Comparison between GOME and DOBSON Observations (PRELIMINARY)
ESA's ERS-2 satellite has been tracking total column ozone over the globe every day since summer 1995, using the Global Ozone Monitoring Experiment (GOME). Due to the orbiter's geometry and due to GOME's field of view, GOME yields global coverage at the equator only after collecting data from three successive days. To overcome this deficiency and to allow to monitor the evolution of the global distribution of the Earth's total column ozone content at least on a daily schedule, DLR's DFD has developed an improved Harmonic Analysis scheme to interpolate data gaps between successive orbits.
The quality of these interpolated total ozone maps are routinely checked by comparing GOME's interpolated total ozone data to the corresponding values reported by the ground-based DOBSON spectrometer located at Hohenpeissenberg (48N, 12E; German Weather Service). Due to the processing scheme to derive GOME's level 3 data, interpolated total ozone data from GOME represent some sort of a daily mean. Therefore, comparison concentrates on daily mean DOBSON data.
Figures 1 and 2 present time series plots of GOME and DOBSON observations of total ozone (quasi daily means) for the location of DWD's Meteorological Observatory Hohenpeissenberg. Triangles represent the ground-based measurements while the solid line indicates interpolated results from GOME. Note that the day to day variability of the series is much more pronounced during winter than during summer time. This is due to the different activity of atmospheric dynamics, especially in stratospheric altitude regions. Figure 2 presents the residuals: GOME minus DOBSON on a percentage scale. Broken lines indicate the +/- 5% -levels.
Figure 3 presents the probability distribution of the residuals shown in figure 2. Note, that the abscissa now is in terms of Dobson units. Obviously, there is a systematic offset of about 5 DU (GOME is higher than DOBSON). The mean deviation is around 3.9%. This is an excellent value and well emphasizes the quality and efficiency of the Harmonic Analysis technique used.