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Number of matched GERB CERES points as a function of GERB detector number obtained from the special scan data used in the pixel level comparison for June (red) and December (blue) datasets. 

Number of matched GERB CERES points as a function of GERB detector number obtained from the special scan data used in the pixel level comparison for June (red) and December (blue) datasets. 

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... GERB / CERES ratio values are shown in table 5 for the SW and table 6 for the LW. In each case the mean radiance for each day of GERB and CERES points is derived and the ratio of these two mean values determined. The mean and standard deviation of the ratio is then calculated from the daily ratios. Table 5 shows the mean ratio and an associated 99% confidence uncertainty determined as 3 time the standard deviation divided by the square root of the number of days. SW radiance data FM2 (Edition 2) FM3 (Edition 1b) GERB/CERES All points June 1.058 ± 0.005 1.067 ± 0.013 Dec 1.048 ± 0.005 1.075 ± 0.006 Overcast June 1.041 ± 0.013 1.039 ± 0.016 Dec 1.032 ± 0.005 1.050 ± 0.017 Cloud cover = 100% τ >7.4 For Clear GERB pixels (GERB cloud cover 0%) Ocean 1.144 ± 0.043 1.084 ± 0.052 Dark Vegetation 1.070 ± 0.017 1.089 ± 0.039 Bright Vegetation 1.062 ± 0.010 1.086 ± 0.013 Dark Desert 1.073 ± 0.019 1.101 ± 0.035 Bright Desert 1.059 ± 0.006 1.082 ± 0.012 Some scene dependence is seen in the radiance comparisons. For the SW agreement between GERB and CERES (in percentage terms) is better for bright scenes than for dark scenes. This is exhibited by a higher SW ratio for ocean than for cloud or bright desert, for example. Separating the GERB SW radiances up into percentiles before making the comparison also illustrates the scene dependence of the comparison results, as shown in table 7 below. GERB/CERES FM2 (Edition 2) GERB/CERES FM3 (Edition 1b) GERB radiance June Dec June Dec percentile 0.0-0.1 1.136 ± 0.012 1.141 ± 0.020 1.132 ± 0.027 1.142 ± 0.014 0.1-0.2 1.073 ± 0.009 1.086 ± 0.032 1.072 ± 0.011 1.102 ± 0.018 0.2-0.3 1.045 ± 0.015 1.060 ± 0.021 1.036 ± 0.010 1.062 ± 0.022 0.3-0.4 1.028 ± 0.008 1.042 ± 0.020 1.029 ± 0.009 1.049 ± 0.008 0.4-0.5 1.014 ± 0.014 1.014 ± 0.026 1.024 ± 0.009 1.041 ± 0.029 0.5-1.0 1.013 ± 0.009 1.014 ± 0.021 1.015 ± 0.014 1.027 ± 0.017 In the LW a scene dependence is also observed. The results of a similar breakdown of the LW radiance ratio by radiance bin is shown in figure 1. This shows that for warmer scenes with radiances above 60 Wm -2 sr -1 the GERB LW radiance is about 1% lower than the FM2 radiance and 2% lower than the FM3. For cold scenes (radiances < 60 Wm -2 sr -1 ) the GERB/CERES ratio increases strongly becoming greater than 1 for radiances below than < 50 Wm -2 sr -1 . It should be noted that the scene dependent differences seen in the LW and SW comparisons may be affected by the comparison methodology and geolocation errors. For the extreme scenes geolocation errors can lead to systematic effects on the inferred average radiance. For example geolocation errors can lead scenes identified as ocean to occasionally contain some cloud; in the SW these cloud points will always be brighter than the dark ocean and therefore act to elevate the average SW radiance inferred. However, the scene dependent differences can also be due to systematic uncertainties in the spectral response of the instrument, as bluer scenes tend to darker scenes, and colder scenes will have a greater proportional of the radiance at longwave wavelengths. Similar comparisons between the GERB reflected solar radiances and the CERES ES8 edition 2 rev1 data from FM2 have been made on an individual pixel basis. These comparisons use a special GERB validation product (directly unfiltered Version 998 GERB L1.5 NANRG) which is not interpolated to a regular grid, to enable each individual GERB pixel to be compared separately. The CERES data were obtained during a period of special operation when FM2 employed a “GERB mode” programmable 10 azimuth scan which increases the number of angularly-matched measurements between the two instruments. The GERB unfiltered radiances compared in this study differ slightly from the level 2 ARG radiances as they do not use SEVIRI in their unfiltering, but are unfiltered based solely on the filtered radiance observed. This different unfiltering method is expected to increase the random error associated with the unfiltering but should not significantly affect the bias except to the extent that SEVIRI inter-channel calibration errors may introduce a bias (estimated to be less than 1%) into unfiltered level 2 ARG radiances. GERB and CERES pixels are matched after adjusting for the fact that the CERES ES8 locations are at 30 km altitude and the GERB locations are at the surface. Using data obtained during the special ‘GERB scan’ mode employed by FM2, points are matched when the view zenith angle and azimuth angle between the CERES and GERB pixels are within 5 ° and the view zenith angle is less than 70 ° . The operation of CERES in the programmed azimuth plane mode assures that the azimuth angles agree well within the five-degree range. When Terra flies nearly under GERB, however, the azimuth angle can vary considerably due to the polar singularity, so if the angle between the ray from the Earth scene to CERES and the ray from the Earth scene to GERB is less than ten degrees, the match is accepted. This treatment is justified by the fact that the reflected solar radiation does not vary significantly within these angle ranges. Thus the error due to the increased angular difference is small, whereas the increased number of measurements obtained with this treatment greatly reduces the scatter in the results. Figure 2 shows the number of matched points as a function of GERB pixel number, used in the June and December comparison. Figure 3 shows as a function of GERB pixel number the resulting average GERB/CERES SW radiance ratio and GERB-CERES SW radiance difference determined by this comparison. Averaged over all pixels the pixel level comparison shows V998 GERB SW directly unfiltered radiances to be 5.5% higher than CERES. A standard deviation of 2% in the GERB/CERES ratio is seen across the pixel array and a noticeable broad scale structure across the pixels is apparent. As the pixel array is oriented roughly north-south with respect to the Earth this structure will be similar to the variation in the GERB CERES ratio with ...

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