VALIDATION OF ENVISAT PRODUCTS USING POAM III O 3, NO 2, H 2 O AND O 2 PROFILES A. Bazureau, F. Goutail Service d Aéronomie / CNRS, BP 3, Réduit de Verrières, 91371 Verrières-le-Buisson, France Email : Ariane.Bazureau@aerov.jussieu.fr ABSTRACT The scope of this paper is to give preliminary results from the validation of ENVIronmental SATellite (ENVISAT) Level 2 products, more particularly ozone, nitrogen dioxide, water vapor, oxygen densities, temperature and pressure profiles measured by the Gobal Ozone Monitoring by Occultation of Stars (GOMOS) instrument. GOMOS is the first stellar occultation space instrument. Its wavelength coverage (between 250 and 952 nm) allows to provide profiles from the tropopause to 120 km with an altitude resolution better than 1.7 km. A comparison analysis is presented between coincident measurements from GOMOS and the Polar Ozone and Aerosol Measurement III (POAM III). First results reveal the expected star dependence in the quality of the retrievals. Preliminary results does not intend to give a definitive conclusion about the validation and need further works. Ozone profiles are well retrieved within 20% between 17 and 50 km, pressure ones within 1% between 10 and 60 km and temperature ones are retrieved within 5% between 10 and 60 km. Considering H 2 O and NO 2 results, unfortunately the quality of the GOMOS data is poor. Finally we discuss about the star dependence on the retrievals quality. 1. PRESENTATION OF POAM III INSTRUMENT POAM III is a nine channels (0.354 to 1.018 µm) solar occultation instrument designed to measure stratospheric profiles of ozone, nitrogen dioxide, water vapor densities, aerosol extinction at five wavelengths and temperature. It was launched on March 1998 aboard the CNES Satellite Pour l Observation de la Terre (SPOT 4) into a sun-synchronous quasi-polar orbit and is currently operational [1]. The sunlight transmission measurement through the earth s atmosphere for SPOT 4 occurs on a circle of nearly constant latitude. The latitude of POAM III measurements varies between 54 and 71 N and between 62 and 82 S throughout the year. All measurements in the Northern Hemisphere (NH) are made at sunset and all measurements in the Southern Hemisphere (SH) are made at sunrise. This correspond to local sunset in the NH throughout the year, to local sunrise in the SH from early April to early September, and to local sunset in the SH at other times of year. POAM III provides 14 profiles of atmospheric constituents (O 3, O 2, NO 2, HO 2 ) densities and aerosol extinction coefficients per day on a latitudinal circle separated by about 25.4 in longitude, alternately in the NH and SH. The instrument vertical resolution is 1 km for O 3 and about 1.5 to 2.5 km for NO 2. Table 1 shows the summary of POAM III validation results as given by the most recent publications. Details of the POAM III version 3.0 O 3, NO 2, H 2 O and aerosol extinction validation analyses can be found elsewhere [2,3,4,5,6].Validation analysis for POAM III show that POAM III measurements accuracy for both hemispheres is estimated to 5-10 % in the 13-60 km altitude range for O 3 [7] and is within 10% in the 20-45 km altitude range for NO 2 [3]. However, it showed a hemispheric dependence in the H 2 O results : its accuracy varies from 5% in NH to 8% in the SH. Table 1 : Summary of POAM III validation results. Values are taken from POAM III Validation web site (http://wvms.nrl.navy.mil/solve/valid/valid.html) and from [2,3,4]. The corresponding altitude range is given between the brackets. Northern Hemisphere Southern Hemisphere Ozone < 5-10 % (13-60 km) < 5-10 % (13-60 km) NO2 < 10% (20-45 km) <10% (20-45 km) H2O 5% (15-45 km) 8% (15-45 km) Aerosol 1.018 micron <30% (10-24 km) <30% (10-21 km) Proc. of Envisat Validation Workshop, Frascati, Italy, 9 13 December 2002 (ESA SP-531, August 2003)
2. SEARCHING FOR COINCIDENCES POAM III/GOMOS POAM III offer the most opportunities for correlative measurements at high latitudes. Because of the unavailability of MIPAS and SCIAMACHY distributed data, we focus in this paper on the comparison between GOMOS and POAM III measurements. All results presented here originates from 5.3 version GOMOS data. The purpose in this paper is to present two comparisons made for each available measurements period : the first one is located in the southern hemisphere in July 2002 and the other in the northern hemisphere in September 2002. The GOMOS and POAM III data sets were searched for spatially and temporally coincident events for these different periods. Coincidences were determined by requiring correlative measurements to occur within 5 in latitude, 12 in longitude and 24 hour in time of GOMOS measurements. Difference profiles between GOMOS and correlative POAM III measurements were calculated using : POAM i GOMOSi i = 100. (1) POAM i Here GOMOS i and POAM i refer to the GOMOS and correlative POAM III measurements respectively at the i th altitude, after interpolating GOMOS data on POAM III altitude grid, every 1 km. The intersection of these two altitude grids give an altitude coverage from 15 km or more, depending on the POAM III involved constituent altitude limitation given by the validation results (see table 1), to 60 km, which is the POAM III maximal altitude. For example, the minimal altitude is fixed to 17 km for O 3, 20 km for NO 2 and 15 km for H 2 O. 3. COMPARISONS GOMOS AND POAM III 3.1 the Southern hemisphere case Within the limits of GOMOS Level 2 products available in December 2002 for the comparison, there were 19 measurements meeting the coincidence criteria in the southern hemisphere in July 2002. The positions of the tangent point for these coincident events are plotted in Figure 1. Average distance between POAM III and GOMOS for all events is less than 450 km. All measurements occurred during twilight or nighttime, when sun zenith angle (SZA) is greater than 90, except one at SZA = 87. Since GOMOS measures atmospheric transmission by stellar occultation using various stars of different magnitude and temperature, it is important to notify the occulted star in all comparison analysis. a) Stars : The reference star name for the 19 events are split between 25 Eta Tau star (13 events) and 87 Alp Tau star (7 events). 25 Eta Tau, identified by the number 146 in the GOMOS data set star catalogue, is a hot star (temperature = 15200K) and a weak star (magnitude = 2.87). 87 Alp Tau, number 13 in the star catalogue, is a cold star (temperature = 3800 K) and a strong star (magnitude = 0.86). Information on star temperature and magnitude is useful to test the inversion algorithm performance : for instance, brighter star allows to retrieve atmospheric constituent from the high altitude down to a minimum altitude of 10 km while pointing system lost weak star for altitudes between 10 and 20 km. b) Temperature and pressure : Figure 2 shows the individual relative differences between POAM III and GOMOS for each constituent profiles in the vicinity of the Antarctic continent in July 2002. The average is also plotted with a different color depending on the star. POAM III temperature and pressure profiles originate from the NMC analysis, they are compared to GOMOS temperature and pressure, which are a composite of the Rayleigh scattering, O 2 densities profiles and the ECMWF a- priori model. It is difficult to see which part of these profiles is coming from the inversion algorithm or which is driven by the input model. However the comparison conducted on temperature and pressure relative difference profiles shows a good agreement between these two sets and an obvious dependence on reference stars. Pressure mean relative difference is within 8% between 10 and 60 km for the star 25 Eta Tau, with an average value of about 0.6% in the same altitude range. This is better than the case of star 87 Alp Tau, where the main relative difference values reach 12% at 60 km and exhibit a positive bias of 10% between 30 and 60 km. For temperature, the mean relative difference values are within 5% between 10 and 60 km, with peak at 6% around 41 km for whichever reference star. This strange behavior is under consideration. The average temperature difference is about 1.4% in the altitude range 14-60 km for the two reference stars.
Fig. 1 : POAM III and GOMOS coincident events over the Antarctic continent in July 2002. The occulted star is notified by two different symbols : blue circles is for 25 Eta Tau (star identifier 146), of magnitude 2.87, and red cross is for 87 Alp Tau (star identifier 13), of magnitude 0.8. Furthermore comparisons between these mean relative difference (pressure and temperature) show a dependence on the quality of the retrievals according to the reference star involved. Comparisons for all other constituents has been conducted taking into consideration each star and considering the mean relative difference. c) Oxygen : Considering O 2 results obtained for 87 Alp Tau, the individual relative differences are within 50% in the 15-60 km altitude range. The mean relative difference is equal to 11% in the same altitude range. Compared to the 25 Eta Tau results, the 87 Alp Tau mean relative difference curve is consistent. Here, 25 Eta Tau individual relative difference curves displays profiles with oscillations, greater than 100%, and exhibits unrealistic values above 40 km and below 30 km. Between 30 and 40 km, average relative difference shows a negative bias of 35%. Considering the comparison between the stars, the retrievals is better for 87 Alp Tau than for 25 Eta Tau. It reveals an obvious reference star dependence in the oxygen retrieval quality. The GOMOS O 2 densities profiles have to be redo, specially for 25 Eta Tau. d) Ozone : For 87 Alp Tau star ozone retrievals, the individual densities agree within 20% between 17 and 50 km, with peaks of 45% to 60% around 35 km. Above 35 km, mean relative difference concerns only 2 events. In the 17-50 km altitude range, average relative difference is less than 5%. Above 50 km, the relative difference reaches values of 100%. For 25 Eta Tau star, more than 12 individual retrieved profiles exhibits larger discrepancies between 17 and 50 km compared to 87 Alp Tau. Considering the mean relative difference, it gives a negative bias of 11% between 17 and 50 km. Above 50 km, the discrepancies increase and reach 100% or more. Globally, ozone retrieved densities agree with POAM III measurements within 20% between 17 and 50 km but this hides large discrepancies in individual profiles. Retrievals above 50 km need to be reprocessed. Unrealistic peaks around 35 km are observed, probably due to the use of cold stars with very low signal in the UV part of the spectrum [Bertaux, private communication].
Fig. 2 : Pressure, temperature and four constituents (O3, NO2, O2 and H2O) profiles results from GOMOS measurements. Individual relative differences are notified by different colors per reference star : red for Alp Tau star (N 13) and blue for Eta Tau star (N 146). The average relative difference for each different star is emphasized by thick lines.
e) NO 2 and H 2 O : Considering all stars, NO 2 individual retrieved densities, between 20 and 45 km restricted to the POAM III NO 2 altitude range, show large deviations (1000%!!) and densities values are not physical : the quality of GOMOS NO 2 retrieved profiles is poor. It needs to be improved. For H 2 O, GOMOS water vapor retrieved densities profiles are compared with POAM III, restricted to the POAM III validated altitude range (15-45 km). The discrepancies for the star 87 Alp Tau are unrealistic below 30 km but between 30 and 45 km the average of the mean relative difference is about -13%. The agreement is reasonable but it concerns only 5 events. In addition, H 2 O results are not available for the 25 Eta Tau star. Water vapor results need to be improved. 3.2 the Northern hemisphere case a) Star : Within the limits of GOMOS Level 2 products available in December 2002, there were 47 measurements meeting the coincidence criteria in the northern hemisphere in September 2002. These coincident events are plotted in Figure 3. GOMOS measurements occurred during twilight (27 events) and daytime (20 events), with SZA varying between 69 and 109. The number of events are sufficient to allow a study on the impact of twilight and bright limb conditions in the GOMOS retrievals. Average distance between POAM III and GOMOS for all events is less than 360 km. The reference star is the same for all events : 49 Del Cap, identified by the number 142 in the GOMOS data sets star catalogue, is a hot star (temperature = 8900K) and a weak star (magnitude = 2.85). Figure 3 : POAM III and GOMOS coincident events over the Arctic continent in September 2002. The occulted star is 49 Del Cap (N 142) of magnitude 2.87.
b) Pressure and temperature : Figure 4 shows the individual relative difference between POAM III and GOMOS for each constituent profiles. The reference star ( 49 Del Cap) is the same for all measurements. As the measurements are split into two different observation conditions, the individual relative difference profiles are plotted with different color depending on the limb condition. In figure 4, the bright limb, when SZA is less than 90, is symbolized by red dashed lines (20 events) and the twilight limb, when SZA is between 90 and 110, by light blue dashed lines (27 events). The average (bright and twilight limb) is also plotted, highlighted by a black thick line. Compared to the southern hemisphere case, individual pressure and temperature relative differences are more consistent with POAM III NMC profiles with smaller dispersions. Individual pressure profiles are within 3.5% for the whole altitude range, while temperature ones are within 2%. In the altitude range 15-60 km, pressure mean relative difference is less than 2%, and temperature one is less than 0.5%. Comparisons between measurements performed during bright limb condition and twilight limb ones show that there were no evidence of a SZA dependence. Relative difference for the GOMOS pressure and temperature shows that the quality of the retrievals is better than in the southern hemisphere case. c) Oxygen : Considering O 2 results obtained, the individual relative differences curves displays profiles with oscillations, greater than 100% and exhibits unrealistic values above 40 km. Between 25 and 40 km, average relative difference shows a negative bias less than 5%. Considering the limb conditions, comparison between 20 and 40 km show a SZA dependence. This small dependence is given by the average relative difference values : when measurements are performed at twilight conditions, the average relative difference (25-40 km) is less than 6% whereas bright condition gives value greater than -40% in the same altitude range. O 2 retrievals are shifted down (to negative bias) when the limb is brighter. GOMOS O 2 is certainly perturbed by the straylight during bright limb observations. The GOMOS O 2 densities profiles need to be reprocessed. d) Ozone : Considering ozone results, the individual densities agree within 50 % in the altitude range 20-60 km, with some sporadic oscillations (100%) around 23 km and around 30 km (at more 2 events). Individual relative difference dispersions are smaller than in the southern hemisphere case. Above 55 km, the discrepancies increase to reach 25%. Between 20 and 55 km, the average relative difference exhibits a positive bias of 12%. Considering limb conditions, comparisons in the same altitude range show no SZA dependence : the average relative difference curves are equivalent and the average relative difference is the same for the two limb conditions. The analysis of this constituent is not perturbed by straylight. e) NO 2 and H 2 O : NO 2 individual densities profiles, between 20 and 45 km restricted to POAM III NO 2 altitude range, are poor above 40 km and densities values are not physical. Below 40 km, NO 2 individual profiles agree within 300%. Considering the average relative difference, between 20 and 40 km, the retrievals exhibits a negative bias (-230%) because of some important individual oscillations (3 events are concerned). These events are under twilight conditions. Considering limb conditions, the average relative difference is better under brighter limb condition within 23% (between 20 and 40 km). However, NO 2 inversion retrievals need to be improved. Water vapor individual relative difference curves display profiles with oscillations, greater than 600% and exhibit unrealistic values. Considering the average relative difference (it concerns at more 12 events), the curve exhibits unrealistic large oscillations. The limb conditions analysis reveals no obvious SZA dependence since the curves are not consistent. Water vapor results need to be regenerated. 4. SUMMARY a) Stars and time Table 2 shows the characteristics of the stars involved in the comparisons between GOMOS and POAM III. For all stars, H 2 O and NO 2 retrieved values are neither physical values nor retrieved (null constant) then discussions about these constituents cannot be conducted. A summary of the mean relative differences for all comparisons (temperature, pressure, O 2 and O 3 ) are plotted in Figure 5, where reference stars are plotted with different colors and symbols. Different dates and locations are merged : it is a raw comparison in spite of the fact that these measurements are made at the same high latitude but not at the same season and not at the same limb condition.
Fig. 4 : Pressure, temperature and four constituents (O3, NO2, O2 and H2O) profiles results. Individual relative differences are notified by different colors per limb condition : blue line for twilight limb and red line for bright limb. The reference star is the same for all measurements Del Cap star (N 142). The mean relative difference is emphasized by black thick lines.
Table 2 : Summary of the reference star involved in the validation. The star identifier is the reference number in the GOMOS data sets star catalogue. Location parameter relies on the geographic location of GOMOS measurements : NH is in the northern hemisphere and SH is in the southern hemisphere. Limb columns gives the number of events for each different limb condition (dark, twilight and bright). Star identifier 13 146 142 Star name Temperature (K) Apparent magnitude Date / Location Dark limb Twilight limb Bright limb 87 Alp Tau 3800 (cold) 0.8 (strong) July 2002 / SH 3 3 0 25 Eta Tau 15200 (hot) 2.87 (weak) July 2002 / SH 8 4 1 49 Del Cap 8900 (hot) 2.85 (weak) Sept. 2002/ NH 0 27 20 b) Limb conditions : As GOMOS provides measurements in different limb conditions, it is important to take it into account in the retrieval quality analysis. The night time measurements provide better atmospheric transmissions since there is less straylight. Table 2 sums up all the number of events for each limb conditions and for each stars. During July 2002, POAM III measurements are all made at local sunrise while the coincident GOMOS events for two different stars are distributed as followed : 1 bright, 7 twilight and 11 dark limb. Limited number of sample (for each star and for each limb conditions) do not allow a qualitative comparison. In the northern hemisphere, GOMOS measurements are performed with the same reference star at bright limb (20 events) and at twilight limb (27 events) but no dark limb matching coincidence criterion with POAM III occurred during this same period. So the analysis is restricted to a comparison between bright limb and twilight limb. Discussion about the dark limb condition in the retrievals accuracy cannot be conducted. Furthermore, more measurements at different limb conditions are needed for a better statistical analysis. b) Pressure and temperature In SH case pressure, two relative difference profiles display curve with a important bias (more than 12%) for altitudes greater than 40 km. In NH case, the bias is less than 2%. Considering temperature results, mean relative difference profiles show a bump between 30 and 40 km for SH case, with large values of 6% for 25 Eta Tau star, but not for NH case. Pressure and temperature retrievals agree with NMC analysis within 12% and 6% respectively. In spite of the low number of cases studied, preliminary results show there were no obvious SZA dependence for temperature and pressure retrievals (a-priori model). c) Oxygen Considering the 25 Eta Tau and 49 Del Cap case, the O 2 densities profiles display large discrepancies, with oscillations greater than 100% and exhibits unrealistic values, whereas O 2 densities are retrieved within 11% between 15-60 km for 87 Alp Tau star. 87 Alp Tau is the only strong and cold star. It reveals an obvious star dependence in the oxygen retrieval quality. The preliminary limb comparison (bright/twilight) made in paragraph 3.2 c) show that there also exist a limb dependence in the retrieval accuracy. d) Ozone Considering O 3 results, significant oscillations are shown in the 87 Alp Tau mean relative difference curve, nevertheless the corresponding bias is less than 5%. For 25 Eta Tau and 49 Del Cap case, the curves are consistent and are in opposition to themselves. Their respective bias are 11% and 12% (between 17 and 50 km). These results show that retrieval accuracy has to be improved. Obviously, the brighter the star is the better accurate the ozone retrieval is. The colder stars, which temperature is less than 7000K, have less UV flux compared to the hot stars, so the inversion products quality is poor specially above 35 km. In preliminary results, the SZA dependence (bright/twilight) was not clearly seen. 5. CONCLUSION This paper does not give a definitive conclusion about the GOMOS validation but highlights that GOMOS data need a new retrieval. The authors recommend to reprocess all GOMOS correlative measurements for a more complete analysis. Concerning the temperature, it is suggested that distributed data reflect the part of retrieved GOMOS measurements
from the a-priori model used (called a measurement response ). Concerning NO 2 and H 2 O, the GOMOS retrievals is not correct. 6. ACKNOWLEDGEMENTS The authors thank Richard M. Bevilacqua and the Naval Research Laboratory (NRL) Team for providing POAM III data and B. Theodore and ACRI for coincidences matching calculations between GOMOS and POAM III positions and providing the corresponding files. This work was supported by the Centre National d Etudes Spatiales (CNES) and the French Programme National de Chimie de l Atmosphere (PNCA). Fig. 5 : Pressure, temperature and two constituents (O 3, O 2 ) difference profiles between GOMOS and POAM III measurements. The mean relative differences profiles are notified by different colors per reference star : red for 87 Alp Tau star (N 13), blue for 25 Eta Tau star (N 146) and green for 49 Del Cap star (N 142). Here, HN acronym refer the northern hemisphere case and HS the southern hemisphere case.
7. REFERENCES [1] Lucke, R. L. et al., The Polar Ozone and Aerosol Measurement (POAM III) instrument and early validation results, J. Geophys. Res., 104, 18,785-18,799, 1999. [2] Randall, C., R. M. Bevilacqua, J. D. Lumpe and K. J. Hoppel, Validation of POAM III aerosols : comparisons to SAGE II and HALOE, J. Geophys. Res. 106, 27,525-27,536, 2001. [3] Randall, C. E. et al., Validation of POAM III NO2 measurements, in press at J. Geophys. Res., 2002a [4] Randall, C. E. et al., Validation of POAM III Ozone : comparisons with ozonesonde and satellite data, submitted to J. Geophys. Res., 12 September 2002, 2002b. [5] Nedoluha, G., R. M. Bevilacqua, K.W. Hoppel, J.D. Lumpe and H. Smit, POAM III measurements of water vapor in the upper troposphere and lowermost stratosphere, in press at J. Geophys. Res., 2002. [6] Bevilacqua, R. M. et al., Validation of POAM III stratospheric water vapor measurements, submitted to J. Geophys. Res., 2002. [7] Lumpe, J. D., R. M. Bevilacqua, K. W. Hoppel and C. E. Randall, POAM III retrieval algorithm and error analysis, J. Geophys. Res., 10.1029, November 2002.