Blumenstock, T.

Borrows, J.P.

Borsdorff, T.

Buchwitz, M.

de Maziere, M.

Dils, B.

Duchatelet, P.

Forster, F.

Frankenberg, C.

Hannigan, J.

Hase, F.

Jones, N.

Klyft, J.

Mahieu, E.

Mellqvist, J.

Notholt, J.

Petersen, K.

Schneising, O.

Strong, K.

Sussmann, R.

Taylor, J.

Vigouroux, C.

Maziere, M. de

Borrows, J. P.

KITopen

Satellite validation of column-averaged methane on global scale:Harmonized data from 13 FTIR ground stations versus lastgeneration ENVISAT/SCIAMACHY retrievalsR. Sussmann(1), F. Forster(1), T. Borsdorff(1), B. Dils(2), M. De Mazière(2), C.Vigouroux(2), T. Blumenstock(3), M. Buchwitz(4), J.P. Burrows(4), P. Duchatelet(5), C.Frankenberg(6), J. Hannigan(7), F. Hase(3), N. Jones(8), J. Klyft(9), E. Mahieu(5), J.Mellqvist(9), J. Notholt(4), K. Petersen(4), O. Schneising(4), K. Strong(10), J.Taylor(10)(Keywords: NDACC, Zugspitze, mid infrared, FTS, FTIR, methane, satellite validation,ENVISAT, SCIAMACHYABSTRACT: Global measurements of column-averaged methane have recently shown astep forward in data quality via year 2003 and 2004 retrievals from two differentprocessors, namely IMAP-DOAS ver. 49 and WFM-DOAS ver. 1.0. Accuracy andprecision have approached the order of 1 %, and can be considered for inverse modellingof sources and sinks. This means at the same time that the quality requirements forground-based validation data have become higher. In order to guarantee a station-to-station consistency of <1 % we performed a harmonization effort for 13 selectedglobally distributed mid-infrared FTIR stations. Station-to-station biases are eliminatedby using identical micro-windows, spectroscopic line lists, retrieval parameters, sourcesof ancillary data like pressure-temperature profiles, and water vapor data for deriving dryair columns. Furthermore, a geophysically consistent set of prior information for theretrievals at all stations was established. Our network validation study utilizes thevalidation strategy developed during the first validation of ENVISAT/SCIAMACHYcolumn-averaged methane by FTIR (Sussmann et al., 2005). The outcome of the newstudy is the accurate determination of the satellite-ground station biases as a function oflatitude on global scale, as well as an assessment of the ability ofENVISAT/SCIAMACHY to measure true day-to-day variability.1 PREVIOUS WORK – VALIDATION OF SCIAMACHY CHANNEL 6 RETRIEVALSIn 2005 methane total-vertical column retrievals from ground-based solar FTIR measurements at the PermanentGround-Truthing Station Zugspitze (47.42 °N, 10.98 °E, 2964m a.s.l.), Germany had been used to validate columnaveraged methane retrieved from ENVISAT/SCIAMACHY spectra by WFM-DOAS (WFMD) version 0.4 and 0.41for 153 days in 2003 (Sussmann et al., 2005). From the FTIR time series of XCH4 an atmospheric day-to-dayvariability of 1% was found, and a sinusoidal annual cycle with a »1.6% amplitude. The bias was WFMDv0.4/FTIR=1.008±0.019 and WFMD v0.41/FTIR=1.058±0.008. WFMD v0.41 was significantly improved by atime-dependent bias correction. It could still not capture the natural day-to-day variability, i.e., the standard deviationcalculated from the daily-mean values is 2.4% using averages within a 2000-km radius, and 2.7% for a 1000-kmradius. These numbers were dominated by a residual time dependent bias in the order of 3%/month.2 NEW GENERATION SCIAMACHY RETRIEVALS (CHANNEL 8)Briefly after our 2005 validation study the SCIAMACHY icing problem was overcome by switching the retrievalalgorithms from channel 8 to channel 6, which is not impacted by the icing issue. Channel 6 retrievals were pioneeredvia IMAP-DOAS and subsequently implemented in WFM-DOAS. This step forward in data quality leads us to thefollowing 3 questions/requirements as to revisiting SCIAMACHY validation for the new algorithms IMAP-DOASv49 (Frankenberg et al., 2008) and WFM-DOAS v1.0 (Schneising et al., 2009):· Can the newest SCIAMACHY channel 6 retrievals now capture true day-to-day variability?· Can the newest SCIAMACHY channel 6 retrievals now reflect the annual cycles as seen by FTIR?one-column lay out Millpress1 von 6 15.10.2015 14:31· Since the dominant source of error in 2005, namely the time-dependent biases due to icing (3%/month withchannel 8) have been overcome with the channel 6 retrievals, it makes now sense to perform a globalvalidation of biases. The question is: is there a latitudinal dependency of biases for IMAP-DOAS v49 andWFM-DOAS v1.0?3 HARMONIZED FTIR RETRIEVAL STRATEGY FOR CH4 COLUMNS AND PROFILES AT 13 FTIRSTATIONS3.1 Harmonized retrieval settingsValidation of the new high-accuracy and precision SCIAMACHY retrievals require that correlative CH4 columns areretrieved consistently at the globally distributed ground-based FTIR stations operated within the Network of theDetection of Atmospheric Composition Change (NDACC).An assessment of consistency between different FTIR stations of the network in May 2008 has shown majordrawbacks as to consistency. Therefore, a harmonized retrieval strategy has been developed which comprises:Fig. 1. Harmonized set of CH4 a priori profiles for different FTIR stations: Toon CH4 balloon profile adapted for mean (NCEP) tropopausealtitude of the different stations via the linear transformation described by Meier (1997).· one common micro-window set for CH4 (5 windows, i.e., 2613.7-2615.4 cm-1, 2650.6-2651.3 cm-1,2835.5-2835.8 cm-1, 2921.0-2921.6 cm-1)· one identical spectroscopic line list (HITRAN 2004 with “Hase update”)· one common source of pT-input profiles (NCEP)· one common a priori profile adapted to the mean NCEP tropopause altitude of the different sites (Fig. 1)· one consistent set of regularization matrices (Tikhonov L1 applied to per cent changes of retrieved VMRprofiles) and altitude grids (1.5 km equidistant):Tests have shown, that it is a good choice to apply the Tikhonov L1 regularization to percentage changes (scalingfactors) of the individual layers of the VMR profiles to be retrieved (another choice would be the application of R toprofiles given in units of absolute VMR). An important argument for the implementation of L1 in units of percentageprofile changes is, that this leads to the limiting case of a VMR-profile scaling, in case the regularization strength a istuned towards infinity (dofsº1): VMR-profile scaling (e.g., used in SFIT 1) is one of the best-tested retrievalapproaches and well known to yield very robust retrieval results for total columns.The harmonized retrieval strategy was implemented for 13 FTIR stations (Table 1) and subsequently applied to CH4retrievals for the years 2003 and 2004.one-column lay out Millpress2 von 6 15.10.2015 14:31Table 1. The 13 FTIR stations of the globally harmonized SCIAMACHY validation effort.station latitude longitude stationaltitudenumber ofcolumns in2003/2004tropopause heightSpitzbergenThuleKirunaHarestuaBremenZugspitzeGarmischJungfrauj.TorontoIzañaParamariboSt-DenisWollongong78.92 °N76.53 °N67.84 °N60.22 °N53.11 °N47.42 °N47.48 °N46.55 °N43.66 °N28.30 °N5.81 °N20.90 °S34.41 °S11.92 °E68.74 °W20.41 °E10.75 °E8.85 °E10.98 °E11.06 °E7.99 °E79.40 °W16.48 °W55.21 °W55.48 °E150.88 °E20 m225 m419 m596 m29 m2964 m745 m3580 m174 m2367 m7 m50 m40 m1131773381234179999498702185207641416338.95 km8.51 km9.62 km10.20 km10.74 km11.25 km11.25 km11.38 km13.25 km14.44 km16.36 km15.66 km12.53 km3.2 Calculation of the dry air columnWe use NCEP pressure temperature profiles and NCEP water vapor profiles to calculate the dry air column. The CH4columns from the MIR retrievals are then divided by the daily dry air column.3.3 Harmonized quality selectionQuality selection of FTIR spectra is performed using the rms of the spectral fitting residuals as a criterion.Harmonized rms thresholds are obtained via elbow plots, see Fig. 3.Fig. 2. Thresholds for quality selection of FTIR retrieval results.4 VALIDATION STRATEGY4.1 Coincidence criteriaTemporal coincidence. SCIAMACHY data and FTIR measured within the same day have been used. The reasons arei) that SCIAMACHY overpasses at one site are limited to one certain hour of a day, and ii) that there is a significantday-to-day variability of XCH4 in the order of 1 %, while there is no comparable true diurnal variation of XCH4(Sussmann et al., 2005).Spatial coincidence. The question of the appropriate size of spatial co-location criteria cannot be uniquely solved, asmany discussions on satellite validation workshops have proven. Therefore in Sussmann et al. (2005) we havedemonstrated a strategy of using different collocation criteria in parallel and, thereby, analyse their impact on thevalidation results. In this study we use three different pixel selection radii, i.e., of 200 km, 500 km, and 1000 km forall sites in parallel.one-column lay out Millpress3 von 6 15.10.2015 14:314.2 Altitude correctionThe altitude of a FTIR located at a mountain or a valley site may differ significantly from the mean altitude of itssurroundings measured by SCIAMACHY. This altitude effect leads to a difference between the CH4 columnmeasured by the FTIR and the mean column derived from SCIAMACHY data within a certain pixel selection radiusaround this site.^Fig. 3. Altitude correction of XCH4 for high-altitude FTIR sites.The altitude effect is reduced by considering column-averaged mixing ratios instead of column densities. However,the effect is perfectly eliminated only in the special case of an altitude constant VMR profile (e.g., approximatelyeliminated for XCO2). In case of XCH4, there is still a residual altitude effect in the order of up to a few percent, dueto the gradient of the CH4 profile above the tropopause.In order to reduce this effect Dils et al. (2006) have applied an altitude correction factor to the SCIAMACHY XCH4data using the ratio of TM4 model data at the station´s altitude and the TM4 VMR at the ground level (as determinedby the model´s orography). However, the spatial resolution of the model (2´3 °) does not correspond with that of aSCIAMACHY pixel and, thus, the correction can never be perfect (Dils et al., 2006).In order to avoid any additional uncertainty introduced by using model data we developed a statistical altitudecorrection using a big ensemble of individual SCIAMACHY XCH4 measurements. A linear fit is performed toindividual-pixel SCIAMACHY XCH4 values plotted as a function of ground-pixel altitude (Fig. 2). The difference inXCH4 derived from this Figure for 745 m a.s.l. and 2964 m a.s.l. is 0.52 %, and agrees very well to the bias of 0.30 %derived from coincident ground-based soundings of the Garmisch FTIR (745 m a.s.l.) and the Zugspitze FTIR (2964m a.sl.). I.e., the altitude dependency of SCIAMACHY is consistent with the ground-based FTIR and thus can be usedfor the altitude correction of FTIR mountain stations (Jungfraujoch, Zugspitze, Izana, see Table 1).4.3 Treatment of intercomparison errorIn order to reduce intercomparison errors, the retrieved FTIR CH4 vertical profiles (dofs ≈2-3) were folded by thetotal column averaging kernels from the SCIAMACHY retrievals (dofs = 1).5 VALIDATION RESULTSCorrelative validation data sets (daily means - satellite versus ground) have been set up separately for- IMAP-DOAS v49 and WFM-DOAS v1.0- 3 different pixel selection radii (200 km, 500 km, 1000 km around each FTIR station)- 2003 and 2004 data- 13 FTIR stationsSo there are 2´3´2´13 = 156 validation data sets. We grouped the 156 validation data sets into 52 validation plots eachcontaining the 3 SCIA data sets for the three selection radii (200 km, 500 km, 1000 km). See Fig. 4 below for someexamples. From each of the 156 data sets the statistics of the diurnal variation, the pixel to-pixel variation, day-to-dayvariability, and biases has been derived.one-column lay out Millpress4 von 6 15.10.2015 14:31Fig. 4. Validation plots SCIAMACHY versus FTIR (daily means; SCIAMACHY data are plotted for 3 different selection radii around eachFTIR ground station).one-column lay out Millpress5 von 6 15.10.2015 14:31The latitudinal distribution of the biases is shown in Fig. 5. Further validation results are given in the Summary.Fig. 5. The bias of each FTIR station relative to SCIAMACHY (IMAP-DOAS v49, 500 km selection radius) plotted against the station’slatitude.SUMMARY AND RESULTSA station-to-station harmonization of XCH4 retrievals from 13 globally distributed ground-based mid-infrared FTIRspectrometry stations of the NDACC network has been performed. The single pixel column measurement precision ofSCIAMACHY XCH4 is »1.5-2.5 % and has improved by a factor 2-4 since 2005 (rel. to WFM-DOAS v0.41).Improved SCIA precision allows to retrieve day-to-day variability (0.7-1.5 %). No significant correlation of theannual cycles between SCIA (“frown”-shaped) and FTIR (“smiley” shape) can be found above clean-air NDACCstations. There is a significant high bias of SCIAMACHY XCH4 of » 2.8±1.4 % (1 sigma uncertainty) relative to theharmonized g.-b. FTIR data set. There is no significant latitudinal dependency of the bias. No significant difference indata quality between IMAP-DOAS v49 and WFM-DOAS v1.0 could be found. WFMD data selection is stricterwhich leads to a slightly better single-pixel precision at the cost of fewer data.ACKNOWLEDGMENTSFunding by the Bavarian Ministery of Economy and the European Commission via the project HYMN (contract037048), and GEOMON (contract 036677) is gratefully acknowledged. F. Forster thanks for funding by the GermanSpace Agency (DLR) within the project SatVal-A (DLR 50EE 0702) via contract with BIfA Augsburg.REFERENCESDils, B., De Mazière, M., Müller, J. F., Blumenstock, T., Buchwitz, M., de Beek, R., Demoulin, P., Duchatelet, P., Fast, H., Frankenberg, C.,Gloudemans, A., Griffith, D., Jones, N., Kerzenmacher, T., Kramer, I., Mahieu, E., Mellqvist, J., Mittermeier, R. L., Notholt, J., Rinsland,C. P., Schrijver, H., Smale, D., Strandberg, A., Straume, A. G., Stremme, W., Strong, K., Sussmann, R., Taylor, J., van den Broek, M.,Velazco, V., Wagner, T., Warneke, T., Wiacek, A., and Wood, S. 2006. Comparisons between SCIAMACHY scientific products andground-based FTIR data for total columns of CO, CH4, CO2 and N2O. Atmos. Chem. Phys. 6: 1953-1976.Frankenberg C., Bergamaschi, P., Butz, A., Houweling, S., Meirink, J. F., Notholt, J., Petersen, A. K., Schrijver, H., Warneke, T., and Aben, I.2008. Tropical methane emissions: A revised view from SCIAMACHY onboard ENVISAT. Geophys. Res. Lett. 35: L15811.Schneising, O., Buchwitz, M., Burrows, J. P., Bovensmann, H., Bergamaschi, P., and Peters, W. 2009. Three years of greenhouse gas column-averaged dry air mole fractions retrieved from satellite – Part 2: Methane. Atmos. Chem. Phys. 9: 443-465.Sussmann, R., Stremme, W. Buchwitz, M., and de Beek, R. 2005. Validation of ENVISAT/SCIAMACHY columnar methane by solar FTIRspectrometry at the Ground-Truthing Station Zugspitze. Atmos. Chem. Phys. 5: 2419–2429.Sussmann., R., Forster, F., Borsdorff, T., Dils, B., De Mazière, M., Vigouroux, C., Blumenstock, T., Buchwitz, M., Burrows, J.P., Duchatelet,P., Frankenberg, C., Hannigan, J., Hase, F., Jones, N., Klyft, J., Mahieu, E., Mellqvist, J., Notholt, J., Petersen, K., Schneising, O., Strong,K., Taylor, J. 2009. A novel Tikhonov-based approach for harmonized high-accuracy retrieval of methane columns and profiles fromNDACC FTIR network measurements. Application to global validation of ENVISAT/SCIAMACHY biases, Geophysical ResearchAbstracts, Vol. 11, EGU2009-7869-2, 2009, http://meetingorganizer.copernicus.org/EGU2009/EGU2009-7869-2.pdf, EGU GeneralAssembly 2009 (Talk).Sussmann., R., Forster, F., Borsdorff, T., Dils, B., De Mazière, M., Vigouroux, C., Blumenstock, T., Buchwitz, M., Burrows, J.P., Duchatelet,P., Frankenberg, C., Hannigan, J., Hase, F., Jones, N., Klyft, J., Mahieu, E., Mellqvist, J., Notholt, J., Petersen, K., Schneising, O., Strong,K., Taylor, J. 2009. Satellite validation of column-averaged methane on global scale: ground-based data from 13 FTIR stations versus lastgeneration ENVISAT/SCHIAMACHY retrievals, in preparation 2009.one-column lay out Millpress6 von 6 15.10.2015 14:31

Satellite validation of column-averaged methane on global scale: Harmonized data from 13 FTIR ground stations versus last generation ENVISAT/SCIAMACHY retrievals

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Satellite validation of column-averaged methane on global scale: Harmonized data from 13 FTIR ground stations versus last generation ENVISAT/SCIAMACHY retrievals

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