Microsoft Word - mipas-val_v3_shortref.doc ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6329 1 Long-term intercomparison of MIPAS additional species ClONO2, N2O5, CFC-11, and CFC-12 with MIPAS-B measurements GERALD WETZEL, HERMANN OELHAF, FELIX FRIEDL- VALLON, ANNE KLEINERT, GUIDO MAUCHER, HANS NORDMEYER, JOHANNES ORPHAL Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Karlsruhe, Germany gerald.wetzel@kit.edu I. INTRODUCTION he Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) aboard the environmental satellite ENVISAT is a limb-viewing Fourier-transform emission spec- trometer working in the mid-infrared spectral region between 685 cm-1 and 2410 cm-1 [Fischer et al. 2008]. During the ten years of operation, MIPAS was able to observe temperature and a large number of climate-relevant trace gases from the upper troposphere up to the meso- sphere (nominal observation mode) and even the thermosphere (special modes). For about two years after the launch of the satellite on 1 March 2002, MIPAS recorded spectra with full spectral resolution of 0.025 cm-1. After anoma- lies in the velocity of the interferometer drive unit and a subsequent interruption of the measurements (March 2004), a lower (so-called optimized) spectral resolution of 0.0625 cm-1 together with a higher spatial resolution was chosen such that operational observations could restart in January 2005. Following the loss of communications with the satellite plat- form, there are no MIPAS data available since 8 April 2012. The description and characteriza- tion of the operational European Space Agency (ESA) processor products have been presented by Raspollini et al. [2006, 2013] and references therein. While previous operational product processing was limited to temperature and the molecules H2O, O3, HNO3, CH4, N2O, and NO2, the new ML2PP V6 data includes also the minor “additional” trace gases CFC-11 (CCl3F), CFC-12 (CCl2F2), N2O5, and ClONO2. In this paper we focus on the comparison of these ad- ditional species. Several flights with MIPAS-B, the balloon ver- sion of MIPAS, have been carried out during the operational period of ENVISAT. Table 1 gives an overview of MIPAS-B flights where MIPAS-B observations coincide in space and time with MIPAS on ENVISAT (MIPAS-E) measurements. MIPAS-B limb sequences re- corded during these flights are compared to the corresponding MIPAS-E overpasses. MIPAS-B was strongly involved in the ENVISAT validation activities and the high ac- curacy of the MIPAS-B data has been assessed in numerous MIPAS-E related validation stud- ies [see, e.g., Cortesi et al. 2007, Höpfner et al. 2007, Milz et al. 2009, Payan et al. 2009, Ridolfi et al. 2007, Steck et al. 2007, Wang et al. 2007a, 2007b, Wetzel et al., 2007, 2013, Zhang et al. 2010a, 2010b]. T ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6329 2 II. MIPAS-B DATA ANALYSIS AND INTERCOMPARISON METHOD MIPAS-B can be regarded as a precursor of the satellite instrument [see Friedl-Vallon et al., 2004 and references therein]. Therefore, a number of specifications are quite similar, such as spectral resolution and spectral coverage. However, for some essential parameters, the MIPAS-B performance is superior, e.g. in terms of the NESR (Noise Equivalent Spectral Radi- ance), and in the case of the pointing accuracy. Further improvement of the NESR is achieved by averaging multiple spectra taken at the same pointing angle. MIPAS-B measures all atmospheric parameters covered by MIPAS-E. Essential for the balloon instrument is the so- phisticated line of sight stabilization system, which is based on an inertial navigation sys- tem and supplemented with an additional star reference system leading to an after all knowl- edge of the tangent altitude in the order of 90 m (3 σ). The MIPAS-B data processing from interferograms to calibrated spectra including instrument characterization is described in Friedl-Vallon et al. [2004] and references therein. The measurements were done typi- cally at a 1.5 km vertical grid. Retrieval calcu- lations of atmospheric target parameters were performed at a 1 km grid with a least squares fitting algorithm using analytical derivative spectra calculated by the Karlsruhe Optimized and Precise Radiative transfer Algorithm [KOPRA; Stiller et al. 2002; Höpfner et al. 2002]. A Tikhonov-Phillips regularization ap- proach [Phillips, 1962, Tikhonov, 1963] con- straining with respect to a first derivative a priori profile was adopted. The resulting verti- cal resolution is typically between 2 and 5 km for the analyzed species and is therefore com- parable to or slightly better than the vertical resolution of MIPAS-E. The species ClONO2, CFC-11, CFC-12, and N2O5 are analyzed in the spectral windows 779.7 - 780.7 cm-1, 840 - 860 cm-1, 918 - 924 cm-1, and 1220 - 1270 cm-1, re- spectively. Different spectral windows within these molecular bands were used for the MIPAS-E data analysis [Raspollini et al. 2013]. Spectroscopic parameters for the calculation of limb emission spectra originate from the high- resolution transmission (HITRAN) molecular absorption database [Rothman et al. 2005]. The MIPAS-B error estimation includes random noise as well as the mutual influence (covari- ance) of the fitted parameters, temperature er- rors, pointing inaccuracies, errors of non- simultaneously fitted interfering gases, and spectroscopic data errors (1 σ). For details on the MIPAS-B data analysis and error estima- tion, see Wetzel et al. [2012] and references therein. Since pressure is the primary vertical coordi- nate of MIPAS-E, vertical profiles of the satel- lite sensor have been interpolated to the MIPAS-B pressure-altitude grid. Differences between measured volume mixing ratios (VMRs) of MIPAS-E and the validation in- strument MIPAS-B are expressed in absolute units and relative differences. The mean differ- ence Δxmean for N profile pairs of compared ob- servations is given as: )( 1 , 1 , nB N n nEmean xx N x    (1) where xE and xB are VMR values of MIPAS-E and MIPAS-B at one altitude level. The mean relative difference Δxmean,rel of a number of pro- file pairs is calculated by dividing the mean absolute difference by the mean profile value of the validation instrument MIPAS-B: ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6329 3 %100 1 1 , ,      N n nB mean relmean x N x x . (2) Differences are displayed together with the combined errors σcomb of both instruments which are defined as: 22 BEcomb   , (3) where σE and σB are the precision, systematic or total errors of MIPAS-E and the validation in- strument, respectively. Precision errors characterize the reproducibil- ity of a measurement and correspond, in gen- eral, to random noise errors. Systematic errors of the V6 processor used for MIPAS-E data have been assessed in a corresponding ESA study [Raspollini et al. 2013]. The uncertainty of the calculated mean difference (standard er- ror of the mean, SEM) is given by σ/N0.5 where σ is the standard deviation (SD). A bias be- tween both instruments is considered signifi- cant if the SEM is smaller than the bias itself. The comparison between the VMR difference and the combined systematic error (for statisti- cal comparisons) or total error (for single com- parisons) is appropriate to identify unex- plained biases in the MIPAS-E observations when they exceed these combined error limits. Since the vertical resolution of the used VMR profiles of both instruments is of comparable magnitude, no smoothing by averaging ker- nels has been applied for the intercomparison of the observed profiles. III. INTERCOMPARISON RESULTS Table 1: Overview of MIPAS-B flights used for inter- comparison with MIPAS-E. Distances and times between coincident trace gas profile pairs observed by MIPAS-E and the validation instrument refer to an altitude of 20 km (Kiruna) and 30 km (Aire-sur-l’Adour and Teresina). Location Date Distance (km) Time dif- ference (min) 20 Mar 2003 16/546 14/15 11 Mar 2009 187/248 5/6 Kiruna (Sweden, 68°N) 24 Jan 2010 109/302 5/6 Aire-sur- l’Adour (France, 44°N) 24 Sep 2002 21/588/4 10/146 12/13/15 /16 14 Jun 2005 109/497/ 184/338 228/229/ 268/269 Teresina (Brazil, 5°S) 06 Jun 2008 224/284/ 600/194 157/158/ 169/170 In this section, ESA operationally processed (ML2PP V6) MIPAS-E VMR profiles of the molecules ClONO2, N2O5, CFC-11, and CFC-12 are compared to coincident MIPAS-B observa- tions. Table 1 summarizes corresponding bal- loon flights in terms of date, time, locations, and differences in time and space to the satel- lite measurements. Mean deviations of all coincident ClONO2 pro- files measured by MIPAS-B and MIPAS-E be- tween 2002 and 2010 are shown in Figure 1. Up to 18 collocations are available per altitude. The agreement between the measured profiles appears to be quite good with a mean devia- tion over all altitudes of 7.2 %. Some biases are visible in limited altitude regions but they are not significant with regard to the combined er- ror limits. Figure 2 displays the differences concerning the molecule N2O5. This molecule exhibits a pronounced diurnal variation in its mixing ra- ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6329 4 tio. Hence, a photochemical correction using a box model [Wetzel et al. 1995], which takes into account the nighttime variation of N2O5, has been applied to the tropical MIPAS-E measurements where both, diurnal variation and the time difference of the observations (see Table 1), are quite large. Apart from the low- ermost altitude region between 15 and 17 km (where only 2 collocations are available) there is no significant bias visible between the bal- loon and satellite measurements of MIPAS. The overall agreement between both sensors is fairly good with a mean deviation of -6.7 %. The comparison for the species CFC-11 is shown in Figure 3. In contrast to the previ- ously discussed molecules, significantly higher values are visible in the MIPAS satellite data in the complete altitude region under compari- son. While below 19 km this positive bias is still within the combined systematic and total errors, it clearly exceeds these error limits above and therefore remains unexplained. Figure 4 displays the observed differences for the molecule CFC-12. Unlike the case of CFC- 11, the agreement of measured CFC-12 VMR is quite good with a mean difference of 11.5 %. No significant biases are visible at all. IV. CONCLUSIONS Vertical profiles of six MIPAS balloon flights between 2002 and 2010 have been used for an intercomparison study of the species ClONO2, N2O5, CFC-11, and CFC-12, which have been processed for the first time by the operational ESA processor (ML2PP V6). For the molecules ClONO2, N2O5, and CFC-12 the agreement between both MIPAS instru- ments is generally good. No clear biases in the observed mixing ratios are visible. Further- more, no seasonal or latitudinal dependence of deviations could be recognized. However, standard deviations are slightly larger than the combined precision errors. Concerning the species CFC-11 a significant positive bias is evident at all altitudes under comparison. Above 19 km, this bias clearly exceeds the combined systematic and total errors and re- mains unexplained. Hence, this molecule can- not be recommended for scientific users at the present state. Investigations are ongoing within the ESA processor development team to understand the reason of the bias. It should also be mentioned that some single MIPAS-E profiles of these additional species exhibit retrieval oscillations which show up in VMR differences (in limited altitude regions) to MIPAS-B observations but which may in turn be smoothed out in comparisons of mean differences. Scientific users should therefore be careful in their interpretation when using sin- gle MIPAS-E profiles of these trace gases. 40 35 30 25 20 15 10 -0.2 -0.1 0.0 0.1 0.2 -0.2 -0.1 0.0 0.1 0.2 -40 -20 0 20 40 40 35 30 25 20 15 10 -40 -20 0 20 40 100 10 2 2 6 6 6 8 10 10 10 10 12 12 14 16 16 16 18 18 18 18 18 18 18 18 18 18 18 2 2 6 6 6 8 10 10 10 10 12 12 14 16 16 16 18 18 18 18 18 18 18 18 18 18 18 A lt itu d e ( km ) A lt itu d e ( km ) MIPAS-E - MIPAS-B Mean diff.: 0.02 ± 0.01 ppbv 7.2 ± 4.0 % difference ( sd) prec. systematic tot. mean comb. err. ClONO 2 VMR Difference (ppbv) ClONO 2 VMR Difference (%) P re ss u re ( h P a ) Figure 1: Mean absolute and relative ClONO2 VMR differences of all collocations between MIPAS-E and MIPAS-B (red solid lines) including standard deviation (red dotted lines) and the standard error of the mean, plotted as error bars around the mean deviation together with precision (blue dotted lines), systematic (blue dash- dotted lines) and total (blue dashed lines) mean combined errors. Red values indicate the number of collocations used for the statistical analysis. Differences of ClONO2 mixing ratios are well within the combined error limits. ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6329 5 40 35 30 25 20 15 10 -0.4 -0.2 0.0 0.2 0.4 -0.4 -0.2 0.0 0.2 0.4 -60 -40 -20 0 20 40 60 40 35 30 25 20 15 10 -60 -40 -20 0 20 40 60 100 10 2 2 2 2 4 8 10 10 10 10 12 14 14 14 16 16 18 18 18 18 18 18 18 18 18 2 2 2 2 4 8 10 10 10 10 12 14 14 14 16 16 18 18 18 18 18 18 18 18 18 A lt itu d e ( km ) A lt itu d e ( km ) MIPAS-E - MIPAS-B Mean diff.: -0.02 ± 0.02 ppbv -6.7 ± 16.7 % difference ( sd) prec. systematic tot. mean comb. err. N 2 O 5 VMR Difference (ppbv) N 2 O 5 VMR Difference (%) P re ss u re ( h P a ) Figure 2: Same as Figure 1, but for the molecule N2O5. The overall agreement of both sensors is fairly good. 40 35 30 25 20 15 10 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 -40 -20 0 20 40 40 35 30 25 20 15 10 -40 -20 0 20 40 100 10 2 2 4 6 7 9 9 10 10 10 10 10 12 14 15 16 17 17 18 18 18 18 2 2 4 6 7 9 9 10 10 10 10 10 12 14 15 16 17 17 18 18 18 18 A lti tu d e ( km ) A lti tu d e ( km ) MIPAS-E - MIPAS-B Mean diff.: 0.032 ± 0.003 ppbv 134.8 ± 66.2 % difference ( sd) prec. systematic tot. mean comb. err. CFC-11 VMR Difference (ppbv) CFC-11 VMR Difference (%) P re ss u re ( h P a ) Figure 3: Same as Figure 1, but for the species CFC-11. A positive bias is visible in the MIPAS-E data, especially above 19 km. 40 35 30 25 20 15 10 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 -0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 -40 -20 0 20 40 40 35 30 25 20 15 10 -40 -20 0 20 40 100 10 2 2 4 7 9 9 10 10 10 12 12 12 14 14 15 16 16 16 17 17 18 18 18 18 18 18 18 18 18 18 18 2 2 4 7 9 9 10 10 10 12 12 12 14 14 15 16 16 16 17 17 18 18 18 18 18 18 18 18 18 18 18 A lt itu d e ( k m ) A lt itu d e ( k m ) MIPAS-E - MIPAS-B Mean diff.: 0.001 ± 0.003 ppbv 11.5 ± 9.4 % difference ( sd) prec. systematic tot. mean comb. err. CFC-12 VMR Difference (ppbv) CFC-12 VMR Difference (%) P re ss u re ( h P a ) Figure 4: Same as Figure 1, but for the molecule CFC- 12. No significant bias is obvious between the measured profiles of both MIPAS instruments. ACKNOWLEDGEMENTS Financial support by the national aeronautics and space research centre DLR (Project 50EE0020) and ESA for the MIPAS-B balloon flights is gratefully acknowledged. 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