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Newsletter #33 January–April 2013
Metop-B/GOME-2 FM2 level 1 — start of operational dissemination of data
Dissemination of level 1b data from the GOME-2 FM2 instrument on Metop-B will start as planned the second week of May 2013. This follows a successful SIOV campaign in October–November 2012 and an intensive commissioning phase that produced updates and improvements to level 1b data quality. Dissemination will also depend on review and approval of the final validation report due the first week of May. This report will be on the EUMETSAT webpage. See the technical documentation section.
All users will be notified of the exact start date of operations, Notice will be posted on the Service Status section of our web-page under Product and Service News and also via the UNS message service.
Level 1 data from GOME-2 FM2 on Metop-B is already available via EUMETCast or the EUMETSAT Data Centre. The data has pre-operational status and dates to mid-February 2013. Daily monitoring reports for both Metop-A and B instruments and detailed information on orbit-to-orbit information are available at gome.eumetsat.int.
GOME-2 Metop-A/B Tandem Operation
As a test, Metop-A and B instruments were operated in tandem with either one or both GOME-2 instruments using a reduced swath of 960 km, instead of the nominal 1920 km swath, during the first half of March 2013. This test was configured as shown in Table 1.
Table 1: GOME-2 tandem operations test schedule. See also the Processor update history.
Figure 1 shows the impact of a reduced swath on the ground-pixel spatial resolution and the one-day global coverage for the nominal wide-swath configuration and the double narrow-swath test sequence number 2. Both are NO2 total column retrievals (“quick-look” retrievals with reduced total column accuracy).
Figure 1: Global NO2 total column 'quick-look' retrievals from both instruments. The left panel uses the current nominal wide swath configuration–1920 km; 40 km × 80 km ground pixel size over the full swath. The right panel shows one day of the tandem-test sequence for which both instrument were operated in narrow-swath mode at 960 km–40 km × 40 km ground pixel size over the full swath.
Recently, the GOME Scientific Advisory group (GSAG) and the O3MSAF consortium made an interim recommendation to EUMETSAT: operate GOME-2 on Metop-A in 960 km swath mode and maintain GOME-2 on Metop-B in 1920 km swath mode for several months. This implementation will gain us experience in the use of the higher spatial resolution data and further improve the consistency of the products from both instruments because it takes advantage of the overlap between the swaths of the two instruments which is still present in this configuration. A more detailed schedule of this implementation of tandem operation will be provided in a separate announcement.
GOME-2 level 1 key-data update: Preparation for Tandem Operations
Metop-B / GOME-2 FM2 level 1
"Cleaning" small-scale spectral features from polarisation key-data
During the pre-operational phase of level 1b commissioning, persistent small-scale structures were identified in the fit residuals of level 2 data. Eventually, these were linked to spectral features introduced by on-ground calibration stimulus sources. These small scale structures were predominantly in key data related to the polarisation correction of the instrument and their angular response data. To remove these small scale structures and reduce outliers in key-data in the angular domain, a combination of fourier-transfer high-pass filtering (spectral domain) and spline interpolation (angular domain) of key-data was applied. The visible effect on the radiometrically calibrated Earthshine data is quite small–but significant, especially at specific viewing angles. See Figure 2.
Figure 2: The top panel shows earthshine spectra at three different viewing angles acquired by GOME-2 FM2 on Metop-B on 26 December 2012 for both old and new key-data sets. The bottom panel shows the residuals between the old and new key-data with “cleaned” polarisation key-data updates.
Metop-A / GOME-2 FM3 level 1
a) Radiometric response calibration
During commissioning of GOME-2 Metop-B level 1 data, we implemented an updated procedure for adjusting the radiometric response functions to account for on-board shifts of the channel separation. This procedure allows the exploitation of level 1b data radiances close to the overlap points (see newsletter #32). This procedure has also been applied to the latest version of FM3 Metop-A radiometric key-data as it also extends the 'valid' spectral regions to be used for level 2 retrievals close to the overlap points and accounts for a long-term drift of the latter that has occurred since the last adjustment of this key-data set shortly after launch in March, 2007). Figure 3 shows the overlap point of Channels 2 and 3 for GOME-2 Metop-A over the full period of the mission. Notice that it reached a stable state only after 2009. At the start of tandem operations, we readjusted the key-data in order to account for this shift.
Figure 3: Overlap point between channel 1 and 2 (left panel) and between 2 and 3 (right panel) for GOME 2 FM3 on Metop-A.
Figure 4: The top two panels show solar mean reference spectrum acquired by GOME-2 FM3 on board Metop-A 26 December 2012 (top panel) compared to a reference spectrum derived by Dobber et al. (<590 nm) and as published by KPNO/AFGL 2004 (>590 nm).The lower two panels show the residual. The left panels show the situation BEFORE the second adjustment of the radiometric response key-data for GOME-2 Metop-A and the right panels show results after this adjustment.
After this second adjustment of GOME-2 Metop-A radiometric response key-data, and when using on-board white-light source spectra from 1April 2013, the overlap region comparisons to the reference data improves significantly. The overall difference to the reference spectrum is due to the instrument signal degradation.
In relation to the previous update on Metop-B key-data (described in newsletter #32), this update will extend the “valid” spectral region of Metop-A GOME-2 level 1b data. Table 2 and Table 3 detail these changes in terms of wavelength (indication depending on actual spectral calibration) and detector fractional pixel number.
Table 2: GOME-2 Metop-A / FM3 Channel overlap point (50/50 signal). Value for the previous key data version is in parenthesis.
Table 3: GOME-2 Metop-B / FM2 validity of etalon correction [detector pixel start/stop]. Value for the previous key data version is in parenthesis.
Table 4 summarizes these results in terms of the “valid” regions in wavelength usable for retrievals before (FM3 key-data version CAL 2.01) and after the update (FM3 CAL 2.02).
Note: Wavelength start/stop is approximate, depending on the actual spectral calibration. Also note that the signal-to-noise ration close to the 50 % overlap point (seen in Table 2 is 50 % lower than in the rest of the channel.
Table 4: GOME-2 Metop-A / FM3 valid spectral regions per channel for use in level 2 retrievals. The table presents the regions before (FM3 CAL 2.01) and after the key-data update (FM3 CAL 2.02)
b) "Cleaning" small-scale spectral features from polarisation key-data
Like the cleaning of GOME-2 FM2 Metop-B key-data described previously, FM3 polarisation related key-data from Metop-A has been cleaned of small scale spectral structures introduced by the on ground calibration sources. The same “cleaning” procedure was used; however, FM3 key-data clearly did not suffer as severely from this problem as FM2 key-data did. The radiometric FM3 key-data upgrade had the biggest impact on the Earthshine reflectivity data, though both the polarisation and radiometric FM3 key-data was updated. Figure 5 shows the combined impact of the cleaned polarisation key-data and the radiometric adjustment on Earthshine reflectivity data from GOME-2 Metop-A.
Figure 5: The top panel shows earthshine reflectivity at three different viewing angles acquired by GOME-2 FM3 on board Metop-A on 26 December 2012 and for the old and new key-data sets. The lower panel shows the residuals between the old and new key-data, which combines "cleaned" polarisation data and radiometric response data updates.
Note: Test data for this key-data update, affecting level 1B quality for both Metop-A and B instruments, is available under ftp://ftp.eumetsat.int/pub/EPS/out/lang/G2AG2B/