GOME-2 COMMISSIONING RESULTS: GEOPHYSICAL VALIDATION OF LEVEL 1 PRODUCTS

Transcription

1 GOME-2 COMMISSIONING RESULTS: GEOPHYSICAL VALIDATION OF LEVEL 1 PRODUCTS Rosemary Munro (1), Rüdiger Lang (1), Yakov Livschitz (1), Michael Eisinger (2), Abelardo Pérez-Albiñana (1) (1) EUMETSAT, Darmstadt, Germany (2) ESA-ESTEC, Noordwijk, The Netherlands Abstract The Second Global Ozone Monitoring Experiment (GOME-2) performs operational global monitoring of ozone column densities and ozone profiles, and column densities of other atmospheric trace gases such as NO_2, BrO, OClO, HCHO, and SO_2. GOME-2 is an improved version of the Global Ozone Monitoring Experiment (GOME-1) launched 1995 onboard the second European Remote Sensing Satellite (ERS-2). It was launched on the first of the METOP series of polar-orbiting operational meteorological satellites on 19th October The remaining two satellites in the series will be launched in 2010 and GOME-2 geophysical validation activities follow successful in-orbit verification of instrument function and performance. Initial checks include validation of the geolocation and spectral calibration parameters, and verification of the cloud and polarisation information. This is followed by validation of the radiance and irradiance products. This paper presents the results of the GOME-2 geophysical validation activities from the METOP commissioning phase. A presentation of achievements and anticipated improvements will be given. GEOLOCATION Verification of geolocation parameters is easily carried out by a visual examination of false colour images for the Polarisation Measurement Devices (PMDs). For GOME-2 the geolocation parameters were quickly shown to be accurate soon after launch. These can be seen in the operational daily reports at gome.eumetsat.int SPECTRAL CALIBRATION GOME-2 spectral calibration parameters are derived from measurements taken using the on-board Spectral Line Source (SLS) lamp. For each GOME-2 channel a low order polynomial is used to describe wavelength as a function of detector pixel. An expected spectral shift is observed between calibration Key data measured on ground and the in-orbit situation due to the change from 1g to 0g (Fig. 1). In-orbit the positions of the SLS lines are fitted to within 0.006nm and the in-orbit spectral stability is very good. The spectral calibration calculated using the on-board SLS compares well to spectral calibration parameters calculated using Fraunhofer lines in the Sun spectrum at known spectral locations (Fig. 2). The maximum deviation of 0.065nm is observed at the shortwave end of channel 2 where there is a known lack of lines in the SLS. Investigation of the quality of the spectral calibration of the PMD measurements, and in particular the impact of the imperfect spectral coregistration of the two PMDs is on-going. An update of the PMD band definition to mitigate, at least in part, the effects of the co-registration errors is planned in the near future.

2 Figure 1: Change in GOME-2 main channel spectral calibration between the Key data measured on-ground and the in-orbit situation. Figure 2: Difference between in-orbit spectral calibration parameters calculated using the onboard Spectral Line Source lamp and spectral calibration parameters calculated using Fraunhofer lines in the Sun spectrum at known spectral locations.

3 CLOUD PRODUCTS The GOME-2 level 1b product includes cloud parameters calculated using the FRESCO algorithm [2]. In addition to comparison with offline FRESCO calculations and FRESCO cloud products from SCIAMACHY (see [3]) the GOME-2 cloud products have been qualitatively compared to AVHRR (see Fig. 3 comparing the FRESCO effective cloud fraction, cloud top pressure and AVHRR reflectance for the 30th October 2006). In general the agreement is good but for a quantitative comparison of AVHRR and FRESCO cloud products AVHRR cloud optical depth would be required. This will be considered in the future. c eff p AVHRR Reflectance Figure 3: Comparison of FRESCO effective cloud fraction, cloud top pressure and AVHRR reflectance for the 30th October DEGRADATION Degradation has already been observed in the GOME-2 Sun Mean Reference spectra. The degradation rate and pattern (see below clearly showing more degradation in the Ultra-Violet) is consistent with that observed from GOME-1. The degree to which the Earthshine data are degrading, and any angular dependencies are still to be examined. Additionally, as GOME-2 includes two PMDs sensitive to orthogonal polarisation directions it has been possible to observe as shown in Fig 4. some differential degradation between the two PMDs in SMR PMD measurements taken in raw mode (high spectral resolution) once per day, but this trend is not consistent. Whether this is also taking place in the Earthshine PMD measurements still remains to be analysed. A full degradation analysis will be instigated following a first reprocessing of the complete data set to remove any discontinuities in the data record due to algorithm corrections and improvements.

4 Figure 4: Ratio of calibrated Sun PMD-p to PMD-s measurements at 350nm from January to September POLARISATION Proposals for the verification and validation of GOME-2 polarisation products have been made in the GOME-2 polarisation study conducted by SRON [4]. These proposals include examination of the Stokes fractions calculated from SMR PMD measurements which would be expected to be zero as the Sun provides an unpolarised input source. In the case of GOME-2 significant deviations from zero are observed in the Sun Stokes fractions (see Fig. 5). These deviations can be directly traced to the fact that the key data describing radiometric calibration of the PMDs for irradiance measurements no longer appear to be completely valid in-flight. In contrast for special Earth viewing geometries, where taking into account the illumination geometry, the Stokes fractions of the light reflected by the Earths atmosphere are expected to be exactly zero [3], there is much better performance for the equivalent Earthshine PMD measurements taken in raw mode (see right). Although there is significant spread, the mean is close to zero. Furthermore if the viewing angle on the scan mirror is restricted to +/- 5 degrees the deviation from zero is significantly reduced (see below right). An analysis of the remaining spread is continuing. There are indications that it may be related to viewing angle however this is not conclusive. Additionally, the spectral calibration of the PMD band measurements and the implications of imperfect PMD spectral co-registration on the Stokes fractions calculated from PMD band data (and used to polarisation correct the Earthshine data) are under investigation. All activities are supported by SRON via the GOME-2 Polarisation Study Scientific Support to Calibration & Validation (see also [5]).

5 Figure 5: Stokes fractions calculated from Sun PMD measurements in raw measurement mode. Figure 6: Earth Stokes fractions for special viewing geometries for all viewing angles (above) and restricted to measurements within 5 degrees of nadir (below).

6 SOLAR MEAN REFERENCE (SMR) SPECTRUM A GOME-2 SMR spectrum is measured daily and reported in the level 1b product. A comparison has been made between the GOME-2 SMR and a selected extraterrestrial reference spectrum [6] & [7]. In general the comparison appears to be good above 300nm with the agreement within a few percent (Fig. 7). Below 300nm the quality of the reference spectrum is not well established and the comparison can be expected to be affected by in-orbit degradation of the GOME-2 Solar spectrum. Figure 7: Comparison between a GOME-2 Sun Mean Reference Spectrum and a selected extraterrestrial spectrum [6] and [7]. CONCLUSIONS Initial calibration and validation activities indicate that much of the GOME-2 level 1 be product can be considered validated, in particular the geolocation, spectral calibration of the main channels and cloud parameters. Work is on-going on the polarisation measurements and products, including spectral calibration. Initial results indicate some problems in the relative radiometric calibration of the SMR PMD measurements although this has no impact on the derived polarisation correction. The performance for Earthshine measurements, critical for the polarisation correction, appears to be significantly better although the angular dependence of the relative radiometric response of the PMD-p and PMD-s needs further investigation. Initial comparison of the GOME-2 SMR with reference spectra shows good agreement. At shorter wavelengths the observed degradation in SMR measurements can be expected to begin to impact the comparison until a degradation correction is put into place. The derivation of degradation correction factors including an assessment of the impact of differences in the observed degradation rate between PMD-P and PMD-S, is planned following an initial reprocessing of the data which is expected to be completed in the near future. A second reprocessing including the derived degradation factors in the processing chain is anticipated.

7 REFERENCES [1] GOME-2 Calibration and Validation Plan, EPS.SYS.PLN [2] Koelemeijer, R. B. A., P. Stammes, J. W. Hovenier, and J. F. de Haan, A fast method for retrieval of cloud parameters using oxygen A-band measurements from the Global Ozone Monitoring Experiment, J. Geophys. Res., 106, , [3] Wang, P. et al., The operational GOME-2 cloud product algorithm and preliminary validation these proceedings. [4] Hartmann, H.W., C.P. Tanzi, J.M. Krijger, and I. Aben, GOME-2 Polarisation Study - Phase C/D: Final Report, RP-GOME2-003SR, SRON, Utrecht, The Netherlands [5] Tilstra, L.G. et al., GOME-2 Polarisation Measurements -Validation Results these proceedings. [6] Wehrli, C. "Extraterrestrial Solar Spectrum", Publication no. 615, Physikalisch Meteorologisches Observatorium + World Radiation Center (PMO/WRC) Davos Dorf, Switzerland, July [7] Neckel, H. and D. Labs "Improved Data of Solar Spectral Irradiance from 0.33 to 1.25 um", Solar Physics, Vol 74, 1981.