ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6333 Results of the preparat- ory study “PREMIER Analysis of Campaign Data” ELISA CASTELLI1,*, SAMUELE DEL BIANCO2, BIANCA M. DINELLI1, DANIEL GERBER3, HERMANN OELHAF4, WOLFGANG WOIWODE4, BÄRBEL VOGEL5, BJÖRN-MARTIN SINNHUBER4, ROLAND RUHNKE4, UGO CORTESI2 1Istituto di Scienze dell’Atmosfera e del Clima del Consiglio Nazionale delle Ricerche (ISAC-CNR), Bologna, Italy, 2Istituto di Fisica Applicata “N. Carrara” del Consiglio Na- zionale delle Ricerche (IFAC-CNR), Firenze, Italy, 3Rutherford Appleton Laboratory (STFC-RAL), Chilton, UK, 4Institute for Meteorology and Climate Research (IMK-ASF), Karlsruhe Institute of Technology (KIT), Karlsruhe, Germa- ny, 5Institute for Energy and Climate Research - Stratosphere (IEK-7), Forschungszentrum Jülich GmbH (FZJ), Jülich, Ger- many e.castelli@isac.cnr.it I. INTRODUCTION he relevance of the UTLS (Upper Tropo- sphere and Lower Stratosphere) region and the impact of limb emission measurements at millimetre and sub-millimetre wavelengths for investigation of chemical, dynamical and radiative processes occurring at these altitudes constitute a major focus of the atmospheric re- T * Corresponding author: Elisa Castelli, Istituto di Scienze dell’Atmosfera e del Clima (ISAC-CNR), Bologna, Italy, email: e.castelli@isac.cnr.it search activities supported by the European Space Agency (ESA) in the last two decades. Since the end of the 1990’s, several studies were conducted on this topic, like those related to the design of the SOPRANO (Submillimetre- wave Observations of PRocesses in the Atmo- sphere Noteworthy for Ozone) and MASTER (Millimetre-wave Acquisitions for Strato- sphere/Troposphere Exchange) instruments [Bühler, 1999], the latter originally conceived for the ESA Explorer mission ACECHEM (At- mospheric Composition Explorer for CHEM- 1 mailto:e.castelli@isac.cnr.it ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6333 istry and climate interaction). Further evolu- tion of the scientific objectives and of the meas- urement concept originally proposed for ACECHEM lead to the selection of the PREMI- ER (Process Exploration through Measure- ments of Infrared and millimetre-wave Emit- ted Radiation) mission as one of the three can- didate core missions of ESA Earth Explorer 7. Within the scientific context of PREMIER, first priority is assigned to the currently open is- sues on the sensitivity of surface climate to UTLS variability and to global circulation, and on the troposphere/stratosphere exchange pro- cesses. PREMIER will address these topics us- ing atmospheric composition data obtained in 3D by an infrared limb-imaging spectrometer and by a mm-wave limb sounder [ESA, 2012] with unprecedented spatial and temporal res- olution. As part of the PREMIER preparatory studies, the PACD project (PREMIER Analysis of Campaign Data) aimed at the analysis of the measurements acquired by the MARSCHALS (Millimetre wave Airborne Receivers for Spec- troscopic CHaracterization in Atmospheric Limb Sounding) instrument, an airborne pre- cursor of the millimetre-wave spectrometer of the PREMIER payload, during the field cam- paigns PREMIER-Ex (Kiruna, Sweden, March 2010) and ESSenCe (Kiruna, Sweden, Decem- ber 2011) with the M-55 Geophysica high alti- tude research aircraft. The scope of the project was to demonstrate the capabilities of MARSCHALS to measure the vertical distribu- tion of atmospheric targets (namely temperat- ure, H2O, O3, HNO3, N2O, and CO) by the in- verse processing of its limb sequences. Moreover, the study included the evaluation of the potential synergy between limb measure- ments in the infrared and in the milli- metre-wave region and the comparison of MARSCHALS retrieval products with the state of the atmosphere simulated using two atmo- spheric chemical transport models: CLaMS (Chemical Lagrangian Model of the Strato- sphere) (e. g., [Grooß et al., 2005], [Konopka et al., 2010], and references therein) and EMAC (ECHAM/MESSy Atmospheric Chemistry) [Jöckel et al., 2006]. In this paper, we provide an overview of the activities covered by the PACD project and present the consolidated results for the analys- is of MARSCHALS measurements, for the comparison with models and for the synergy with infrared measurements, the latter ob- tained exploiting the measurements of MIPAS- STR, an infrared interferometer on board the same aircraft. Full details on the results from investigation of the individual aspects ad- dressed by the PACD project are reported in dedicated papers, already published [Castelli et al., 2013] or currently in preparation. II. THE MARSCHALS INSTRUMENT MARSCHALS is a millimetre-wave hetero- dyne spectrometer that operates on board the M-55 Geophysica as a demonstrator of the mil- limetre-wave limb sounder for the proposed Earth Explorer 7 Mission PREMIER. It acquires atmospheric emission spectra over three spec- tral bands around 300 GHz (Band B), 325 GHz (Band C) and 345 GHz respectively (Band D). Limb scans consecutively iterate over single spectral bands. A description of the spectro- meter can be found in Oldfield et al., [2001] and in Dinelli et al., [2009], while Castelli et al., [2013] report on the instrument upgrades per- formed before the PREMIER-Ex and the ES- SenCe campaigns. The MARC code (Millimetre-wave Atmo- spheric Retrieval Code, [Carli et al., 2007]) was designed for the analysis of MARSCHALS measurements. The code can retrieve vertically distributed quantities such as Temperature, Volume Mixing Ratios (VMRs) and external continuum (a retrievable profile used to ac- 2 ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6333 count for continuum effects from cloud opaque in the millimetre-wave frequency range, [Dinelli et al., 2009], [Castelli et al., 2013]), as well as scalar quantities like frequency shift, pointing bias and radiative gain and offset. All spectra acquired during a limb scan are ana- lyzed simultaneously. The following list de- notes all the different species measured by MARSCHALS, grouped together in corres- ponding frequency bands. Band B: O3 and N2O; Band C: Temperature, H2O, O3 and HNO3; Band D: HNO3, O3 and CO. Band B con- tains some information on HNO3, but it is not sufficient to produce significant results. The external continuum, which is retrieved for every scan, provides information on the im- pact of clouds on MARSCHALS measure- ments. MARSCHALS has been deployed on board the M-55 Geophysica in several meas- urement campaigns: SCOUT-O3 (2005) in the tropics [Dinelli et al., 2009]; PREMIER-Ex Test Campaign (2009) at mid-latitudes; PREMI- ER-Ex Science Campaign (2010) and ESSenCe Campaign (2011), both at polar latitudes. In the following sections we describe the results ob- tained by MARSCHALS during the PREMIER- Ex and ESSenCe Science campaigns. III. MEASUREMENT CAMPAIGNS RESULTS III.I PREMIER-EX CAMPAIGN: FLIGHT ON 10 MARCH 2010 FROM KIRUNA (SWEDEN) The PREMIER-Ex campaign took place on the 10 March 2010 from Kiruna (Sweden) and aimed at the quantification of horizontal mix- ing processes in the UTLS. The flight track fol- lowed a triangular pattern, aiming toward the West and then turning back to Kiruna. During the flight MARSCHALS sampled air masses of different origin (polar vortex as well as mid- latitude and mixed air masses). From MARSCHALS spectra we retrieved VMR pro- files of temperature, H2O, O3, HNO3, N2O, CO. In particular, the HNO3 retrieved profiles along the flight show that the MARSCHALS instrument was able to resolve regions of vor- tex and filament of vortex air. The results ob- tained during this campaign have been thor- oughly described in Castelli et al., [2013]. During the campaign the MIPAS-STR instru- ment was deployed aboard the M-55 Geophys- ica [Woiwode et al., 2012], as a demonstrator of the infrared limb sounder instrument pro- posed for the Earth Explorer 7. MIPAS-STR is a high resolution Fourier-Transform limb sounder with an unapodized spectral resolu- tion of 0.036 cm-1. The spectral channel 1 util- ized for the retrievals considered here covers the spectral range from 725 to 990 cm-1. De- pending on the measurement scenario and flight altitude, typically one limb scan is ob- tained each 25 to 45 km along flight track. The MIPAS-STR measurements are inverted using the Tikhonov-Phillips regularization method, yielding vertical profiles of the target paramet- ers with typical vertical resolutions between 1.0 to 2.5 km between flight altitude and the lowest tangent point (depending on the re- trieval target). During the flight, MIPAS-STR detected dense clouds in the first leg below 8 km and in the final leg up to an altitude of 10 km. H2O, O3 and HNO3 profiles retrieved from MARSCHALS were then compared with MI- PAS-STR results, using only the collocated res- ults. Since the synergy between IR and mm- wave measurements is one of the main charac- teristic of the PREMIER mission, it was ex- plored using the VMR profiles retrieved from the two limb sounders, using two data fusion techniques: the Level 1 plus Level 2 technique (L1+L2) and the Measurement Space Solution (MSS) method [Ceccherini et al., 2009]. 3 ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6333 Table 1: DOFs, GI increase in % and total retrieval error reduction in % with respect to MIPAS-STR obtained from the (L1+L2) data fusion method for H2O, O3, and HNO3 in each MARSCHALS Band. Quantifier Band H2O O3 HNO3 DOFs B 2 7 negligible C 8 5 2 D 2 4 3 GI increase [%] B 3 13 negligible C 11 7 8 D 3 5 12 Total error reduction (10-20 km range) [%] B negligible 60 negligible C 50 60 30 D negligible 60 30 Figure 1: Comparison of number of O3 DOFs from individual retrievals for MARSCHALS Band B (red), MIPAS-STR (green) with the number of DOFs from synergistic retrieval performed using the L1+L2 (blue) and the MSS (purple) methods. III.I.I INVESTIGATION OF POTENTIAL SYNERGY BETWEEN MM-WAVE AND INFRARED LIMB-SOUNDING DURING THE PREMIER-EX CAMPAIGN: L1+L2 AND MSS METHODS The potentiality of data fusion between mm- wave and infrared limb-sounding of the UTLS was investigated combining collocated data ac- quired by MARSCHALS and by MIPAS-STR and comparing the quality of the products from individual and synergistic retrieval of se- lected targets, i.e. H2O, O3, and HNO3. The per- formance of the different methods was estim- ated using a set of quality quantifiers that in- cluded the Degrees of Freedom (DOF) of the retrievals, the Gain of Information (GI) (both described in Rodgers, [2000]) and the total re- trieval error comprising the random and sys- tematic components. Both data fusion meth- ods, L1+L2 and MSS, were tested and a brief description of the results is given below. The L1+L2 method consists in repeating the analysis of MARSCHALS Level 1 (L1) data us- ing the Optimal Estimation (OE) method 4 ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6333 [Rodgers, 2000] with the MIPAS-STR Level 2 (L2) products (with the associated covariances) used as a priori information. The MSS method is a post-retrieval processing and it has been thoroughly described in Ceccherini et al., [2009, 2010]. This method is based on the concept that a retrieved profile is the sum of a component belonging to the measurement space, the MSS, and a component belonging to the null space. The observations only provide information on the MSS and leave completely undetermined the component belonging to the null space. If we compute the MSS in the space that is the sum of the two independent atmo- spheric measurements spaces, we obtain the fused data. This new MSS includes all the in- formation contained in the observations without any bias due to the a priori informa- tion used in the single retrievals. The L1+L2 data fusion was performed on H2O, O3, and HNO3 VMR vertical profiles retrieved from the best matching sequences of MARSCHALS and MIPAS-STR. Table 1 sum- marizes the results obtained for each retrieval target separated for the three MARSCHALS bands (B, C and D). Since MIPAS-STR L2 data are used as a priori, we report the improve- ments due to data fusion to those data. A signi- ficant improvement is found for H2O in Band C for all the three considered quantifiers, while for the other two bands we have a slightly lower improvement for the DOF and GI. For O3 we found an improvement (with a reduc- tion of the total error of up to 60%) in all spec- tral bands with the best results obtained for the GI and DOF in Band B. In the case of HNO3 re- trievals, as expected, we did not obtain any im- provement from Band B since there is negli- gible information on the HNO3 in this band. However an improvement is obtained in both Bands C and D with a total error reduction in the altitude range from 10 to 20 km of up to 30%. The evaluation of the performances of the MSS method has been performed for the O3 VMR only. Here, we briefly report a direct comparis- on of the results obtained in Band B, highlight the basic differences between the two ap- proaches and describe the potentialities of syn- ergy between mm-wave and infrared limb- sounding measurements. In Figure 1 the number of DOFs from indi- vidual retrievals is compared with the number of DOFs from synergistic retrieval performed using the L1+L2 and the MSS methods. We re- call that the number of DOFs obtained using the L1+L2 method represents the independent information that can be retrieved from MARSCHALS measurements in addition to the a priori knowledge provided by MI- PAS-STR L2 data, while the MSS number of DOFs are related to the fused data. The im- provement obtained with the MSS approach is evident. This is due to the fact that the MSS ap- proach is capable to evaluate the contribution of the measurement space component with no impact of the used external constraints, such as the Tikhonov regularization used for the MI- PAS-STR retrievals [Woiwode et al., 2012], or the a priori information used in MARSCHALS retrievals. The conclusions of this exercise are that the ap- plication of the L1+L2 method for the mm- wave and infrared measurements qualitatively demonstrates the improvements due to the synergy between the different techniques, but quantitative comparisons are difficult since the characteristics of the different retrieval meth- ods are not fully compatible. In contrast, the MSS method allows for a more clear comparis- on of the individual results and the results of the data synergy, since the measured compon- ents from the different techniques are treated in a similar way and the same external con- straint is applied to both. The investigation of potential synergy between mm-wave and in- 5 ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6333 frared limb-sounding measurements shows that the fused products (obtained with both MSS or L1+L2 techniques) provide an im- provement with respect to the single sensor re- trieval. In particular, from Figure 1 the im- provement given by the use mm-wave with re- spect to infrared measurements in case of pres- ence of high altitude clouds is evident (last part of the flight, from MARSCHALS scan 31 onward). III.II ESSENCE CAMPAIGN: FLIGHT ON 16 DECEMBER 2011 FROM KIRUNA (SWEDEN) The ESSenCe campaign consisted of two flights. The first flight was performed on the 11 December 2011 at the boundary of the Arctic polar vortex while the second flight was per- formed on the 16 December 2011 inside the vortex. During the second flight the M-55 fol- lowed a rectangular flight pattern. MARSCHALS spectra acquired during the second ESSenCe flight were used to retrieve VMR profiles of temperature, H2O, O3, HNO3, N2O, and CO inside the Arctic vortex. These profiles are compared with results of the chem- istry transport models CLaMS and EMAC. III.II.I COMPARISON BETWEEN MARSCHALS AND MODEL RESULTS FROM CLAMS AND EMAC In order to assess the capability of the MARSCHALS instrument to resolve UTLS processes the retrieval results are compared to results from established models that allow to characterize the atmospheric conditions ob- served by measurements. While the primary goal of this cross-validation is to prove that at- mospheric structures are represented ad- equately by the MARSCHALS results, these results in turn provide information to which degree these structures and the associated pro- cesses are captured by the models. The EMAC model is a numerical chemistry and climate simulation system used to investigate the past and future evolution of the atmosphere. The model includes sub-models for the description of tropospheric and middle atmosphere pro- cesses and their interaction with oceans, land and human influences. The CLaMS model, ori- ginally developed for the stratosphere, was ex- tended to the troposphere and can be used to investigate atmospheric processes in high res- olution studies. Both models are well suited for the comparisons with the MARSCHALS data and complements each other. The comple- mentarity is due to different model resolutions and initialization, to differences in treatment of tropospheric processes and to differences in model architecture/transport scheme, which allows to identify shortcomings in the data sets. The comparison between MARSCHALS measurements and model results has been per- formed through the use of a common grid hereafter indicated as the Vertical and Hori- zontal Grid of the Comparison or VHGC. The models results were extracted on their own ho- rizontal (2.8°x2.8° for EMAC and about 100 km for CLaMS) and vertical resolution and were interpolated considering the horizontal distri- bution of the MARSCHALS tangent point geo- locations. The VHGC ranges from 0 km to 26 km, in steps of 0.5 km, with spatial (longitude, latitude, altitude) and temporal coordinates as- sociated to each MARSCHALS limb scan. For each altitude level of the VHGC in the range between the lowest and highest MARSCHALS tangent points, we used the interpolated time, latitude and longitude of the tangent points immediately above and below the selected alti- tude, while for each altitude level of the VHGC 6 ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6333 Figure 2: From Top to bottom: EMAC (left) and CLaMS (center) results after convolution with MARSCHALS AK and MARSCHALS retrieved VMRs (right) for O3, HNO3, H2O and N2O. above (or below) the highest (or the lowest) MARSCHALS tangent point, we used the time, latitude and longitude values of the highest (or of the lowest) MARSCHALS tangent point. The models provide the atmospheric status on a very fine vertical grid. Retrieved data are ob- tained on a coarser grid, optimized for each target. In order to compare the model data with the retrieved vertical profiles we need to take into account the smoothing introduced by the retrieval procedure due to the information content of the MARSCHALS measurements. Using the method described in Ceccherini, [2012] we can take into account the retrieval ef- fect on the model profiles using the expression reported below: xmodel=AKh xmodel,h+(I-AK)xa+AK x̂ -AKh x̂h (1) where xmodel is the vertical profile obtained by the models and reported on the vertical grid of the retrieval, xmodel,h is the vertical profile provided by the models on the high resolution altitude grid, x̂ is the MARSCHALS retrieved profile on the vertical grid of the retrieval, x̂h is the retrieved profile interpolated on the high resolution altitude grid, xa is the a priori profile represented on the vertical grid of the retriev- al, AK is the square Averaging Kernel matrix on the vertical grid of the retrieval,is AKh the rectangular high resolution Averaging Kernel matrix. Figure 2 shows the comparisons of the convo- luted EMAC (left) and CLaMS (center) model results with MARSCHALS (right) retrievals for O3, HNO3, H2O and N2O. The Figure shows that there are some differences between the two models and that in general, the agreement between models results and MARSCHALS measurements is quite good. The observed dif- ferences between the two models can be due to 7 ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6333 both the different chemical initialization (the CLaMS model was initialized using MLS data on the 1st November 2011 while EMAC was initialized with model data on the 1st January 2011) and to differences in the Polar Strato- spheric Clouds (PSC) NAT particles sediment- ation scheme. For ozone the models are in good agreement below 17 km, while above 17 km EMAC results are systematically larger. Within this uncertainty MARSCHALS ozone is in good agreement with the models. The HNO3 comparison shows that below 15 km the mod- els are in good agreement and the agreement with MARSCHALS retrievals is reasonably good. Above 15 km the differences among MARSCHALS and the models are larger; how- ever these differences are present also between the model profiles with CLaMS results system- atically larger then EMAC ones. Nevertheless both MARSCHALS and the models consist- ently show a HNO3 reduction at 20 km. Mod- els H2O is in quite good agreement above 12 km, while below 12 km CLaMS results are sys- tematically larger. At lower altitudes, MARSCHALS is in better agreement with EMAC results. The model results for N2O are in good agreement, while MARSCHALS res- ults are systematically lower than the models below the flight altitude. CO values from both models agree quite well in the stratosphere above 10 km, however CO from MARSCHALS retrievals assumes much lower values (not shown here) substantially because of the inter- ference of CO with CH3Cl [Castelli et al., 2013]. IV. SUMMARY AND CONCLUSIONS In the frame of the PREMIER preparatory activities, the activities conducted for the ESA project “PREMIER Analysis of Campaign Data” aimed at demonstrating the capabilities of MARSCHALS to measure the vertical distri- bution of atmospheric targets (also through the comparison with chemical transport model results) and to evaluate of the potential syn- ergy between limb measurements in the in- frared and in the millimetre-wave region. The inverse processing of MARSCHALS limb spectra, during the PREMIER-Ex 2010 and the ESSenCe 2011 campaigns, clearly demon- strated the capability of the instrument, when operating on the three spectral bands (i.e. Band B, C and D, centered respectively at 300, 325 and 345 GHz) to measure vertical profiles of temperature and VMR of Water Vapor, Ozone, HNO3, N2O and CO. This capability was suc- cessfully tested both in clear-sky and cloudy- sky conditions and proved the advantages of using mm-wave limb measurements to sound the UTLS altitude range in presence of clouds that are opaque to the infrared sounding. Sig- nificant results were obtained in the investiga- tion of the potentiality of combining the in- formation from collocated and independent data sets measured by mm-wave and infrared limb sounders. By using MARSCHALS and MIPAS-STR data on the flight of 10 March 2010, we compared the performances of indi- vidual and synergistic retrieval based on dif- ferent approaches to data fusion. Both the L1+L2 and the MSS methods showed that the fused data have higher quality than the single one. The improvement was quantitatively as- sessed through selected quantifiers (total re- trieval error, DOFs and GI). In addition, we proved that the MSS method enables an easier evaluation of the quality improvements of the data fusion products. Finally, the comparison of MARSCHALS retrieved profiles with results from CLaMS and EMAC models showed that 8 ANNALS OF GEOPHYSICS, 56, Fast Track-1, 2013; 10.4401/ag-6333 for O3 a generally good agreement is found between retrieved profiles and model outputs. The agreement is good also for H2O and HNO3, with discrepancies between model val- ues and MARSCHALS products at some alti- tudes. For N2O and CO MARSCHALS re- trieved profiles are in general low compared to model simulations. ACKNOWLEDGEMENTS We acknowledge ECMWF for having granted direct access to their archives of atmospheric data and J. Langen for useful comments. REFERENCES [Bühler, 1999] Bühler, S. (1999). Microwave Limb Sounding of the Stratosphere and Upper Troposphere, Shaker-Verlag, Aachen, ISBN 3- 8265-4745-4. [Carli et al., 2007] Carli, B., Bazzini, G., Castelli, E., Cecchi-Pestellini, C., Del Bianco, S., Dinelli, B.M., Gai, M., Magnani, L., Ridolfi, M., and Santurri, L. (2007). 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