokes parameter in the AF-FoVonly, afterhavingremoved all snapshotswith maximum brightnesstemperature (Txx,Tyy, or |Uxy|) exceedingsomethreshold. Also, only one OTT wasapplied in order to reveal the drift from the NH summer 2012 to the present. Jun,Jul,Aug2012 Nov,Dec,Jan 2012/3

  11. (Tx+Ty)/2 bias +1 K SSS bias -2 psu (independent of incidence angle)

  12. THREE-MONTH MEAN MAPS (USING ONLY ALIAS-FREE FIELD OF VIEW) SHOW CLEARLY THE GLOBAL BIAS EVOLUTION Jun,Jul,Aug2012 Nov,Dec,Jan 2012/3

  13. SALINITY BIAS TRANSFORMS INTO FIRST STOKES BIAS NEARLY LINEARLY Jun,Jul,Aug2012 Nov,Dec,Jan 2012/3

  14. SUBTRACTING MEAN SSS BIAS FOR NOV-DEC-JAN 2012/3 FROM SSS BIAS FOR JUN-JUL-AUG 2012 SHOWS LATITUDINAL DEPENDENCE OF THE EVOLUTION OF SPATIAL BIAS DISTRIBUTION BIAS EVOLUTION EXHIBITS DOMINANT ZONAL SYMMETRY

  15. SAME EVOLUTION AS IN PREVIOUS SLIDE BUT FOR THE FIRST STOKES PARAMETER DIVIDED BY TWO BIAS EVOLUTION EXHIBITS DOMINANT ZONAL SYMMETRY

  16. ANALYSIS APPROACHES • Look at the standard AF FoVbiasevolutionboth as a function of latitude and time and averaged over the South Pacific. Review qualitative relationshipwith Tp6 and Tp7 (two of the measuredphysical instrument temperatures). • Look at the possible sun impact on the image and antennatemperaturebiases. Compare the NIR drift to the AF FoV drift, and try to assess the impact of the direct sun correction by comparingbiasevolutionswith and without the correction. Use JRECON solutions to obtainbiaseswithoutsun. • Compare seaonalbiasevolution in successive years.

  17. THERMAL IMPACT OF THE SUN: TP6 Time-latitude evolution of both Tp6 and Tp7 show clearseasonal patternswithsimilar amplitude fromyear to yearatanygiven time of year. There isnot a clearinterannual drift:

  18. THERMAL IMPACT OF THE SUN: TP7 Time-latitude evolution of both Tp6 and Tp7 show clearseasonal patternswithsimilar amplitude fromyear to yearatanygiven time of year. There isnot a clearinterannual drift:

  19. THERMAL IMPACT OF THE SUN: TP7 AF-FoV first Stokes parameterbiasevolutionresemblessomewhat the evolution of Tp7 in descending passes, but the relationship in ascending passes is not as clear. Also, unlike Tp6 and Tp7, the first Stokes biases in asc and desc passes exhibit a long-termdownward trend thatis not present in Tp7.

  20. THERMAL IMPACT OF THE SUN: TP7 Figures below compare trends of both one-slope and calibrated L1 solutions for AF-FoVmeanbias in (Tx+Ty)/2 to trends in latitudinally-averaged Tp7 deviations. Tp7 curves are offset to fit on these figures. First Stokes biases in asc and desc passes exhibit a long-term trend thatis not present in Tp7. Also amplitude of bias drop late in year in descending passes isincreasingfromyear to yearwhile the variation of Tp7 is not. So perhaps a thermal effectis not the whole story; perhaps L-band brightness of sunplays a role.

  21. ANALYSIS APPROACHES • Look at the standard AF FoVbiasevolutionboth as a function of latitude and time and averaged over the South Pacific. Review qualitative relationshipwith Tp6 and Tp7 (two of the measuredphysical instrument temperatures). • Look at the possible sun impact on the image and antennatemperaturebiases. Compare the NIR drift to the AF FoV drift, and tryto assess the impact of the direct sun correction by comparingbiasevolutionswith and without the correction. Use JRECON solutions to obtainbiaseswithoutsun. • Compare seaonalbiasevolution in successive years.

  22. CAN WE HOPE TO SEPARATE THE THERMAL IMPACT OF THE SUN FROM THE IMPACT OF THE SUN L-BAND BRIGHTNESS ON THE IMAGES? The sun L-band brightnessexhibitsstrong quasi-monthly oscillations over the last couple of years. Note in particular the recentstrong oscillation with a nearly one-monthperiod:

  23. CAN WE HOPE TO SEPARATE THE THERMAL IMPACT OF THE SUN FROM THE IMPACT OF THE SUN L-BAND BRIGHTNESS ON THE IMAGES? The L-band brightnesstemperature of the sunoscillatesbetween about 15,000 K and 200,000 K with a veryregular cycle fromAugthroughNov 2012:

  24. CHECK FOR EVIDENCE OF POSSIBLE SUN IMPACT ON NIR AND AF FOV BIAS EVOLUTION FROM JUNE 2010 THROUGH DECEMBER 2012 COMPARE SOLUTIONS WITH AND WITHOUT (JRECON) DIRECT SUN CORRECTION APPLIED TO THE IMAGES

  25. Green curve = AF Biaswithout– withthe currentDPGSdirectsun correction • Blue and redcurves: Biasevolution of the NIR antennatemperatures (zerobaseline) • Yellowcurve: Computedsun impact on NIR CA antennatemperature (Tx+Ty)/2 Overall, for ascending passes thereis good correspondencebetween NIR TA evolution (red and bluecurves) and AF Fovbiaswith and without direct sun correction (cyan and green curves).

  26. Green curve = AF Biaswithout– withthe currentDPGSdirectsun correction • Blue and redcurves: Biasevolution of the NIR antennatemperatures (zerobaseline) • Yellowcurve: Computedsun impact on NIR CA antennatemperature (Tx+Ty)/2 In descending passes thereis an increase in temporal ripplesduring the periodwithstrongsunbrightness oscillations. AF-FoVbiaseswith and withoutsun correction (cyan and green curvesrespectively) are nearlyidentical.

  27. A zoom on the year 2012 for descending passes shows more clearly the oscillations in the NIR antennatemperaturebiases, which are well-alignedwith the oscillations in the computed contribution. Note howeverthat I have alreadysubtracted the yellowcurvefrom the antennatemperaturebiases to obtain the biascurves. One possible explanationisthat the antenna patterns used to compute the sun contribution to antennatemperature are in error. Also note that oscillations in the AF FoVbiascurve are muchsmallerthanthose for the NIR antennatemperatures.

  28. ANALYSIS APPROACHES • Look at the standard AF FoVbiasevolutionboth as a function of latitude and time and averaged over the South Pacific. Review qualitative relationshipwith Tp6 and Tp7 (two of the measuredphysical instrument temperatures). • Look at the possible sun impact on the image and antennatemperaturebiases. Compare the NIR drift to the AF FoV drift, and try to assess the impact of the direct sun correction by comparingbiasevolutionswith and without the correction. Use JRECON solutions to obtainbiaseswithoutsun. • Compare seaonsalbiasevolution in successive years.

  29. Comparingbias trends in 2011 and 2012, the seasonal trends in the NIR TA and AF FoVbiases are similar but the amplitude of the bias drop towards the end of the yearisstronger in 2012 than in 2011. Also, in Oct-Dec of bothyears the NIR TA biasesbegin to shift upwardbeforethose of the AF FoV.

  30. Comparingbias trends in 2011 and 2012, the seasonal trends in the NIR TA and AF FoVbiases are similar but the amplitude of the bias drop towards the end of the yearisstronger in 2012 than in 2011. Also, in Oct-Dec of bothyears the NIR TA biasesbegin to shift upwardbeforethose of the AF FoV.

  31. CONCLUSIONS • There is not a reallynoticeableinterannual trend in Tp7 but thereis a significantdownward trend in the ascending and descendingpass AF FoV and NIR antennatemperaturebiases (in first Stokes parameterat least). • There seems to beverylittle impact of the direct sun correction upon the AF FoVbias trends. More impact in seen in the NIR antennatemperatures, but the NIR antennatemperaturebiases, for which the direct sun contribution has already been removed, exhibitstrong temporal ripplesthat are alignedwithsun Tb variations. • Comparingbias trends in 2011 and 2012, the seasonal trends in the NIR TA and AF FoVbiases are similar but the amplitude of the bias drop towards the end of the yearisstronger in 2012 than in 2011. Also, in Oct-Dec of bothyears the NIR TA biasesbegin to shift upwardbeforethose of the AF FoV.

  32. AN ASIDE: A STRANGE DISCONTINUITY IN THE ESTIMATED SUN BRIGHTNESS TEMPERATURE IN ASCENDING PASSES IN THE CURRENT OPERATIONAL DPGS

  33. Whenwecollect all ascending passes since Jan 2010 and plot the estimatedsunbrightnesstemperature as a function of time and boresight latitude, a verystrongdiscontinuityappearsbetween May and September of everyyear:

  34. To determinewhatthesediscontinuities correspond to we first add to the previous plot curvesalongwhich the direct sun passes across the line eta=0 in directorcosinecoordinates. Thesecurves are shownbelow in gray. They are similar in shape to a portion of the discontinuities:

  35. If wethen overlay curves (in magenta) showingwhere the direct sun crosses eta=0.08, weseethatthesecurvesalignnicelywith the northern portion of the sun Tb discontinuities:

  36. If wethen overlay, in dahsed magenta, curvesalongwhich the direct sun crosses xi=0.985, weseethatthesecurvesalignnicelywith the remaining portions of the discontinuities:

  37. If wethenmapthesediscontinuities back onto directorcosinecoordinates, weobtain the black curveshownhere, whichclearly shows how the discontinuities correspond to the suncrossingspecific portions of the FOV. For referencewealso show the paths of the sunthroughexampleascending passes on 15 May and 20 July 2010.

  38. Zoom-out to show wherethisdiscontinuityoccurs relative to the entire FOV:

  39. Nextconsider an examplehalforbitfrom May 16, 2012 thatexhibits a strong jump in the estimatedsunbrightnesstemperature:

  40. Whenwe do not attempt to remove the sun, the brightnesstemperatures in the vicinity of the sunaliasesissimilarbefore and after the jump: BEFORE JUMP

  41. Whenwe do not attempt to remove the sun, the brightnesstemperatures in the vicinity of the sunaliasesissimilarbefore and after the jump: AFTER JUMP

  42. Application of L1OP sunremovalalgorithmdoeseffectivelyremove the sunaliases on bothsides of the jump: BEFORE JUMP

  43. Application of L1OP sunremovalalgorithmdoeseffectivelyremove the sunaliases on bothsides of the jump: AFTER JUMP

  44. There is no correspondingdiscontinuity in the AF-FoVmeanbiases, however:

  45. There is no correspondingdiscontinuity in the AF-FoVmeanbiases, however:

  46. EXTRA SLIDES

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